THE UPPER SLATE MEMBER. 337 



numerous small {)articles of n l)l;u-k substance which is taken to be partly 

 altered pyrite or marcasite, and perhaps partly carljonaceous material. 

 The induration in the rock is due to tlie formation in situ of (quartz, ()r in- 

 filtration of silica, from which finely crystalline quartz has formed, assisted 

 by the partial transformation of the feldspars into interlocking complex 

 areas of quartz, mica and feldspar, which also interlock with the material 

 of the matrix. 



(3) Biotitic f/raywaeke (PI. xxxiii, Fig. 2). — Macroscopically, this rock 

 differs from (2) only in lieing of a darker gray color; and under the micro- 

 scope also it is nmch the same, except that micaceous alteration of the feld- 

 spars has been carried fai'ther. Fragments of feldspiu-, with its second;n-y 

 products, compose most of the section. The alterations of feldspar to I)io- 

 tite and quartz are beautifully shown. The freshest of the feldsj)ar grains 

 are surrounded and more or less deeply ])enetrated by secondary biotite. 

 These grains yet retain their well roiuided forms. However, in many cases, 

 the original outlines of the grains of feldspar are lost, although often the 

 complex aggregate of resulting biotite folia and quartz, mingled with the re- 

 maining feldspar, retain very perfect general roinidish or oval forms. Often 

 the entire surfaces of the feldspars include very numerous particles of the 

 biotite and quartz — the former usually nuich the more jjlentiful — thei'e re- 

 maining through such areas here and there little spots of feldspar which act 

 as a unit in each area. With a low power such areas api)ear to be roundish 

 aggregates of biotite. It is only with a higher power that tlie remaining 

 feldspar and its true relations to the biotite are discovered. In this rock all 

 the stages of the process of alteration of the feldspars are nicely shown 

 from those areas in which the secondary biotite forms l)ut a him around 

 the feldspars to tho.se in whicli no feldspar remains, there being in the 

 place of the fragmental feldspars interlocking aggregates of biotite and quartz. 

 Even these areas .often so perfectly retain their general oval or roundish 

 character tluit, taken l>y themselves, they woidd lie regarded in the sections 

 as peculiar complex fragments, which in this condition liad lu-comc worn and 

 depo.sited. Taken in connection with these other grains present, there can 

 be no doid)t of tlicir formation l>v tlu- alteration of feldspar. As from a 

 single large ))article of feldspar many individuals of (puirtz and mica ai'e 

 ?ioN XIX 3a 



