Plate 22 



Photomicrograph of bluestone, x 25 diameters. The clearer grains are 

 quartz and indicate the approximate size of other original constituents. 

 In this case the alteration of the feldspars and ferromagnesian originals 

 is so complete that their products form an indeterminable complex aggre- 

 gate of closeh' interlocked granules, flakes, and fibers of extremely fine 

 texture. 



Photomicrograph of first grade medium grain bluestone, x 25 diameters. 

 Taken to show angular and interlocking grains indicating secondary 

 growth and a coinplete lack of reeding structure. The clear grains are 

 quartz; the rest of the field is made up chiefl\' of secondar}" derivatives 

 from the original feldspars and ferromagnesian minerals. 



