Composition of Certain Mesozoic Igneous Rocks of Virginia
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BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l . 2, pp. 339-348 March 18, 1891 COMPOSITION OF CERTAIN MESOZOIC IGNEOUS ROCKS OF VIRGINIA. BY H. D. CAMPBELL AND W. G. BROWN. (Read before the Society December 30, 1890.) CONTENTS. Page. Generally uniform Composition of Mesozoic T rap ___________________________ 339 Traps of exceptional Composition________________ _______________ _________ 340 V arieties____________________________________________________________ 340 Localities of Occurrence______________________________________________ 341 Description of the Hypersthene-Diabase________________________________ 342 Constituents of the Hypersthene-Diabase_______________________________ 342 Characters of the Hypersthene_________________________________________ 344 Associated M inerals__________________________________________________ 345 Description of the Olivine-Hypersthene-Diabase________________________ 346 Discussion______________________________________________________________ 348 G e n e r a l l y u n if o r m C o m po sit io n o f M esozoic T r a p . No one has ever studied the earlier Mesozoic formation along the Atlantic border without being struck by the numerous ridges and dikes of trap which are found in connection with it, and no one has ever studied the trap found in these ridges and dikes without being impressed by its uniformity of com position. J. D. Dana * has pointed out the wonderful uniformity of these eruptive rocks, calling them all dolerites; rocks made up essentially of labradorite and pyroxene, with more or less magnetic iron ore in disseminated grains or crystals. He says that the aspect of specimens from Nova Scotia to North Carolina is closely the same, and that the density varies little from 3. The lowest density which he quotes is 2.94, and the highest 3.16. Chemical analyses of these rocks by G. H. Cook,f W. G. Mixter,J S. T. Tyson,§ F. A. Genth || and G. W. Hawes all show their chemical composition ♦Am. Jour. Sci., 3d ser., vol. VI, 1873, p. 105. f Geology of New Jersey, 1868, p. 215; Am. Jour. Sci., 3d ser., vol. VI, 1873, p. 106. i Am. Jour. Sci., loc. cit. \Loc. c it, p. 107. fltfeol. Surv. Pa., Report C, 1874, p. 122; and Report CCC, 1877, p. 275. ft Am. Jour. Sci., 3d ser., vol. IX, 1875 p. 185. L—B ull. Geol. Soc. Am., V o l . 2 , 1890. (3 3 9 ) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/2/1/339/3412704/BUL2-0339.pdf by guest on 24 September 2021 3 4 0 H. D. CAMPBELL AND W. Or. BROWN— COMPOSITION OF TRAP. to be nearly the same. The large amount of magnesia (10.3 per cent.) in G. H. Cook’s analysis of the trap from the palisades of the Hudson is worthy of notice. E. S. Dana * made a microscopic examination of numerous specimens of trap from Connecticut, and of a few individual specimens from Nova Scotia, New Jersey, Pennsylvania and North Carolina, and found that, so far as microscopic structure goes, the rock from these distant localities is hardly to be distinguished from the trap of the Connecticut valley. He gave the composition of the rock to be “ pyroxene, labradorite and magnetite, with also occasionally some chrysolite and apatite.” He, too, called it dolerite. G. W. Hawes t discovered a glassy ground-mass in certain modifications of these rocks, and mentioned the occasional presence of biotite and horn blende. Excepting local modifications, he considered the rocks to be like the ordinary old diabases, and in microscopic features to be monotonously alike wherever fresh stones occur. J. P. Iddings % examined microscopically the igneous rocks occurring in the earlier Mesozoic area at Orange, New Jersey, and found some of them to be holocrystalline and others to contain glass. He says the former should be called dolerites and the latter basalts. N. H. Darton, § in speaking of the igneous rocks of the New Jersey Meso zoic region, says that they are remarkably uniform petrographically, as they are all basalts, varying mainly in structure and development. These references are sufficient to show that the trap rocks of the earlier Mesozoic areas upon the Atlantic border have been considered essentially alike in mineral and chemical composition, whether called dolerites, diabases, or basalts. T r a p s o f exceptional C o m po sit io n . Varieties.—We are glad to be able to bring to notice two interesting varieties to break the monotony of these igneous rocks. On account of the conspicuous occurrence of hypersthene in one variety, and of olivine together with hypersthene in the other, we have called the former hypersthene-diabase and the latter olivine-hypersthene-diabase. The palisade area of Triassic rocks extends from the Hudson river, through New Jersey, Pennsylvania and Maryland, into Orange county, Virginia. As early as 1839, W. B. Rogers || called attention to the trap of the part of this area lying in Virginia as being a very conspicuous feature from a geo logical point of view. He mentioned the occurrence of ridges, knobs and *Am. Jour. Sci., 3d ser., vol. VIII, 1874, p. 390. fProc. U. S. Nat. Mus., 1881, p. 129. i Am. Jour. 8ci., 3d ser., vol. XXXI, 1886, p. 331. 3 Bulletin U. S. Geol. 8urvey, no. 67, 1890, p. 15. 1 Geology of the Virginias, 1884, p. 475. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/2/1/339/3412704/BUL2-0339.pdf by guest on 24 September 2021 LOCALITIES OF HYPERSTHENE-DIABASE. 341 dikes of “ greenstone,” composed of hornblende and white feldspar. It was in a specimen from Culpeper county, Virginia, near the southern end of this Triassic area, that we first noticed the presence of hypersthene. Localities of Occurrence.—A few localities where the rocks to be described below occur in their typical form may be mentioned: About three miles north of Rapidan, a station on the Virginia Midland railroad, are to be seen two rounded knolls of similar shape and size, as rather conspicuous objects in the landscape. They are sometimes called the “ Twins,” although several other names are given to them by the people in the neighborhood. On the top of the knob nearer to the railroad, beautiful perpendicular, pentagonal and hexagonal basaltic columns form the face of a cliff. Some of these columns are sixty feet high and from twenty to twenty- five feet in diameter. A ridge runs around from one knoll to the other, making a curve like a horseshoe. The cliffs face the center of the curve. The stone from the side of the “ Twins ” is quarried under the name of granite. It is in reality hypersthene-diabase. Along the railroad between Rapidan and Mitchell stations for a mile or more the same rock appears in the cuts. A few miles east of Culpeper Court-House, Mount Pony stands out prom inently as an isolated peak, commanding such an extensive view from its summit that it has been used as a signal station upon various occasions. The greater part of this peak is composed of a rock of the same character, although the hypersthene is less conspicuous in it than in the other speci mens. Not far east of Brandy station, on the same railroad, a hummock called the “ Dumpling ” is hid away in the woods. This hillock is composed of the same rock. Other knobs and dikes occur in this region. Similar rocks occur in New Jersey and Pennsylvania, as we have learned through the courtesy of Mr. N. H. Darton, who kindly put into our hands for examination the collection of New Jersey trap rocks made by him for the United State Geological Survey. In the thin section number 152, from “ three miles south of Milford, New Jersey, in Pennsylvania,” we find hypers thene to be abundant. The rock seems to be identical with that from the “ Twins.” Section number 294 shows the same rock as occurring at Point Pleasant, New Jersey. With reference to the latter rock Mr. Darton says * that it is very similar to the typical palisade trap. Several other of these sections show the probable presence of an orthorhombic pyroxene. These localities will suffice to show that the hypersthene-diabase is not merely a local variety. A t only one locality have we thus far noticed the olivine-hypersthene-dia- base. A dike of considerable width occurs crossing the Virginia Midland * B ulletin U. S. Geological Survey, no. 67,1890, p. 68, Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/2/1/339/3412704/BUL2-0339.pdf by guest on 24 September 2021 3 4 2 H. D. CAMPBELL AND W. G. BROWN— COMPOSITION OF TRAP. railroad in Culpeper county, Virginia, about two miles north’ of Rapidan station. This rock weathers into globular masses and contains olivine through out, so far as we could judge from making numerous thin sections. Description of the Hypersthene-Diabase.—The hypersthene-diabase has a medium grain and is of a dark-gray color. The darker varieties have a somewhat greasy luster. The unaided eye can detect two dark minerals, the one nearly black, and the other deep honey-yellow or brown in a very light-colored background. Sometimes the light material occurs as small irregular veins running through the darker rock. In thin sections we were able to distinguish triclinic feldspar, diallagic augite, hypersthene, biotite, apatite and occasional quartz, hornblende and probably zircon. Black opaque grains were also present, and as these were magnetic and some of them showed a trace of titanium, they were considered to be magnetite and ilmenite. The structure is generally ophitic. It seems to be intermediate between the granular structure of gabbros or norites and the porphyritic structure of the holocrystalline varieties of the augite-porphyrites, shading sometimes into the former, sometimes into the latter.