562 STUDIES FOR STUDENTS 



EXPLANATION OF PLATE X 



Fig. 1. — Section No. 4733. (x 12.) Berlin rhyolite. This figure 

 shows the exceedingly fine grained matrix and the porphyritic indi- 

 viduals of feldspar, which are characteristic of the rock in the hand 

 specimen. The mica which occurs in small flakes is also nicely shown. 

 The parallel arrangement of the small flakes, which is evidently a cause 

 for the "rift" in the rock, is well brought out. The cracking of the 

 feldspar, referred to in the text, is also seen in this figure. 



Fig. 2. — Section No. 4704. (X 12.) Utley rhyolite. Porphyritic 

 crystals of quartz and feldspar and a small portion of the fine, dense 

 matrix are shown in this figure. The compactness of the rock, with 

 the consequently minute pores and low porosity are very evident. 



