PLATE XT. 



PHOTOMICROGRAPHS OF PELITES. 



A. Slate from Mesabi district of Minnesota. This is the normal, fine-grained Virginia slate, consisting 



of quartz and feldspar, chlorite, muscovite, biotite, and iron oxide, all the constituents arranged 

 with their longer diainete'rs roughly parallel, though the chlorite and iron oxide are arranged to 

 a less extent than the other constituents. After Leith. 



B. Biotite-slate from Penokee-Gogebic district of Michigan and Wisconsin. Shows the alteration of 



the feldspars to the micas. The crystalline character of the developing rock is well shown. 

 There remain, however, areas of feldspar which act as a unit, although they are cut in every 

 direction by the alteration products, quartz and mica. 



C. Muscovite-biotite-quartz-schist, showing uniformity of texture. Black Hills, South Dakota. 



D. Albite-gneiss from Hoosac Mountain, Massachusetts. Shows no trace of clastic origin. After 



Wolff. 

 902 



