70 C. W. lioness — Stanley Shale of OMahoma. 



Above the quartzite ledge, at the base of the Stanley, 

 occur some siliceous shales and sandstones which are 

 micaceous, ripple-marked, and cross-bedded. They are 

 greenish gray in color when fresh but Aveather to a dingy, 

 bronzy appearance upon exposure. There are, however, 

 only a few feet of these and they are quickly followed by 

 a considerable thickness, perhaps 200 feet or more of 

 schistose, red, soft, sandstones and grits, the red color 

 being due to oxidation. These are usually coarse to 

 medium-grained, are often micaceous and seem to be de- 

 veloped best within the eastern half of the area mapped 

 (see map). 



A heavy bed of very resistant tuff intervenes at this 

 place in the section whose thickness approximates 90 feet 

 on Mountain Fork River, sec. 27, T. 2S., K 25E. The 

 material consists, for the most part, of fresh feldspars and 

 resembles a coarse graywacke or arkose in the hand speci- 

 men but is in reality a volcanic ash. It is usually gray in 

 color, flecked or mottled with green blotches of chlorite, 

 but there are as many varieties and degrees of fineness 

 and coarseness of this rock almost as one might collect 

 specimens. 



This tuff is a very important layer of rock, for it is the 

 only recognizable horizon in the lowermost 3,000 feet of 

 the Stanley and makes a good identifiable key ledge from 

 the distribution of which it will be possible to decipher 

 the major structural features of the lower part of this 

 formation if they ever are to be made out. 



That this rock is a tuff and not a graywacke or arkose 

 of sedimentary origin, as has hitherto been supposed^^ 

 was discovered by Dr. C. P. Berkey in 1917, when he re- 

 ported to me in a letter the results of an examination of 

 some rocks sent to him at the close of my first summer's 

 work in the region. 



In plane-polarized light, under the microscope, almost 

 any of this material thus far examined may be seen 

 plainly to be made up wholly or partially of angular frag- 

 ments which are bounded by broadly curved concave lines 

 meeting in sharp points, characteristic of volcanic ash 

 fragments and known as ^^bogen structure.'' The ma- 

 jority of the fragments have devitrified and under crossed 

 nicols these lose their identity in an aggregate of quartz, 



" Miser, H. D. : loc. eit. 



