PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 579 



Jaspilyte. Qreenwacke.) 



Meg. The usual red, white and black, banded jaspilyte. The white bands are 

 of quartz in fine grains, the black of hematite with a little magnetite, and the red 

 of quartz and hematite. 



Mic. The sections show the typical jaspilyte, composed of quartz, hematite and 

 some magnetite. The quartz is a little coarser grained than in the usual jaspilyte 

 and has a tendency to a common hexagonal outline. The grains of quartz are pretty 

 uniform in size, averaging about .05 millimetre, and the extremes are about one-half 

 and one and a half times as large as the average. 



Four sections. 



Remarks. In the description of this rock in Bulletin vi, the silica was regarded 

 as not completely crystalline or partly amorphous. That the silica is ordinary quartz 

 in small grains is clear and has been so stated by several, including C. R. Van Hise,* 

 J. E. Spurrf and the writer. u. s. G. 



No. 868. GREENWACKE. 



From a low ridge southeast of the main jasper ridge, N. W. % N. E. % sec. 32, T. 62-15, northwest from 

 Tower. 



Ref. Annual Report, xv, pages 266-268, 387, 389; Annual Report, xvii, pages 194, 215; Bulletin vi, pages 

 25, 42, 43, 68, 421. 



Meg. The rock is fine grained and nondescript. It evidently has been sheared 

 and altered much. Its color is light gray, but in many places it is brown with iron 

 oxide, and has visible pyrite crystals. 



Mic. The- rock is much chloritized, having a fibrosity due to the elongation 

 of the minerals prevailingly in one direction. This is quite conspicuous in the 

 section. Besides chlorite, Muscovite is also present in considerable amount. Calcite 

 is shown by its high polarization and black cross in convergent light. It is quite 

 abundant and in large areas. Quartz, frequently embracing a considerable amount 

 of the other minerals poikilitically, is secondary and quite abundant. A plagioclase 

 feldspar is also common, but it is completely obscured by the. growth of minute crys- 

 talliths which fill its area. Zoisite is in small isolated grains, scattered everywhere. 

 The rock is difficult to characterize because of the extreme alteration. It was 

 originally probably a debris of eruptive materials, mainly basic. Occasional lighter 

 areas, in natural light, show the forms of large feldspars. Small flakes of a cherry- 

 red to brownish-red and opaque mineral, indicating a ferruginous product of change, 

 may be hematite. Zoisite and epidote grains show as globules or short, blunt rods, 

 the former having no color between crossed nicols, white and colorless in natural 

 light, and of high refractive index. 



Five sections examined. 



Age. Keewatin. N. H. w. 



* Annual Report Arkansas Geological Survey, 1890, vol. ill, p. 184. 

 x, p. 222. 



