COMPARISON WITH OTHER ROCKS 371 



As developed from the chemical and microscopic study of the rocks of 

 the Virgili na district, the present much altered rock, greenstone, clearly 

 indicates its derivation from an original andesite of an intermediate 

 basic type as contrasted with the similar Catoctin schist or greenstone, 

 which from Keith's * description, is derived from a more basic ande- 

 site, diabase, or basalt. 



According to Keith, the rocks of the Catoctin area are igneous in origin 

 and represent probably two different flows — the upper, basaltic, and the 

 lower, dioritic. In general the rocks are much altered through dynamic 

 metamorphism and secular decay and now largely form greenish epidotic 

 and chloritic schists, designated by Keith as the Catoctin schist. The 

 fine-grained varieties are composed of quartz, plagioclase, epidote, mag- 

 netite, and chlorite. In the coarse-grained types the original nature 

 of the rock is well indicated. The ophitic arrangement of the coarse 

 feldspars is definitely marked. The additional minerals in the coarse 

 rocks are calcite, ilmenite, skeleton olivine, biotite, hematite, and, in a 

 few instances, hornblende. The alteration products, chlorite and epidote, 

 are abundant and characteristic. An analysis of the fresh rock by George 

 Steiger is shown in column VIII of table of analyses, on pages 364-365. 



The South Mountain area is shown by Williams f and Bascom J to 

 consist of the acid volcanic rhyolite and the basic types, diabase and 

 basalt, the latter yielding on alteration the greenstones of the region. 

 Bascom § further describes the basic types of this area as holocrystal- 

 line, effusive, plagioclase-augite rocks, with or without olivine, the essen- 

 tial characteristics of the diabase group. 



After establishing the igneous origin of the greenstones of the Menom- 

 inee and Marquette districts of Michigan, Williams || shows the different 

 rock types to have been olivine-gabbro, gabbro, diabase, diabase-por- 

 phyry, glassy diabase and melaphyre, diorite, diorite-porphyry, and 

 tuffs, with the two districts limited on their north and south sides by an 

 acid series consisting of granite, granite-porphyry, and quartz-porphyry. 



The original mineral constituents of these rocks are described b}' Wil- 

 liams ^[ as labradorite, quartz, biotite, hornblende, diallage, augite, oliv- 

 iue, zircon, apatite, sphene, ilmenite, and magnetite. The secondary min- 

 erals produced by metamorphism and weathering are albite, saussurite, 

 zoisite, quartz, hornblende, epidote, chlorite, biotite, talc, serpentine, 

 carbonates, iron oxides, and pyrite.** 



*14th Ann. Rept. U. S. G. S., 1894, p. 304 et seq. 



f Op. cit. 



JOp. cit. 



§ Ibid., p. 69. 



|| Bull. U. S. Geol. Survey, no. 62, pp. 197-199. 



II Ibid., pp. 199, 200. 



** Ibid., pp. 213, 214. 



