144- GEOLOGY OP THE COMSTOOK LODE. 



the expense of the chlorite. At least a part of the quartz is primitive. It 

 contains fluid inclusions, gas-pores, and possibly also glass-inclusions. The 

 larger feldspars are well developed, very closely striated, and appear to give 

 angles of extinction of about 18° 80' in orthopinacoidal section. The 

 smaller feldspars are granitoid rather than microlitic in their development, 

 and so much obscured by decomposition-products as to make it uncertain 

 whether they also are referable to oligoclase. The iron ore is probably 

 titanic, and is accompanied by both leucoxene and ferric oxide. The slide 

 contains much apatite, a large part of it in unusually long mici'olites, and a 

 little sphene. The rock appears to me to be a decomposed diabase. 



Geology of the High Plateaus of Utah. Slide and specimen No. 274. Gate of Mun- 

 roe, Sevier Plateau. 



The general character of this rock, both macroscopically and micro- 

 scopically, is almost identical with that last described, but the bisilicates 

 have been entirely decomposed, and the chlorite is much disseminated; 

 there is a strong probability, however, that the original mineral was augite. 

 The feldspar best answers in its optical characters to labradorite. It con- 

 tains fluid inclusions. The apatites are extraordinarily large and abundant, 

 and, strange to say, contain numei'ous fluid inclusions. 



