BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL.. 43. PP. 965-992:14 FIGS. DECEMBER 30. 1932

INTRUSIVE RELATIONSHIP OP THE TO THE (PORPHYRY) OF SOUTHEASTERN MISSOURI *

BY W. A. TARR (Bead before the Geological Society December 81,1931)

CONTENTS Page Introduction ...... 965 General geology ...... 968 Distribution of and porphyries...... 974 Intrusive relationship of the granite to the porphyry...... 974 General statement ...... 974 Localities where sharp contacts are found...... 975 General statement...... 975 Knoblick M ountain...... 976 Graniteville ...... 980 Cornwall ...... 981 Matthews Mountain ...... 983 Areas of probable intrusive contacts ...... 987 Cedar Creek ...... 987 Mount Devon ...... 987 Buckner Mountain...... '...... 987 Cold water, Wayne County ...... 987 Contacts in and around the large central area of granite...... 987 General discussion ...... 987 Evans Mountain and hill to the w e s t...... 988 Skrainka Hill, Bald Mountain, and Fizzell Branch...... 989 Stouts Creek south of Roselle ...... 990 Stono Mountain ...... 990 Stouts Creek “shut-in” ...... 990 Summary ...... 991

I ntroduction

Although the general distribution of the igneous rocks of southeastern Missouri has been known since the publication of Schoolcraft’s report, “ Mines of Missouri,” in 1819, they were first studied and mapped in detail by Erasmus Haworth about 70 years later. In one of his ,

* Manuscript received by the Secretary of the Society December 31, 1931.

(965)

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published in 1888,1 he states that “a comparative study of the porphyries and granites of this district reveals the fact that there is no sharp division line between the two, at least, so far as can be judged from the hand specimens.” He further raised the question as to whether these rocks in Missouri illustrated “the gradual transition from a holocrystalline, coarse­ grained through the fine-grained ones to those which originally were vitrophyres.'” He concluded, tentatively at that time, that such a grada­ tion existed. Haworth’s next contribution was a paper2 in which he presented the arguments in favor of an Archean age and an igneous origin for these crystalline rocks. The Missouri State Report on the Iron Mountain Quadrangle was published in January, 1894, in which Haworth briefly discussed (pp. 15-27) the igneous rocks of the quadrangle. He concluded (pp. 24-25) that the granites and porphyries are gradational in their relationship, and that they have different textures in different parts of the same because of different conditions during solidification. He says: “There are literally hundreds of places where the phenomenon 3 can be observed, with only here and there one in the whole Missouri Archean which bears evidence of the granite having been forced up through the porphyry.” He cites various localities where gradation is found. The final summary of Haworth’s work, which extended over several years and, in considerable part, at least, was done independently, appeared in 1895.4 Since then, no important contributions to the igneous history of the State have been made; only a few brief notes have appeared in miscellaneous reports, most of which have incorporated material from Haworth’s work on the crystalline rocks. Although the entire trend of Haworth’s thought in his last is in favor of a gradational relationship between these two groups of rocks and he devotes considerable space in all to proving it, he cites one locality, 2 miles south of Cornwall in Madison County, in which he thinks the granite is intrusive. Also, he suggests, but does not state positively, that three small granite outcrops on Black River in Reynolds County might “represent independent eruptions which took place after the surround-

1 Erasmus Haworth: A contribution to the Archean geology of Missouri, University Press, State University of Minnesota, 1888, pp. 1-40. See also : Am. Geol., vol. 1, 1888, pp. 280-297, 363-382 ; Johns Hopkins Univ. Circ., vol. 7, 1888, pp. 70-71. 2 Erasmus Haworth : The age and origin of the crystalline rocks of Missouri. Mis­ souri Geol. Survey Bull. no. 5, 1891, pp. 5-42. 8 1, e., gradation. * Erasmus Haworth : Crystalline rocks of Missouri. Missouri Geol. Survey, vol. 8, 1895, pp. 81-224, pis. 1-30, with map of the area.

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ing masses 5 were formed/’6 a conclusion in which Dake 7 concurs. On pages 209-219 of his paper, he goes into detail as to localities where grada­ tion may be seen. To make Haworth’s interpretation of the relationship clear the following quotations are needed: “Early in the course of field work, it was decided to advance as a working hypothesis the idea that the granites and the porphyries belong practically to the same general period of eruption, and that in many cases, from a particular magma a granite was produced in one portion and a porphyry in another.8 It was not thought that this was true in every case of contact between the two rocks, but that it probably was in a majority of cases. Since it was produced the hypothesis has been strengthened by many observations, and it is now offered as the most probable explanation for the intimate relations of the two kinds of rocks.” 9

On page 218 of the same report, after stating that some granites may be intrusive, as indicated above, the following statement occurs: “It is not to be understood that all the granites in the big area were brought to the surface at once; but rather that the granites and the porphyries were formed simultaneously and came from the same general magma, and conse­ quently are of the same geological age. It is therefore useless to attempt to decide which is the older, the granite or the porphyry.10 This view is placed in contrast with the one that the two types of rocks are of two different periods, and possibly of different origins.” In the next quotation, however, which represents the closing sentences of the paragraph quoted above, Haworth recognizes that there were prob­ ably different eruptions of both porphyries and granites which, if true, could mean only that certain of the igneous rocks are younger than others and thus that they are of different ages. “From statements already made, it will be seen that there are good reasons for believing that many of the prominent porphyry hills are the result of dif­ ferent individual outbursts. In a similar way, different granite areas may also have resulted from a number of different eruptions.”

There can be no doubt after reading Haworth’s account of the occur­ rence of these igneous rocks that he fully believed their relationship throughout the area to be dominantly gradational, yet there is also the

5 I. e., the porphyries. 6 Idem, p. 180. 7 €. L. Dake: Potosi and Edgehill Quadrangles, Missouri Bur. Geol. and Mines, vol. 23, series II, 1930, p. 38. 8 Italics are the present writer’s. 9 Erasmus Haworth: Crystalline rocks of Missouri. Missouri Geol. Survey, vol. 8, 1895, p. 209. 10 The italics are the present writer s.

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evident attempt to leave open the possibility of another explanation while maintaining his gradational hypothesis. It seems probable that Haworth’s use of textural terms as general rock names has led to some confusion on His part. For example, many of his “granite porphyries,” such as the one found at the Graniteville quarries, would undoubtedly be called granites at the present time. Others in which the groundmass, although crystalline and composed of and , is fine grained would really be granite porphyries. It is entirely possible for a granite to be porphyritic and not be a granite porphyry. Likewise, some of the true granite porphyries of southeast­ ern Missouri resemble, megascopically, at least, some of the rhyolite porphyries that comprise essentially all the so-called “porphyries” of the area. Some confusion may have arisen from this fact in naming these rocks are the rhyolite porphyries (sometimes referred to simply as “por­ phyries” or “the porphyry”) and the .granites. The writer, many years ago, after seeing the two groups of rocks, felt that the gradation of the granites (especially the coarse-grained granites) into the porphyries, which showed all the characteristics of eruptive rocks, was improbable, and hence sought for evidence bearing on the problem. In 1919 positive evidence of the intrusive character of the granite was found on Knoblick Mountain and supplementary evidence elsewhere. In 1929 careful search at the contact of the two rocks in several localities furnished more positive proof, and detailed work during the past year has furnished confirmatory evidence at widely scattered points throughout the whole district in which igneous rocks occur. This paper presents this evidence and proves that the granites of southeastern Missouri are intru­ sive into the eruptive porphyries and hence are younger.

G e n e r a l G e o l o g y

Only a brief statement of the geology of the area and the distribution of the rocks will be given. The igneous rocks involved are all older than the Cambrian sediments, although basic igneous rocks cutting the Cam­ brian beds occur in an adjacent county. The two major groups of igneous rocks are the rhyolite porphyries (sometimes referred to simply as “por­ phyries” or “the porphyry”) and the granites. The porphyries consist of a great series of rhyolitic that un­ doubtedly poured out upon the surface (pre-Cambrian), as they usually show flow structures, , and spherulites, and, less frequently, are interbedded with tuffs. The extreme commonness of flow lines can leave no doubt of the eruptive character of these rocks. An almost universal

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feature is the presence of , usually of both quartz and feldspar (orthoclase) but also of either alone. Nonporphyritic do occur and they may be closely associated with the porphyritic varieties. The position of the flow lines of these rocks varies throughout the district, and they may be due to variations in the surface, to the intrusion of the granites, or to diastrophic changes.

F i g u r e 1 .—Secondary Enlargement of the Phenocrysts in a Rhyolite Porphyry Specimen obtained at Buckner Mountain, 2 miles west of Fredericktown, Missouri. Note inclusion of other materials. X 27.

There is no evidence of separate flows as far as the writer could deter­ mine, save where tuff beds or brecciated zones occur. Very probably, the entire series represents a rapid outpouring of a great quantity of rhyolitic , but detailed mapping may reveal more evidence bearing upon this question. The point is not pertinent, however, to the problem of the relationship of the porphyries to the granite, as there is little dcubt but that they all belong to a closely timed series of eruptions, if not to one single outpouring.

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The great variety of textures of the porphyries offers a fertile field for detailed pétrographie studies. The groundmass of the porphyries is typically aphanitic. In some of them, the groundmass is microcrystalline and microgranitic ; in others it is cryptocrystalline. There are grada­ tions between the two kinds. Some of the cryptocrystalline varieties are undoubtedly devitrified glasses, as they exhibit the palimpsest structures of the original glasses. One of the outstanding features of some of the porphyries is the

F i g u r e 2 .— Micrographic Texture developed by Recrystallization of the Porphyry Specimen obtained 3 feet from the contact with the granite, at the Cornwall, Missouri quarry. X 75.

secondary growth of the feldspar and quartz. These have grown during the last stage of cooling and have incorporated the smaller grains adjacent to them (figure 1). Another feature of much interest is the development, at the contact of the porphyries and granite, of micrographic textures in the recrystallized porphyries (figures 2 a n d 3 ) .

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The granites are astonishingly similar throughout the area. However, during this study of their relationship to the porphyries sufficient evi­ dence has been collected to prove that there is actually more than one granite in the region. The granites consist dominantly of feldspar and quartz, with a little mica which is mostly biotite. Most of them are some shade of red, but there are local variations where the biotite or hornblende or both are so abundant as to give the rock a gray color. These dark minerals are usually

F i g u r e 3 .— Micrographic Texture This texture is due to recrystallization of the porphyry at the contact of the porphyry with the granite on Matthews Mountain, about 7 miles southwest of Fred- ericktown, Missouri. X 75.

accompanied by a lighter colored feldspar. The granite that outcrops over a considerable area about halfway between Fredericktown and Knoblick Mountain is unusually rich in biotite. Most of the biotite in all the rocks has altered to chlorite. Biotite appears to be more abundant near the contacts of the granite with the porphyries. The feldspar is largely orthoclase and microcline, but the soda-rich members are present in some local outcrops. Other minerals occurring in m inor amounts are magnetite, specularite, fluorite, epidote, and apatite. Graphic granite is well developed in a few localities. Miarolitic cavities containing fluorite and mica in addition to quartz and feldspar are common near some contacts, as those at Stono Mountain,

LXIII—B u l l . G e o l . S o c . A m ., V o l . 4 3 . 1 9 3 2

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near Doe Bun, on Matthews Mountain, and all along the contact from Stono Mountain southeast to Silver Mine. A feature of much interest in the granites of the entire district is the presence of two generations of quartz. Some granites (as those north, east, and south of Stono Mountain and those to the southeast as far as Silver Mine) show the first generation as “quartzoids,” usually doubly terminated with rhombohedral faces or, more correctly, trapezohedrons, since they probably developed about 575° C. and hence were originally B-quartz which crystallizes in the trapezohedral class. On weathering, this first generation of quartz stands out prominently on the surface of the granites, though in the coarse-grained varieties it does not show so plainly* of course. Many of the granites contain also two generations of feldspar. For example, the large phenocrysts in the granite at the Graniteville quarries represent a first generation of feldspar. The texture of the granites ranges from the very coarse-grained (size of grains well over 5 mm.) varieties at Graniteville, Cornwall, and Cold- water down to those varieties having grains about 1 millimeter in diam­ eter, but most of the granites are medium-grained. Some aplitic phases are fine-grained. Porphyritic phases of the granite and granite porphy­ ries are fairly common. The coarse-grained granites at Graniteville and at the old Pilot Knob quarry, a mile south of Graniteville, contain pheno­ crysts of orthoclase as much as 2 inches long. Such a coarse-grained rock should be called a “porphyritic granite,” not a “granite porphyry” as Haworth calls it. The rock on the south side of Mount Devon, which is mapped as granite by Haworth, is a good example of a granite porphyry. Some of the rhyolite porphyries are almost coarse enough to be called granite porphyries, but their other features indicate that they belong with the rhyolites, the textures of which, as already indicated, cover a wide range of grain size. Basie dikes are surprisingly abundant throughout the area. They occur cutting both granites and porphyries and hence are younger than both. Aplite dikes in the granite are fairly common. The field evidence thus far gathered indicates that a number of dif­ ferent granite intrusions occurred, although further laboratory studies may show that two apparently different types of granite are actually merely two phases of one intrusion. Thus, it may very well be that the blue-gray biotite-hornblende granite found on the northeast side of Knob- lick Mountain is a segregated phase of the grayish-red granite to the west on the same hill. The heavy mantle rock in the locality, as in other places throughout the district, prevents the contact of the two rocks being

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seen. A reddish-gray granite, occurring locally along the south side of Evans Mountain just east of the St. Francis River, may be either a biotite (chlorite)-rich phase of the larger mass of granite of that locality,

F ig u r e 4 .—Map showing the Distribution of the Granite (dark gray) and Porphyry (light gray) in southeastern Missouri Arrows indicate localities where sharp contacts of the two rocks may be seen. Photo­ stat copy of Haworth’s map in Missouri Geol. Survey, vol. 8, 1895.

or a separate intrusion. The fine-grained, dark, biotite-rich granite covering several square miles to the northwest of Fredericktown is so

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uniform throughout the area where it has been studied and so different from the other granites of the district that it can scarcely be doubted that it represents a separate intrusion. The chemical composition of the rhyolites and granites is so strikingly similar that one does not wonder that Haworth sought to prove the two rocks to be merely different phases of the same rock mass. The silica content ranges from 70 to 77 per cent in the granites and from 69 to 77 per cent in the porphyries. There is also a similar agreement in the amounts of the other constituents. Insufficient analyses are available, however, for a detailed discussion of the composition. Both rocks are quartz-rich, and both show phenocrysts of feldspar, usually orthoclase. All these facts indicate that both rocks were derived from a deep-lying silica-rich magma. Hand in hand with these facts goes the further fact that several of the basic dikes are extraordinarily rich in quartz; some of them commonly containing grains of quartz 2 inches across.

D istribution o f G r a n i t e s a n d P o r p h y r i e s The distribution of the granite and the porphyry as mapped by Haworth is shown in figure 4. The writer’s studies call for a revision of this map, which will be undertaken as soon as further studies are completed. The black area in the north-central part of the map, beginning about 2 miles west of Fredericktown and extending northwest to Doe Run and Stono Mountain, represents the major granite area, which may well include several granites. The small black areas indicate other granite outcrops. The granites in the vicinity of Fredericktown resemble those of the larger area, whereas those of the outlying areas are the very coarse-grained granites. The distribution of these coarse-grained granites is significant and calls for further study. They seemingly occur chiefly around the central granite area. Examples of places where the coarse-grained granites are found are the Graniteville quarries, an area near Hogan, Coldwater, Corn­ wall, and along Doe Run Creek north of Wachita Mountains.

I n t r u s iv e R elationship o f t h e G r a n i t e to t h e P o r p h y r y

GENERAL STATEMENT The writer in his studies of the relationship of the granite and porphyry of southeastern Missouri (in Iron, St. Francois, Madison, and Wayne counties) encountered the same obstacle as did Haworth, that is, a wide­ spread mantle rock that prevented access to many contacts at just the points where it was most desirable to find them. However, a diligent

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and careful search in quarries and along innumerable streams that crossed contacts, as well as along the faces of bluffs and other steep slopes, re­ sulted in the discovery of a sufficient number of sharp contacts to furnish the clews necessary to the interpretation of those places where the evi­ dence was favorable but the contact inaccessible. The finding of a single contact made possible a determination of the textural differences of the rocks on each side, thereby permitting the location of the contact else­ where with a considerable degree of accuracy. The initial studies were

F i g u r e 5 .—Contact of the Granite (below) and dense Rhyolite (above) in quarry on Knoblick Mountain The granite contains inclusions of rhyolite. Photograph by Tarr.

made of contacts where the coarsest granites were adjacent to the porphy­ ries, as in such places the contrast would be greatest and the gradation, if it occurred, could most readily be detected. The ease with which sharp contacts were found soon led to the study of ths relationship of the two rocks at any point where they were associated.

LOCALITIES WHERE SHARP CONTACTS WERE FOUND

General statement.— Brief descriptions of contacts in the following localities are given because these contacts are so clear cut and distinct and thus show so conclusively the intrusive character of the granite.

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Knoblick Mountain .— The exposure on Knoblick Mountain that shows the contact so excellently occurs in a small quarry toward the top of the hill. Along the southwest side of the quarry is the face (6 to 8 ft. high) in which the intrusive relationship of the granite and rhyolite is so plainly shown (see figures 5 and 6). This contact strikes northwest-

F i g u r e 6 .—Contact of Rhyolite (top of picture) with Granite (below) Inclusions of rhyolite are seen as dark streaks in granite. Miarolitic cavity containing epidote to right of hammer. Quarry on Knoblick Mountain. Photograph by Tarr.

southeast. Another exposure of the contact on Knoblick Mountain was found to the southeast of the quarry (see figure 7), but it became lost under the talus on the southeast side of the hill. The granite at the contact is medium-grained, porphyritic, and of a flesh-red color having a grayish tinge due to the biotite (chlorite) in the rock. A great deal of epidote and considerable fluorite occurs in the

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granite. Micrographic texture is well developed in it (see figure 8). The rhyolite is very dense; only slightly porphyritic; shows flow lines; and ranges from red to black in color. Farther up the hill, the rhyolite is porphyritic with phenocrysts of feldspar and quartz. Float was. found of material having the same phenocrysts as the granite at the contact but with a finer groundmass, which suggests that some of the granite was injected as a dike into the rhyolite of the upper part of the mountain, the consequent rapid cooling causing the formation of a granite porphyry.

F i g u r e 7 .—Contact of Granite (below and to right of hammer) with Rhyolite (back and above hammer) The locality is on Knoblick Mountain. The contrast of the two textures is notable here. Photograph by Tarr.

Some slight bleaching in the color of the rhyolite occurred along the contact. The line between the two rocks is sharp and irregular. Frag­ ments of the rhyolite as long as 5 inches occur in the granite, largely within 2 or 3 feet of the contact; and stringers of the granite penetrate short distances into the rhyolite. The sharpness of the contact is well shown in figures 8 and 9, which are microphotographs of thin sections of the contact. Haworth has mapped the top of Knoblick Mountain as porphyry, show­ ing the lower lim it to he about 200 feet below the top of the knob. He also extends this limit down the north side of the mountain to a point

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about a quarter of a mile from the old village of Syenite where the large granite quarries are located. This lim it he has located entirely too far down on the north and northeast sides. The actual contact plunges steeply down the southeast side and continues southeastward across the relatively flat country instead of curving around the south side as Haworth has it mapped.

F i g u r e 8 .—Microphotograph of the Contact between the Oranite and Porphyry in the Knoblick Quarry Note the micrographic texture in the granite, a common feature in the granite at other contacts. X 27.

Haworth maps the granite as beginning just below the 1100-foot con­ tour line on the south side of the mountain and as continuing several miles to the south. This is incorrect, as the rhyolite porphyry is con­ tinuous from the top of the mountain down the south and southeast sides and across the valley (figure 10), and thence southward 3 miles ap­ parently without a break. The eastern limit of this great porphyry block has not been determined, hence its width is unknown.

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Before passing to the description of another contact, it should be noted that the granite forming Butler H ill (which is the westward continua­ tion of Knoblick Mountain) is coarse-grained, and is similar to that at the old Abbot’s quarry, a mile and a half southwest, to the red granite in Milne and Gordon’s quarry at Syenite, and to the granite across the St. Francis River to the northwest. The contact of this coarse-grained

F i g u r e 9 .— Microphotograph of the Contact between the Granite and the dense Rhyolite The locality is at another point in the Knoblick Mountain exposure. X 27.

granite with the rhyolite porphyry on Knoblick could not be located exactly, but it was evident that the two rocks are in contact. About a mile to the southwest of Butler Hill, the rhyolite and granite again outcrop, and although in places the contact can be located within a few feet, the actual contact was not found. The medium- to coarse­ grained granite continues to the southwest and is a part of the large area of granite. The rhyolite is a part of the large block that extends to the south of Knoblick Mountain.

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Graniteville.—The rhyolite lies to the northeast of the granite outcrop at Graniteville. A very good contact can be seen on the upper side of two or three large, flat, bare granite outcrops whiclj occur on the slope lying above and between the Sheahan and Schneider quarries. The line of contact can be followed easily all along the hill, but the actual contact can be seen only at these bare outcrops. The Graniteville granite is very coarse-grained, the grains averaging between % and % inch across. It is exceeded in coarseness only by that at Cornwall. The granite usually is as coarse at the contact as elsewhere, though locally there is a slight reduction in size of the grains at the im-

H— 1 Mile —H

F ig u r e 1 0 .—Profile of KnobUck Mountain Showing the relationship of the rhyolite to the granite.

mediate contact (figure 11). Stringers of granite 2 inches (and less) wide cut the porphyry. These may be coarse- or fine-grained, depending upon their thickness. The rhyolite shows flow lines and phenocrysts of feldspar (and some quartz) a few feet from the contact. At the contact, it is bleached and dense but, in local areas, has undergone some slight recrystallization. Some epidote has developed in the rhyolite, especially along joints. The sharpness of the contact at Graniteville makes it unmistakable, and the presence of granite stringers in the porphyry is further proof of the intrusive character of the granite. A mile south of Graniteville is another outcrop of coarse-grained por- phyritic granite very similar, though slightly finer in grain, to that at the quarries in Graniteville. The area of this granite outcrop comprises about 200 acres. It is surrounded on the east, south, and west by a very dense rhyolite porphyry, which locally is almost free from phenocrysts.

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The rhyolite is slightly bleached at the contact with the granite, and the flow lines in it extend to the contact. A single exposure of the contact was located in the low col connecting the granite area with the rhyolite hill to the southeast. The contact was vertical (see figure 12) and very sharp. This exposure of the contact of the two rocks, although small, is very good and shows conclusively that this outcrop was a stock (or better a cupola) of granite intruded

F i g u r e 11.— Contact of Granite and Porphyry at Outcrop between Sheahan’s and Schneider’s quarries, Graniteville, Missouri The section parted at the contact during its preparation. Phenocrysts of feldspar are more abundant in this granite than in that in the quarry below. X 27.

into the porphyry. The line of contact could be traced, within narrow limits, around nearly the entire outcrop, so readily was the coarse-grained granite distinguishable from the dense rhyolite with its feldspar pheno­ crysts and flow structure. Cornwall.— The granite in the Cornwall quarry (in eastern Madison County) is probably the coarsest in the state, the grains averaging be­

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tw een y 2 and 1 inch across. Part of the quartz is white, and as the feldspar is a deep red the granite has a striking appearance. The granite forms the northeast end of a small hill and the rhyolite the southwest part. The entire granite outcrop is limited to a few acres. In discussing the Cornwall area, Haworth 11 states: “Here also are fine examples of transition from granite into porphyry. The east end of the hill is granite, and the rock at the eastern part is the coarsest of any known in the state. In passing to the west the texture becomes finer, and by the time the middle of the hill is reached it has changed into a tolerably coarse-grained porphyry.”

F ig u r e 1 2 .— Vertical Contact of Granite (left) with Porphyry Locality is on the southeast side of the granite outcrop 1 mile south of Granite- ville. Hammer head at line of contact. Photograph by Tarr.

Again Haworth was mistaken, for the coarse-grained granite is in con­ tact with the rhyolite porphyry which, at the contact, has undergone some recrystallization that increased the size of the grains there and developed some fine micrographie textures. This increase in size of the grains in the porphyry extends only a foot or two from the contact, where it becomes the typical dense porphyry.

11 Erasmus Haworth : Crystalline rocks of Missouri. Missouri Geol. Survey, vol. 8, 1895, p. 177.

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Lower down the slope from the exposure of the contact just mentioned, a dikelike mass of granite porphyry lies between the granite and the rhyolite, but it pinches out near the top of the hill. This granite por­ phyry contains inclusions .of the rhyolite but none of the granite, which suggests that it is older than the granite. Both this granite porphyry and the rhyolite are cut by a jet-black to greenish black, extremely dense felsite that includes fragments of devitrified glass. No attempt was made to work out in full the details of these intrusives, as the chief problem under consideration was the actual contact of the granite with the other rocks. Epidote has been intensively developed in both of the dikes and in the rhyolite, evidently by the solutions from the granite intrusion. Thus, the granite is the youngest of all these rocks. It is very possible that Haworth found this granite porphyry next to the rhyolite porphyry, and not noting the inclusions in it concluded that the granite porphyry (which, however, is an entirely different type of rock) graded into the rhyolite. He states that the granite in a quarry (which could not be located) about 2 miles away is intrusive in the porphyry. The contact in this Cornwall area appears to be nearly vertical. This contact of the extremely coarse granite with the dense rhyolite can be interpreted only to mean that the granite is intrusive, and the increase in grain size in the rhyolite at the contact can be due only to the effect of heat and solutions from the granite intrusion. Matthews Mountain.—Matthews Mountain is about 7 miles southwest of Fredericktown in sections 34 and 35 of township 33 noTth, range 6 east. Only part of the mountain is shown on the Mine La Motte sheet. This locality was visited because H aw orth12 cites it as an excellent ex­ ample of the gradation of granite into porphyry. In discussing transi­ tion zones, he states: 13 “On the northwestern part of Matthews Mountain is an exceptionally fine illustration. The granite (Tp. 33 N., R. VI E., Sec. 34) covers a space of a little more than ten acres. It reaches up into the porphyry hill, and all along the line gradually passes into porphyry in such a way that there is no indica­ tion whatever of its belonging to a separate eruption.” In tracing out the contact on Matthews Mountain, it was soon found that Haworth’s estimation of the extent of the outcrop was much too low, for the granite was followed eastward nearly a mile and a half and the outcrop is nearly half a mile wide. This exposure proved to contain an abundance of splendid evidence supporting the intrusive character of the granite.

12 Idem, p. 177. 13 Idem, p. 217.

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Sediments form the northern boundary of the granite outcrop. The La Motte member rests against the foot of the hill and consists of sand­ stones and conglomerates. In the conglomerate are pebbles of rhyolite porphyry but none of granite. The porphyry to the east of the granite (a valley had developed along the eastern contact) was a dense rhyolite porphyry containing abundant evidence of flow structure. The dip of these flow lines was dominantly east and southeast and from 10 to 90 degrees. The contact was traced up the west side of the valley, and it was dipping steeply to the east. At the top of the ridge, a sharp contact of the granite and porphyry was found. Its strike was northwest and southeast but it curved back to the northeast and within a short distance passed down the side of the ridge. The flow lines of the rhyolite here were roughly parallel to the contact and their dip was as high as 60 degrees to the south and south­ west. The granite was medium- to slightly coarse-grained until within 6 inches of the contact it became finer. The porphyry had undergone recrystallization at the contact producing a splendid development of micrographic texture (see figure 3) which extends «back for 50 feet. This recrystallization resulted in a slight increase in granularity near the contact, making the porphyry there a fine-grained rock. The granite is medium- to slightly coarse-grained until, within 6 inches of the contact, it becomes finer. Well-developed graphic intergrowths are common in the granite at the contact (figure 13) but disappear within 50 to 100 feet below it. The contact between the two rocks could easily be traced along the top of the mountain, but the actual contact was not seen again until the west end of the mountain was reached. There an excellent exposure of it was found. The contact was irregular, as it is everywhere. The granite is medium- to coarse-grained at the actual contact. The rhyolite is fel- sitic (see figure 13), and considerably bleached at the contact but becomes darker within 15 to 18 inches of it. Miarolitic cavities in the granite of Matthews Mountain are abundant and well developed near the contact with the rhyolite. Some, 12 inches long and 4 inches wide, are lined with quartz and feldspar and occur within 6 inches of the contact. Some of the cavities are completely filled with quartz and thus form quartz veins. The latter often attain a width of 4 or 5 inches and extend into the porphyry. The restriction of the cavities to the top of the granite intrusion and their maximum development near the contact with the porphyry (taken in connection with the recrystalliza­

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tion of the porphyry) show that the granite was rich in gases that ac­ cumulated at the top of the magma. These gases aided also in making the granite coarser in the upper part. The outcrop of granite on Matthews Mountain is elongated and rela­ tively narrow. Its contact along the south side is more or less irregular. Near the west end of the mountain the contact swings to the south and

F i g u r e 1 3 .—Microphotograph of the Contact of the recrystallized Porphyry with the Granite (below) Shows development of graphic texture in granite, and recrystallization of the porphyry. Contact on west side of Matthews Mountain. X 27.

then turns west and is lost under sediments. The position of the Cam­ brian sediments in valleys now occupied by streams shows that there were valleys in the same location in pre-Cambrian times. As no granite fragments were found in these sediments, though the slope and the sedi­ ments themselves are now covered with granite boulders, it is evident that the original topography was closely sim ilar to that of the present, yet the granite was not exposed, at least on the higher slopes. Since the Cam brian

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period, the thin veneer of porphyry has been removed and the granite exposed. I t seems probable, then, that the original intrusion of granite had an outline on this side roughly similar to the present form of the exposure on the mountain. In other words, it was an elongated stock thrust up into and displacing the porphyry, causing the wide variability in the dip of the rhyolite adjacent to the granite. The steeply dipping contacts formed here, and elsewhere, are further evidence of the intrusive

N A

A - P l a n of Outcrop.

JVope o f m t t Onqinaf , ' S u r fa c e d / 180' \ R hyoH te i Gron/te\

F ig u r e 1 4 .— Sketch Map of Outcrop of Granite on Matthews Mountain, and Sections Showing the probable original Shape of the Granite Intrusion.

character of the granite. The position of the contact on the west end of the mountain indicates that the granite cuts under the porphyry and very probably then plunges steeply downward (figure 14). Matthews Mountain shows so many features that prove or favor an intrusive origin for the granite that it is one of the outstanding localities of the district in this respect.

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AREAS OF PROBABLE INTRUSIVE CONTACTS Cedar Creek.—Haworth has mapped a small outcrop of granite along Cedar Creek in section 31, township 33 north, range 6 east. There is no granite on the north side of the creek; instead, the rhyolite comes down to the valley floor. The granite along the south side is medium-grained, but the attempt to find the contact up the hill to the south resulted in finding an even coarser-grained rock at the top of the hill, which is mapped as porphyry by Haworth. The granite is similar to that at Stono Moun­ tain. This granite extends to the east along Cedar Creek and is prob­ ably connected with the granite hill about 1 mile east. The texture of the rocks on the two sides of Cedar Creek is opposed to the contact being gradational at this outcrop. Mount Devon.—Mount Devon is a prominent hill about 3 miles south­ west of Fredericktown. A small area of granite is mapped by Haworth along its foot on the south side, but if this rock is a granite then that at the top is, also; in fact, the proper name for the rock is “granite por­ phyry.” Buckner Mountain.—Along the east base of Buckner Mountain, across the Little Saint Francis River north of Mount Devon, Haworth has mapped a small area of granite. Actually, the entire mountain is a rhyolite porphyry in which the phenocrysts show secondary growth and are embedded in a fine-grained groundmass. This rhyolite porphyry is closely related to the rhyolite of Skrainka Hill to the west. Coldwater, Wayne County.—An outcrop of coarse-grained, porphyritic granite covers an area of about 2 square miles along the Saint Francis River in the northeastern part of township 30 north, range 5 east in north­ ern Wayne County. Though the actual contact was not found, it was located within 15 or 20 feet, and small inclusions of porphyry were found in boulders of granite. The evidence strongly favors the view that this granite mass is intrusive in the porphyry.

CONTACTS IN AND AROUND THE LARGE CENTRAL AREA OF GRANITE General discussion.—An examination of Haworth’s map (figure 4) will reveal that the southwest and western margins of the central granite area is in apparent contact with the rhyolite on these sides. It is in these places that further evidence of the intrusive character should be found. The map reveals other suggestive places for study. We have already shown that granite is intrusive into the rhyolite porphyry on Knoblick Mountain, and that this same porphyry extends to the south about 3 miles. On the Mine La Motte and Ironton sheets,

LXIV —B u l l . G e o l . S o c . A m ., V o l . 4 3 , 1 9 3 2

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porphyry is shown as capping many of the higher hills and mountains that lie wholly within the granite area to the southwest of Knoblick. These, then, should be excellent places in which to find the contact. Haworth has located the contact, between the granite below and the porphyry above,, as roughly following the contour lines—a position that might be correct if the granite represented the deeper (and therefore more slowly cooled) portions of a thick flow and the hills were erosion remnants. A study of part of these hills did not support Haworth’s interpretation. The hill in section 2, township 34 north, range 4 east is mapped as porphyry above the 1300-foot line. If the base of the hill is granite, so is the top, for the same rock occurs in both places. Actually, the rock is granite-porphyry. A small outcrop of the same rock occurs on the west side of Bucks Mountain to the southeast. A medium- to coarse-grained granite occurs as the floor of the valley between this hill and Bucks Moun­ tain. Some nonporphyritic phases on this small hill resemble a fine-grained granite on the top of the hill east of Stono Mountain to the north. Bucks Mountain has a coarse-grained granite at the base but it becomes medium- grained higher up and is only moderately porphyritic. Haworth mapped the top of Bucks Mountain as porphyry. The hill east of Stono Mountain is also mapped as porphyry, the lower limit of which is drawn to agree with the lower limit of the porphyry on Stono Mountain to the northwest. This entire small hill consists of the same medium-grained granite as does the surrounding area, with the exception of a small area of fine-grained granite (almost an aplite) on the very top of the hill. Bald Mountain was not visited, but the writer ventures to predict that the part mapped on it as rhyolite porphyry will not prove to be so. This hill is nearly 200 feet lower than Bucks Mountain and only about a mile away. Across Wachita Creek, 2 miles east of Bald Mountain, is an elon­ gated hill, the upper part of which is mapped as porphyry. The north end proved to be entirely granite, and the remainder of the hill probably is also. Evans Mountain and hill to the west.—Farther to the south along Highway 70, Evans Mountain and the small hill west of the Saint Francis Eiver are largely composed of rhyolite into which the granite has been intruded. Excellent evidence of the intrusive action was found in many places on both hills. As shown on Haworth’s map, the porphyry caps each hill. Actually, the two hills are connected by the rhyolite porphyry which is continuous under the river. This is not the only place where the map needs revising around Evans Mountain. The rhyolite crosses to the north of the Saint Francis River, and likewise

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is found on the southeast side, where it extends more than a quarter of a mile beyond the highway in section 2, township 33 north, range 5 east. The line of contact of the granite and rhyolite can be found on both hills. The actual contact is well shown on the east side of the small hill along the Saint Francis River and about half a mile north of the bridge. The contact is nearly vertical, and strikes down the face of the bluff and crosses the river to the east side. The rhyolite is dense, almost black, and shows flow lines that are vertical; the granite is medium- grained and red. Therefore the contact is very distinct. The black color of the rhyolite is bleached to a distance of 3 inches or more from the con­ tact. The south contact on the small hill is easily followed, as the granite there is coarser-grained and is adjacent to the dense rhyolite. In a shallow valley that has been cut back into the rhyolite of the west face of Evans Mountain about 100 feet above the river, there is a small exposure of a medium- to coarse-grained granite. This is evidently a cupola from the underlying granite mass. Stringers and dikes of the granite cut the dense rhyolite porphyry in every direction. Some of these dikes are actually , as 6 inches long of orthoclase and quartz are common in them. Solutions from the granite veins have bleached the rhyolite. Miarolitic cavities are common in the granite. There can be no doubt of the intrusive character of the granite here. Evidently, hot solutions emanating from this granite instrustive, or from other nearby masses, were the cause also of the great quantities of intensely seriticized rhyolite that are found all along this face of Evans Mountain, as well as at other points around it. An excellent contact occurs on the south side of Evans Mountain. The exposure is in a small valley, and shows the coarse, gray phase of the granite, mentioned above, in actual contact with the dense rhyolite. Blocks of rhyolite are entirely surrounded by granite. These blocks may be xenoliths that were formely roof pendants. The rhyolite has been moderately bleached. About a mile southeast of this contact, granite of a much finer grain may be seen in contact with the rhyolite. Both rocks are badly weathered, and the microscope was of great assistance in interpreting their textures. Shrainlca Hill, Bald Mountain, and Frizzell Branch.—On the south­ east side of the large granite area lie Skrainka Hill and Bald Mountain. The latter is capped by rhyolite, but a remarkable, nearly black rock lies between the granite and the porphyry. The exact relationship of the three rocks has not been worked out as yet, but there has evidently been some assimilation there, and if so, the granite is intrusive. Granite ex­

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tends much higher up the side of Skrainka Hill than it is mapped. This area is remarkable for the very large basic dikes that cut the granite and rhyolite. Further studies of these dikes are being made in an attempt to determine their relationship. An excellent contact of the granite and a felsitic rhyolite occurs just below the schoolhouse on Frizzell Branch. The original rhyolite has been completely recrystallized to a fine-grained granite. The contact is readily seen, however, as the granite is medium- to coarse-grained. The character of this rhyolite suggests that it was a roof pendant that became deeply buried in the granite; hence its complete recrystallization. Stouts Creek south of Roselle.—A fine contact is exposed along the north side of Stouts Creek, half a mile south of Roselle, in section 6, town­ ship 33 north, range 5 east. Haworth 14 discusses this locality in detail and gives four analyses of the granite and porphyry to prove their grada­ tional relationship. Had he examined the rocks exposed along the north bank, he would have found splendid sharp contacts. The coarse-grained granite is in contact with a dense or fine-grained, recrystallized rhyolite. The contacts are so plain, they could not be missed. It may be that they were covered by flood-plain deposits when Haworth visited the locality. High water prevented the writer from crossing to the south side of the creek and examining the contact there, but the evidence for an intrusive relationship on the north side is so positive that the need of doing so does not seem imperative. Farther to the north the contact between the granite and rhyolite was limited to a space of 4 or 5 feet along the west line of section 14, town­ ship 34 north, range 4 east. Stono Mountain.—Still farther north, an excellent contact is found near the 1200-foot contour line on the south slope of Stono Mountain. The granite is medium-grained but is finer where it is in contact with the dense porphyritic rhyolite. Evidently, the intrusive granite had been chilled along its contact with the rhyolite. The rhyolite is much bleached and recrystallized for several feet from the contact. The rhyo­ lite of Stono Mountain is similar to that on Knoblick Mountain, Evans Mountain, and in other places to the south. Locally, it varies consider­ ably and may be nearly free from phenocrysts. Stouts Creek “Shut-in."—One more locality where a sharp contact was seen is at the bridge over Stouts Creek at the “Shut-in.” A small area of granite, to the northwest of the bridge, has been intruded into the dense rhyolite porphyry which is well exposed in the new highway cut

«O p. cit., pp. 212-213.

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by the bridge. This intrusion was accompanied by some mineralization, as both pyrite and specularite are common in the granite and adjacent porphyry. The granite is medium- to fine-grained and becomes finer near the margin of the mass. It shows well-developed graphic texture. The rhyolite has been so completely recrystallized near the contact, in some parts, that all evidence of the phenocrysts has been lost.

S u m m a r y Positive proof that the granites are intrusive into the rhyolites in the area of igneous rocks in southeastern Missouri is given in this paper. The facts cited as evidence may be summarized as follows: 1. In many localities the medium- to coarse-grained granites are in actual contact with the dense rhyolites, a condition that is commonly recognized as indicating an intrusive relationship. 2. Contacts were found in which the granite was in contact with a rhyolite showing flow structure, an impossible gradational relationship. 3. Most of the contacts found are vertical, or nearly so, which is a common position for the contacts between intrusive rocks. 4. Nearly horizontal contacts are found, and these, also, are common relationships of intrusive rocks. o. The contact of the granite with the rhyolite cuts across the contours; it should follow them if the granite was the result of a slower cooling of the lower part of a widespread, essentially horizontal lava flow. 6. The contact is extremely irregular in direction as well as in dip— further conditions that should not exist under the gradational hypothesis given above in 5. 7. The flow lines in the rhyolite, where determinable, lie at all angles, showing evidence of displacement by the intrusive granite. 8. The recrystallization of the rhyolite adjacent to the granite indi­ cates a process induced in the rhyolite by the heat and the solutions from the intrusive granite. This recrystallization dies out at a short distance from the granite. 9. Bleaching of the rhyolite near the contact is another change due to heat from the intrusive. 10. Development of micrographic textures in the rhyolite adjacent to the granite is due to the same cause as the recrystallization. 11. Inclusions of rhyolite in the granite furnish indisputable evi­ dence of the existence of the rhyolite as a solid rock while the granite was still in a liquid state.

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12. Stringers and dikes of granite cutting the rhyolite prove that the former is intrusive in the latter. 13. The presence of miarolitic cavities near the margin of the granite mass indicates the confinement of gases (and liquids) within the granite, whereas if the two rocks were gradational these cavities would be dis­ tributed through the gradational zone. 14. Fluorite, epidote, and quartz occur primarily in the granites but have also been introduced into the rhyolite from the granite intrusion. 15. The boss- or cupola-shape of many of the smaller granite masses (in connection with the more or less vertical walls of the masses, and other relationships) becomes strong evidence in favor of the intrusive character of the granite. Some of these lines of evidence are widely accepted as proof of the intrusive character of a rock mass. Others, in themselves, are not proof, but when associated with some of the positive elements become strong supporting evidence. Taken as a whole, these lines of evidence would be regarded as undoubted proof of the intrusive character of any in such a relationship to another rock. It is impossible to tell how much younger the granites of this area are than the rhyolites. As yet, no attempt has been made to differentiate between the granites. Numerous other problems needing solution are connected with these igneous rocks.

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