PLATE X. 



PHOTOMICROGRAPHS OF GRAYWACKES. 



A. Graywacke from Penokee-Gogebic district of Michigan and Wisconsin. The orthoclase is in most 



cases separable from the quartz, in that it lacks the perfect clearness and uniformity of color 

 which each grain of that mineral shows. The striated feldspars are nicely shown. 



B. Biotite-graywacke from Mesabi district of Minnesota. This is the normal phase of Lower Huronian 



graywacke, consisting of quartz and feldspar grains, mainly the former, very imperfectly rounded, 

 and a considerable amount of secondary biotite and muscovite or sericite. All the constituents 

 have a dimensional parallelism, and the micas have also a crystallographie parallelism. After 

 Leith. 



C. The same, showing development of new hornblende near contact with intrusive granite. After 



Leith. 



D. The same, showing development of new hornblende to the practical exclusion of mica at contact 



with intrusive granite. After Leith. 



