PLATE IX. 



PHOTOMICROGRAPHS OF SANDSTONE AND QUARTZITES. 



A. Sandstone from Arlington, Columbia County, Wis., showing enlargement ot quartz grains in 



optical continuity, and crystal facets so common in sandstones which have been partly cemented. 



B. Quartzite from Penokee-Gogebic district of Michigan and Wisconsin. The grains have been 



thoroughly cemented by enlargement, having grown by adding new quartz material to them- 

 selves in optical continuity until they met adjacent grains growing out in a similar manner. 

 The rock is a thoroughly crystalline quartzite. 



C. Completely recrystallized quartzite in which all trace of rounded grains is lost. From Rib Hill, 



Wis. The rock has been so thoroughly recrystallized that the quartz grains form an irregular 

 mosaic, like vein quartz or recrystallized chert. 



D. The same, granulated. In the field, in hand specimens, and under the microscope all stages of 



the mashmg of the rock are to be observed. Each of the granules is a minute part of the quartz 

 individual from which it was derived. 



872 



