Plate 27 



Photomicrograph of no. 432-a. Taken with crossed nicols, magnification 

 about 25 diameters. 



Showing the microstructure in this specimen. A single original quartz 

 unit covers a large portion of the field. It includes other matters among 

 which are grains of feldspar, separate quartz grains, biotite and black gran- 

 ules and the principal quartz unit is itself much strained and preserves evi- 

 dence of fracturing and rehealing. 



Although the hand specimen has all the usual megascopic appearaiice of a 

 quartzite, it is rather plain from a microscopic examination that the material 

 is not of such simple origin. The quartz has all the habit of vein quartz, and 

 its tendency to include other grains and 'be intergrown with other units of 

 the same quartz make one believe that it is a differentiation extreme of mai;- 

 matic origin, similar to the quartz stringers described and illustrated in 

 nos. 163 and 389. 



The feldspar grains and some of the other grains included in the quartz 

 of this specimen have the same habit as have the original rock materials noted 

 in no. 163 and are believed to be remnants or representatives of remnants of 

 the original rock which was invaded by this quartz. 



