160 GEOGRAPHIC DISTRIBUTION. 



space included in division 4. Near the lower part two layers 15 to 18 inches 

 each, of sandstone, containing Ostrea soleniscus, Trapezium micronema, &c. 

 Altogether 90 to 100 feet or more. 



No. 25 (4), 300 feet. Covered space, probably occupied by clays, 

 but showing some sandstone that may or may not be in place; perhaps 

 room enough for 250 to 300 feet. 



No. 26 (3), 90 feet. Soft light grayish sandstone, nearly vertical. 



SUMMIT OF SULPHUR CEEEK GROUP. 



No. 27 (2), 100 feet. Slope apparently occupied by clays, thickness 

 " perhaps 100 feet or more. 



No. 28 (1). Black shale, only seen in bottom of Sulphur Creek, thick- 

 ness unknown. 



BLACK BUTTE QUARTZITE. 



Southwest of Black Butte Station on the Union Pacific Railroad there 

 is a conspicuous topographic feature known as Black Butte. It is carved 

 from beds of the Point of Rocks Group and is crowned with a dark indurated 

 and exceedingly tough quartzite, which, in the distance, has the appearance 

 of a bed of extra vasated material, and even on closer examination I was 

 deceived by its apparent similarity to some rhyolites. 



Rocks having a similar appearance and structure are found on the sum- 

 mit of Aspen Mountain, and I unhesitatingly considered the latter to be 

 quartzites; but, on visiting Black Butte, the general weathering and exceed- 

 ingly dark appearance of these beds created the impression that they had 

 been extravasated, though hand specimens had the appearance of quartzite, 

 and I left the field with many doubts as to their nature. Captain Dutton 

 has since made sections of this rock and examined them under the micro- 

 scope and pronounces it a quartzite. 



Such a conclusion in connection with the geological relations of these 

 beds is very interesting Quartzites, from geological considerations of 

 structure and erosion, are usually supposed to have been at some time deep 

 seated and are often shown to have been involved in profound plication, or 



