Thomsonite, Mesolite, and Chabazite Prom Golden, Colorado

BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

V o l . 11, PP. 461-474, PLS. 43-49 JUNE 30, 1900

THOMSONITE, MESOLITE, AND CHABAZITE PROM GOLDEN, COLORADO

BY HORACE B. PATTON

(jRead before the Society, December SO, 1899)

CONTENTS Page Work of previous investigators...... 461 Place and mode of occurrence...... 462 Thomsonite...... 463 General description...... 463 Type 1...... 464 Type I I ...... 466 Type 11a...... 466 Type III...... 467 Type Ilia ...... 467 Mesolite...... 468 Chemical analyses...... 469 Chabazite...... 470 Other zeoli tes...... 472 Analcite...... 472 Apophyllite...... 472 Stilbite and laumontite...... : ...... 472 Calcite and aragonite...... 473 Order of deposition...... 473 Summary...... 473

W o r k o f P r e v io u s I nvestigators

The zeolites of North and South Table mountains at Golden, Colorado, have already become known to mineralogists mainly through the ex­ cellent descriptions of these minerals and of their occurrence by Cross and Hillebrand in Bulletin 20 of the United States Gelogical Survey.* In this bulletin the above named authors t have given a brief description of

♦Contributions to the Mineralogy of the Rocky Mountains, Bull. no. 20, U. S. Geol. Survey, t A briefer, preliminary paper by the same authors was published in A.m. Jour. Sci., 3d ser., vol. xxiii, 1882, p. 452, and 3d ser., vol. xxiv, 1882, p. 129.

LXVI—Bum.. Geoi.. Soc. Am., Vol. 11, 1899 (461) 462 H. B. PATTON— THOMSONITE, MESOLITE, AND CHABAZITE the Table mountains, with their basaltic caps and a much more detailed description of the various zeolitic minerals contained in the amygdaloidal cavities. They also discuss the results of the investigations of Professor Carl Klein on the optical anomalies of analcite and apophyllite from this locality* During the past two years the Colorado State School of Mines, located at Golden, has been conducting quarrying operations for the purpose of securing for its mineral cabinet specimens of these Table Mountain zeolites. A new locality opened up on the east faGe of North Table mountain proved to be very prolific of these zeolites and produced many specimens of ex­ treme beauty. In most respects the zeolites here developed correspond closely with the occurrences described in the above mentioned paper by Cross and Hillebrand, and it is hardly worth while to duplicate what they have already written on the subject. The minerals thomsonite and mesolite, however, not only show extraordinary beauty, but occur in a great variety of forms; habits, and associations, and in the case of the former also present features that do not entirely accord with the descriptions of Cross and Hillebrand.

P l a c e a n d M o d e o f O c c u r r e n c e

As the description of these zeolites and of the Table mountains in which they occur are so readily accessible to all, a very brief reference to the surroundings will suffice for our present purpose. At Golden are two socalled table mountains, designated North Table mountain and South Table mountain. Geologically they are but one mountain, con­ sisting of soft, nearly horizontal beds of Middle Tertiary age, capped with a thick lava sheet and cut in two by Clear creek. The soft bedded rocks that form the base of the mountain are almost entirely composed of fine andesite ash beds belonging to the Denver Tertiary.f The lava cap consists of two flows of feldspar-basalt that together reach a thick­ ness of about 100 feet at the place where the minerals under discussion were obtained. The second flow followed so closely upon the first that no erosion of the first sheet occurred before it was covered by the second sheet. The two flows are of about equal thickness and form a nearly vertical cliff of 100 feet, in the center of which is a horizontal band of very scoriaceous basalt that belongs mainly to the top of the lower flow. This porous band is some 15 feet thick and contains caCvities of all sizes up to 6 or 8 feet. The large cavities are drawn out flat in the direction

♦Neues Jahrbueh fur Mineralogie, etc., vol. i, 1884, p. 250. fSee Whitman Cross: Geology of the Denver basin, Monograph xxvii, U. S. Geol. Survey, p. 155. PLACE AND MODE OF OCCURRENCE 4 6 3 of flow; the smaller ones may be flattened oval in shape. Even in the weather-beaten face of the cliff the presence of white zeolitic minerals filling the cavities is very noticeable. At a depth of 2 or 3 feet they are usually quite fresh. Cross and Hillebrand divide the zeolites filling the amygdaloidal cav­ ities in the basalt of North and South Table mountains into two groups, based upon their method of occurrence. The minerals of the first group are laumontite and stilbite, and are to be found only on the floor of the cavities. On the floor of many of the cavities, especially of the larger ones, there is to be seen a very peculiar bedded deposit of yellowish or reddish yellow color, which closely resembles a friable sandstone, but which is shown to be composed of mixtures of these two minerals. Included in the minerals of the first group are also to be mentioned occasional minute, reddish spherules of thomsonite. These latter have not been observed by the writer in the newly opened locality ; but with this exception the minerals of the first group are quite in accordance with the descriptions given by Cross and Hillebrand and may be passed over without further comment. The second group is made to include all those that are not confined to the floor of the cavities. They occur on the roof and sides, as well as on the floor composed of zeolites of the first group, or they completely line cavities that do not contain these bedded floors. The minerals of this second group are thomsonite, stilbite, chabazite, analcite, apophyllite, and mesolite, to which maybe added calciteand aragonite, as both these minerals occur associated with the zeolites of the second group.

T h o m s o n it e

GENERAL DESCRIPTION

It is universally recognized that specimens of any given species com­ ing from the same locality usually have the same habit and general appearance. This similarity affects crystal form and habit, color, luster, size, and association, and is so marked that mineralogists do not hesitate to identify localities by such peculiarities, such identification of localities usually being entirely justified by the facts. Exceptions to this rule will doubtless suggest themselves to the reader, but it is doubtful if a more striking one can be found than is shown by the mineral thomsonite in the particular locality under discussion. It would not be difficult to select four or more specimens of this mineral coming from immediately adjacent cavities so markedly different in habit and general appearance as to suggest their occurring in widely different localities. In fact, one 464 H. B. PATTON— THOMSONITE, MESOLITE, AND CHABAZITE may be justified in stating the contrast in still stronger terms, inasmuch as it is difficult to realize that the several specimens really represent the same mineral, irrespective of their common origin. This great diversity is undoubtedly due to the fact that the several varieties have been formed under varying conditions. It is, indeed, possible and easy to recognize several generations of thomsonite, each generation having its own habit. As a matter of fact, two such generations were recognized and described by Cross and Hillebrand. Their description of the first generation is very exact, and specimens of this type may easily be identified from such description. It is not so easy, however, to identify their second genera­ tion with any individuals of the later ones described in the following pages. Without the aid of photographs or other illustrations, it is no easy matter to convey an accurate conception of minerals whose forms are hot clearly cut. It is to be hoped, therefore, that the reproductions of carefully made photographs accompanying this paper will give some­ thing like an accurate idea, impossible through the printed page alone, of the occurrences under discussion. Altogether three distinct types may be distinguished, representing successively later generations. In at least two cases one or two sub- types are recognized and are designated by the letter a. It is not, how­ ever, always possible clearly to distinguish these subtypes, as they pass into each other by many intermediate stages. In case of the three main types, designated as I, II, and III, there is ordinarily no difficulty in making the proper distinction. This may be done by the peculiarities of development, as well as by the association with the other zeolites and with each other.

T Y P E I

This represents the ordinary habit of thomsonite as it occurs in many places on both Table mountains, and is very characteristic of the locality. The following description of this habit, taken from the paper by Cross and Hillebrand, fits the occurrence admirably: ‘‘ The mineral occurs in very minute rectangular blades, which are placed upon each other like the leaves of a closed fan, and the very compact combinations of such aggregates are usually arranged in a more or less distinct radiate manner. Sometimes spherical forms result, in other cases columns, by radiation from an axis, or, less frequently, walls, the blades standing at right angles to the central plane. . . . When a large surface of chabazite has been completely coated by the more or less radiate aggregates of thomsonite, forming an undulating surface, the whole has a most delicate silken luster, while that on a fractured surface of a spherical mass is more satin-like.” BULL. GEOL. SOC. AM VOL. 11, 1899, PL. 43

F i g u r e 1. —Thom sonite o ? T y p e I Partly covered by Thomsonite of Type Ilia. Chabazite underlies the whole. Two- thirds natural size

F i g u r e 2 . — T h o m s o n i t e o f T y p e II Underlain by Thomsonite of Type I. One-half natural size

THOM SO NITE OF TYPES I AND II BULL. GEOL. SOC. AM. VOL. 11, 1899, PL. 44

Figure 1.—Thomsonite of Type \\a Underlain by Thomsonite of Type I. One-half natural size

Figure 2.—Thomsonite of Type III One-half natural size

THOMSONITE OF TYPES lla AND III THOMSONITE OP TYPE I 465

Figure 1 of platé 43 gives an excellent idea of this habit. .The glob­ ular appearance is very characteristic of many of the specimens. In. the lower part of the figure may be seen two or three of the spherical aggre­ gates broken open so as to show the radiated structure and the satin-like luster. In this specimen the surface of the cavity was first coated by a continuous thin layer of chabazite, which does not show in the figure. This, in turn, was completely covered by the thomsonite. A later generation of thomsonite, to be described under type Ilia, forms rough surfaced globular aggregates grown on the surface of the first generation. In their description of the individual minute leaves that make up these radiate aggregates, Cross and Hillebrand refer to the macropinacoid as the dominant form with subordinate prism and brachypinacoid, the terminal face being the basal plane. As the prism is nearly at right angles and the leaves examined measure only about 0.01 millimeter in thickness, this determination is naturally only a surmise, based on the generally accepted forms given in the standard mineralogies. In spite of the minuteness of the constituent leaves, it is quite possible, however, by optical investigations to show that the brachypinacoid rather than the macropinacoid is the dominant form. This determination is based on the genérally accepted fact that the best cleavage is parallel to the brachypinacoid, and the plane of the optical axes is parallel to the basal pinacoid, with the acute positive bisectrix parallel to the macro-axis. According to these accepted properties of thomsonite, one would expect to observe a positive bisectrix, with the axial plane at right angles to the vertical crystal axis on the best cleavage planes. These observations have been verified in numerous cases on individual leaves and on care­ fully selected cleavage pieces, as well as on random cleavages produced by pulverization. The mineral cleaves very easily parallel to the leaves and only indifferently at right angles to the same. This determination of the position of the positive acute bisectrix was also supported by the behavior of the same material in parallel polarized light. Individual leaves and cleavage pieces invariably show the axis of least elasticity parallel to the vertical axis. The extinction is always parallel. It should be stated, however, that when the cleavage pieces are not selected, but are obtained by pulverization and examined at random, occasion­ ally one may lie on the poorer macropinacoidal cleavage plane. In this case the vertical axis is the axis of greatest elasticity. It may be well to state here in anticipation that in case of all the other types of thom­ sonite, wherever the forms are tabular, the brachypinacoid may be shown in the same way to be the dominant form. 4 6 6 H. B. PATTON— THOMSONITE, MESOLITE, AND CHABAZITE

Thomsonite of this habit is very likely to be the first mineral deposited on the roof and sides of the cavities, and, where there is no bedded floor composed of mixed zeolites, this is also true of the bottom of the cavities. Usually, however, it is laid down on a thin coating of chabazite.

T Y P E I I

Thomsonite of this type never forms compact aggregates. The indi­ vidual crystals are usually rectangular tablets of small size (1 millimeter or less), but still they are relatively much larger than the individual leaves of type 1. These little tablets bunch themselves together into delicate, snow white, prismatic aggregates that frequently taper to a point or branch into smaller offshoots. The whole interior of a cavity may be thickly covered with a growth of the delicate! tapering and branching prisms such as is shown in figure 2 of plate 43. Sometimes these com­ plex prisms are so fragile that a slight pressure of the fingers will crush them. More frequently they are firm enough to stand considerable pressure, such as would be produced by wrapping in cotton and tissue paper. The complex prisms usually attain a length of a quarter to half an inch. Not infrequently thomsonite of this type forms more or less parallel continuous prismatic aggregates a quarter to half an inch thick, completely coating a cavity. In this case the appearance described above is produced by the continued growth of some of the complex prisms form­ ing the base. Occasionally one may notice this variety of thomsonite directly coating the surface of the cavity or lying on a thin film of chabazite. Nearly always, however, a close examination will disclose an underlying layer of thomsonite of the first type. The aggregates com­ posing this second growth do not, however, continue the growth of the first generation, but rest quite indifferently on this as a base.

T YP E H a

No sharply defined line can be drawn between this and type II. In its best developed form it consists of similar complex, prismatic aggre­ gates diverging regularly and symmetrically from common points so as to form beautiful delicate hemispherical bunches. At the same time the delicate prisms become longer and straighter and taper out into hair­ like forms, and thus appear to pass insensibly into the mineral mesolite. These hemispherical balls may sometimes coalesce, but usually they lie quite distinct and beautifully regular. Figure 1 of plate 44 presents an excellent likeness of this habit where the prisms are fairly coarse and firm. It lies here on a continuous layer of thomsonite of the first type. Apparently types II and Ila belong to the same generation. They do not F i g u r e 1.—Thomsonite of T ype I li a Two-thirds natural size

Figure 2.—Cavity lined with Chabazite and Thomsonite of Type I and of Type III Mesolite needles continue growth of Thomsonite of Type III. Two-thirds natural size

THOMSONITE OF TYPE Ilia AND MINERAL LINED CAVITY BULL. GEOL. SOC. AM. VOL. 11, 1899, PL. 46

F i g u r e 1.—Cavity lined with Thomsonite of Types I a n d I la, and filled with Mf.solite One-third natural size

Figure 2.—M esolite growing on Thomsonite of Types I a n d II One-half natural size

MINERAL LINED CAVITY AND MESOLITE THOMSONITE OF TYPES Ilffl, III, AND Ilia 467

appear to occur together, but they bear the same relationships to the older and younger minerals, and are closely connected by intermediate stages. A careful investigation of individual leaves of II and of Ila, in parallel and convergent polarized light, revealed the same characteristics as are described under type I, with this exception: that the optical axial angle is considerably greater in leaves of types II and Ila than in type I. Means for the determination of the optical axial angle were not at the disposal of the writer.

T Y P E I I I

Thomsonite having this habit occurs commonly filling cavities nearly or quite full. It forms solid masses formed of radiating fairly coarse white blades that may reach a length of two or more inches. Where the cavity is not completely filled, these radiating aggregates assume hemi­ spherical shaped forms, measuring from one to three inches across. They never terminate smoothly, but the fibers may gradually taper out into fine hairs resembling those of mesolite, or more commonly they may become covered with the rough bladed aggregates forming type Ilia. As against the older generations of thomsonite, this variety is quite dis­ tinct, although it may merge into mesolite varieties by insensible grada­ tions. Figure 2 of plate 44 illustrates a solid amygdule composed of thomsonite of this character. It is deposited upon a coating of thom­ sonite of the first type, and this upon chabazite. In figure 2 of plate 45 we have a section through one of theSe radiated aggregates which rests upon thomsonite of the first type. The extremities of the radiating blades extend into delicate separate fibers that resemble the fine hair-like growth to be described as mesolite. Some of these fibers penetrate crystals of analcite that do not show plainly in the figure. In this case therefore the thomsonite is older than the analcite. This, however, is not always the case, for thomsonite of this habit has been observed resting upon analcite as well as on chabazite and on thomsonite of types I and II.

T Y P E I lia

The mineral is composed of thin rectangular leaves, confusedly inter­ laced, so as to form a very rough porous aggregate that bears a resem­ blance to grated cocoanut. Almost always this cocoanut-like mass is to be found on the surface of hemispherical aggregates of the type just de­ scribed. Still thomsonite of type III does not always pass into this variety, and Ilia has been observed occasionally quite unconnected with III. Whether it really forms a generation by itsplf may be doubted. Figure 1 of plate 45 gives a fairly good idea of this habit. 468 H. B. PATTON— THOMSONITE, MESOLITE, AND CHABAZITE

Cleavage flakes and individual leaves taken from both III and Ilia show the same optical properties as do those from II and Ila.

M e so l it e

This mineral occurs as extraordinarily delicate aggregates, composed of long, slender, hair-like fibers. They are often exquisitely beautiful, and the variety of structure and general appearance appears to be almost infinite. All the specimens thus far obtained may, however, be con­ veniently classified under three general types, which are designated re­ spectively as types a, b, and c. First, type a. The fibers form loosely felted masses that resemble fine cotton, wool. Second, type b. The fibers are distinct and separate, and.form delicate brushes with parallel or nearly parallel bristles. This particular variety is not common. I t is apt to grow on the hemispherical bunches of thom­ sonite of type Ila. Third, type c. The felted aggregate is. composed of fibers that lie approximately in one plane, so as to form a fragile gauze or cobweb-like membranes. Figures 1 and 2 of plate 46 and figure 1 of plate 47 represent three specimens typical of type a, and figures 1 and 2 of plate 48 exhibit two phases of type c. In figure 2 of plate 47 there is to be seen an excellent example of type b on the right side, while the specimen on the left is intermediate between b and c. These three types of mesolite, as well as the numerous intermediate varieties, may be seen growing on analcite and on thomsonite of all types, but not on chabazite, as this last named zeolite is invariably first covered over by another mineral. They are very closely associated with thomsonite of types II, Ila, III, and Ilia, and their extremely deli­ cate fibers often appear to be but continuations of the coarser thomsonite growths that support them. More especially is this intimate association to be seen between mesolite and thomsonite. of type III. Not infre­ quently cavities several inches in diameter (4 to 8 inches) have their centers filled with a rather dense, wool-like, matted aggregate of mesolite) while the outer portion is composed of thomsonite of type III. But the determination of this fine fibrous mineral as mesolite depends entirely on the chemical analysis, as the fibers are far too fine to admit of a decisive physical or optical1 investigation. Optically about all that can be observed is a parallel extinction, with usually positive extinction. It is perhaps well to state that at the time when this paper was pre­ pared it was supposed that this mineral, mesolite, was really thomsonite, and specimens sent out from the Colorado School of Mines were so labeled. BULL. GEOL. SOC. AM. VO L.11, 1899, PL. 47

Figure 1.—M esolite growing on Thomsonite of Type II One-half natural size

Figure 2.—M esolite growing on Thomsonite of Type I la Two-thirds natural size

MESOLITE GROWING ON THOMSONITE BULL. GEOL. SOC. AM. VOL. 11, 1899, PL. 48

F i g u r e 1.—M esolite growing on Thomsonite of Type Ila and Analcite Two-thirds natural size

Figure 2.—M esolite growing on Thomsonite of Type II One-half natural size

MESOLITE GROWING ON THOMSONITE AND ANALCITE CHEMICAL ANALYSES 4 6 9

This determination as mesolite was based on a carefully made chemical analysis, which apparently closely coincided with analyses of. undoubted thomsonite from Table mountain and elsewhere; but inasmuch as Cross and Hillebrand had published analyses of an apparently identical mineral from Table mountain, and had determined the mineral to be mesolite, it seemed advisable to submit the material in question to a renewed investigation. The result showed that the material used for the first analysis was not absolutely pure, but contained just enough carbonate of calcium to make the analysis closely approximate that of thomsonite. The material on which the later analysis was made was obtained at a later date and proved to be perfectly pure.

C h e m ic a l A na lyses

The chemical analyses given below were carried out in the chemical laboratories of the Colorado State School of Mines. Analyses 1 and 2 were made by President Regis Chauvenet, and number 3 by Doctor Robert N. Hartmann. In presenting these analyses the author wishes herewith to acknowledge gratefully the valuable assistance thus ren­ dered. In all three cases great care was exercised to secure absolutely pure material. Number 1 was taken from thomsonite of type II and was obtained by carefully breaking off the freely projecting needles and prisms. Number 2 was taken from the center of a mass some 2 inches in diameter and represents thomsonite of type III. Number 3 repre­ sents a fine cotton-like mass of mesolite. For purposes of comparison, five analyses are also given which were made by W. F. Hillebrand* from material collected by the U. S. Geological Survey from Table moun­ tain. Judging from the descriptions accompanying these analyses, IX and X represent thomsonite of type I, while XI and XII represent either type II or type III. Number XVIII is mesolite.

Num­ Num­ Num­ IX.X. XII. XVIII. ber 1. ber 2. ber 3. XI.

Si02...... 41.34 41.59 45.59 40.88 40.68 41.21 42.66 46.17 30.35 30.59 25.18 29.68 30.12 29.71 29.25 26.88 CaO...... 11.20 11.15 8.93 11.88 11.92 11.34 10.90 8.77 Na*0...... 5.04. 4.66 7.65 4.72 4.44 5.62 4.92 6.19 H 20 ...... 12.27 12.24 12.67 12.91 12.86 12.20 12.28 12.16 100.20 100.23 100.02 100.07 100.02 100.08 100.01 100.17

♦ B u ll . No. 20, U . S. Greoh Survey, pp. 25, 35.

LXVII—B um -. G rol. S oc, Am., Vol. 11, 1899 470 H. B. PATTON— THOMSONITE, MESOLITE, AND CHABAZITE

For the purpose of checking analysis number 3 a corroborative test for silica was made on another sample of mesolite, which gave SiO = 44.83. The following description of the method pursued in the analysis of mesolite has been furnished by Doctor Hartmann. The mineral was air-dried and analyzed by the method customarily employed with sili­ cates decomposed by HC1. H20 was determined by cautious ignition at a low red heat, and the residual silicate used for further analysis. The silica was treated with ammonium fluoride and Na2S04 to test the purity. The sodium was- weighed as sulphate obtained by ignition of the salts of the evaporated filtrate from the calcium determination with H2S04. Platinum vessels were used wherever possible.

Ch a b a z it e

Chabazite is the commonest of the zeolites found on the Table moun­ tains at Golden. It forms thin crusts lining the smaller cavities, and is invariably the oldest mineral deposited. It is difficult, however, to secure good specimens of this mineral without retaining a large piece of the inclosing basalt. This is due to the brittleness of chabazite and to the fact that the crusts are always very thin, so that when separating from the rock the fragile crusts break into small fragments. In their description of this mineral as it occurs on North and South Table mountains Cross and Hillebrand state that it occurs in simple rhombohedrons or in plain rhombohedrons twinned parallel to the basal plane. With the exception of the locality from which the zeolites de­ scribed in the present paper were obtained, it is true, as far as the obser­ vation of the writer goes, that the chabazite crystals are invariably simple or twinned rhombohedrons. It is all the more surprising, therefore, to find that at this particular quarry the chabazite crystals possess a far more complicated form than has been observed before. The chabazites of this locality vary from white to reddish in color, and in size from 5 to 10 millimeters. Not infrequently both white and reddish crystals may be found on the same specimen. There are a few simple rhombohedrons that show interpenetrating twinning such as are to be seen at other localities on the same mountain; but in nearly all cases the form is not only twinned, but is quite complex. The fol­ lowing forms may be identified on most of the crystals: 4- ii(1011), — 2 R (0221) = s, — IR (0112) = e, oo P 2 (1120) = a; also two scaleno- hedrons. Figure 2 of plate 49 is intended to represent one of these twinned crystals in its customary development. In the general distribution of F ig u r e 1.— F loor o f m ix e d L a u m o n tite a n d S t il b it e Floor is composed of layer of Laumontite, % inch thick, isolated Analcite crystals and layer of Thomsonite of Type II, % inch thick. One-third natural size

Figure 2.—Drawing of Crystal of Chabazite

FLOOR OF LAUMONTITE AND STILBITE AND DRAWING OF CHABAZITE CRYSTAL

CHABAZITE 4 7 1 the faces and slight distortion it fairly well represents an actual crystal, although the crystal as it occurs does not show all the faces represented in the drawing. It is very common to find one of the interpenetrating crystals smaller than the other, as is shown in figure 2 of plate 49, but other crystals show every possible degree of distortion of one or both of the two individuals. In many cases one of the interpenetrating indi­ viduals may be almost suppressed or, what amounts to the same thing, may show just a corner of the rhombohedron projecting from the face of the other and larger individual; but, with all the varying degrees of dis­ tortion and suppression, three of the forms are almost always to be seen. These are R, — 2 R, and oo P 2—that is, R, s, and a. The flatter rhombo­ hedron e is very often missing, or is present on one and missing on the other twinned individual. The two scalenohedrons are never present as distinct faces, but as strongly developed striations. Nevertheless they are well enough devel­ oped to give distinct flashes of light. By their intersections they form flat ridges upon the faces of the rhombohedron R. Almost always one of these striating scalenohedrons may be detected, while the other one is mainly absent. The more common of the two scalenohedrons striates R parallel to the rhombohedron edges that meet at the vertex of the crystal—that is, parallel to the edge formed by R and e. It forms by the intersections of the striations a very flat ridge on the face of R, running from the top of the crystal down toward the middle of the face. The other scaleno- hedron striates R on the lower part of the face parallel to the zigzag edges—that is, parallel to the edge formed by R and a. It forms a flat ridge that runs from the point of intersection of two lateral edges up­ ward till it meets the ridge formed by the other scalenohedron. By their mutual intersections these two scalenohedrons also form two hori­ zontal flat ridges. Thus four ridges are formed that divide the face R into quadrants. The above description of the striating scalenohedrons applies, of course, only to ideally, or symmetrically developed crystals, such as are never realized in nature. As a matter of fact, only occa­ sionally do the striations approach to symmetry. In nearly all cases the flat striation ridges are shoved to one side or the other or are entirely crowded off. An attempt to illustrate this distortion of the scalenohedral striations is made in the shading of the faces of R in figure 2 of plate 49. As far as is known to the writer, the chabazite crystals that are most nearly akin to these from Golden are the crystals from the phonolite near Riibendorfel, in Bohemia.* These crystals from Riibendorfel (com­

* Doctor C. Hintze : Handbuch der Mineralogie, Leipzig, 1897, p. 1777. 472 H. B. PATTON----THOMSONITE, MESOLITE, AND CHABAZITE

monly labeled as from Aussig) show sealenohedral striations and also similar forms and twinning, but lack the prism a.

O t h e r Z eo lit es

ANALCITE

The other zeolites present may be dismissed with a very brief mention, inasmuch as they have already received ample description. Next to thomsonite analcite is the most abundant zeolite present. The milky white variety is the most abundant, but clear glassy crystals are not want­ ing. In size they vary from 1 millimeter up to 2i inches in diameter. Specimens measuring 1 inch are common. They sometimes occur alone, lining the cavities on all sides, but more frequently they lie on a coating of chabazite or of thomsonite of types I and II. The form is the char­ acteristic trapezohedron 2 02 (211), sometimes with the edges running to the center of the octant slightly truncated by the trisoctahedron f 0 (332). Its period of growth about corresponds with that of thomsonite, type III, as it occurs sometimes beneath this variety, sometimes penetrated by its needles. In one case a few minute crystals of analcite were ob­ served growing upon the very delicate hairlike fibers of mesolite. This would indicate that analcite occurs here in two generations.

APOPHYLLITE

Apophyllite is very sparingly represented by snow-white opaque crystals, with the characteristic steep pyramid and indirect prism. It occurs in crystals from a quarter to half an inch in length ; is always associated with analcite and always imbedded in and therefore older than that mineral.

STILBITE AND LA UMONTITE

Stilbite and laumontite occur, as described by Cross and Hillebrand, forming the bedded floor of many cavities, and also as larger and dis­ tinctly crystallized specimens growing freely on the upper surface of the bedded floors. A second generation of both of these minerals has been noted in a few cases. In one case snow-white crystals of laumontite showing the customary square prism and steep orthodome occur, grow­ ing on a quarter-inch layer of thomsonite, which forms the uppermost deposit of the bedded floor. These laumontite crystals measure from 5 to 10 millimeters in length and half a millimeter or more in thickness. The position of this thomsonite layer with reference to the order of OTHER ZEALITES 4 7 3 deposition is not clear, but it resembles type II more closely than any other. CALCITE AND ARAGONITE Calcite is only occasionally present in scalenohedral crystals. Its position appears to lie between thomsonite, type III, and mesolite. Aragonite is more commonly present. It is the very latest mineral de­ posited, and occurs in thin transparent or whitish coatings on most of the younger minerals.

O r d e r o f D epo sit io n The minerals here described, as well as their different varieties, occur so frequently associated together that, for most of them, the order of deposition is very readily observed. For the purpose of ready compar­ ison, the order as determined in the new locality and that given by Cross and Hillebrand are placed in parallel columns.

Order of deposition in new locality. Order of deposition observed by Cross and Hillebrand. 1. Laumontite. 1. Laumontite. 2. Stilbite. 2. Stilbite. 3. Chabazite. 3. Thomsonite. 4. Thomsonite, type I. 4.Calcite (yellow). 5. Apophyllite. 5. Stilbite. 6. Thomsonite, types II and Ila. 6. Chabazite. 7. Laumontite. 7. Thomsonite. 8. Stilbite. 8. Analcite. 9. Analcite. 9. Apophyllite. 10. Thomsonite, type III. 10. Calcite (colorless). 11. Calcite. 11. Mesolite. 12. Thomsonite, type Ilia. 13. Mesolite. 14. Analcite. 15. Aragonite.

The differences between the two will be seen to be very slight, and can mostly be accounted for by the recognition of a second period of laumon- tite deposition and of the different stages of thomsonite formation.

S um m ary

In a newly opened quarry on the east face of North Table mountain at Golden, Colorado, are found a great variety of zeolites of which thom- sonite, mesolite, and chabazite are the most important. These occur sometimes separately, filling or lining adjacent cavities, but more often 474 H. B. PATTON— THOMSONITE, MESOLITE, AND CHABAZITE in successive deposits in the same cavity. The thomsonite is chiefly remarkable for the great variety of types in which it occurs, each type representing a distinct generation. A microscopical investigation indi­ cates that the brachypinacoid is the dominant form, instead of the macropinacoid, as formerly supposed. The mesolite is distinguished both by great variety of type and by exquisite beauty and delicacy. Chabazite occure in crystals that show twinning parallel to the basal plane and an unusual development of forms. The zeolites, together with calcite and aragonite, as they occur here, indicate no less than fifteen distinct stages or periods of mineral depo­ sition.

BULL. GEOL. SOC. AM. VOL. 11, 1899, PL. 50

Figure 1.—Samples of L»ionite Ore N um bers 1-5, bom bshell ore ; 6-11, pipe ore ; 12, brecciated ore

Figure 2.—View in the Hunter Ore Bank Showing an ore pocket in eroded cavity in the limestone

LIMONITE ORE AND ORE POCKET