LITHOLOGY. 179 
Iron sesquioxide, . ‘ F . 3 ‘ : ; . . 15 
Iron protoxide, eo <a So ae ep ap aw Ss 5.76 
Manganese protoxide, 022 
Lime, : : . : A , : ‘ 2.86 
Soda, a ‘ 3 bs g - P 3 , ; é ‘ 4.52 
Potash, . . - ‘ . ; ' 3 ; ‘s < 5-54 
Ignition, . 3 . Fi ‘ ‘ 5 . ‘ ‘ ‘ 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 Smong 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 
sone 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- 
