LITHOLOGY. 1/9 



Iron sesquioxide, . . . . . . . . . . .15 



Iron protoxide, . . • 5.76 



Manganese protoxide, ......... .22 



Lime, 2.86 



Soda, 4.52 



Potash, 5.54 



Ignition, .15 



100.25 



This analysis confirms the examination of the section, showing that 

 the rock is mainly feldspar ; and, indeed, this analysis does not vary 

 widely from that of orthoclase. Only here and there among the porphy- 

 ritic crystals is there a black glassy grain of quartz, and it is as sparingly 

 present in the ground mass. A porphyry of the same nature occurs at 

 Lincoln. In thin sections of this rock the hornblende is conspicuous in 

 crystals of some size. Augite was originally present, but is now mostly 

 changed into chlorite and epidote. The rock also contains magnetite 

 and pyrite. 



The black color of these quartz porphyries depends upon the circum- 

 stance that the orthoclase is clear. When this character is absent, and 

 the feldspar is no longer transparent, the rock, which is otherwise the 

 same, is gray in color. Such a gray quartz porphyry from Mt. Lyon has 

 some interesting microscopic features. Although many crystals of ortho- 

 clase are porphyritically developed, they are not at all conspicuous in 

 specimens. This is because they are so impure that they are scarcely 

 individualized from the rest of the rock. Through the felsitic ground 

 mass are distributed numerous rounded and angular grains of augite, bio- 

 tite, chlorite, and magnetite ; and through portions of the orthoclase 

 crystals these grains are as thickly distributed as they are through the 

 ground mass. The appearance of a section of this rock, as it looks in 

 ordinary light, is represented in Fig. 2 on PL 10. If, now, polarized light 

 be employed, the form of a large crystal of orthoclase is brought to view 

 where nothing was visible before. The appearance of the same section 

 in polarized light is represented in Fig. 3 on the same plate. This shows 

 us that a pure, clear feldspar crystal began to grow in the rocks, but, for 

 some reason, a change took place in the conditions at a certain period of 

 its growth, whereby in its further increase it was obliged to include par- 



