270 W. Gross — Denver Tertiary Formation. 



agencies and upon the rate and conditions of deposition. 

 Abrasion and attrition or conditions favoring chemical decom- 

 position will destroy the accompanying minerals more rapidly 

 than the quartz and will hence tend to increase the proportion 

 of the latter in resulting sandstones. 



These generalizations are illustrated in all the groups of sed- 

 imentary formations known at the eastern base of the Moun- 

 tains, from the Cambrian up to the Denver beds. The fine 

 grained Dakota and Laramie sandstones are almost exclusively 

 made up of quartz. In the Willow Creek grits and sandstones 

 appears a variety of materials mentioned by Mr. Eldridge in 

 the statement already given. 



In view of the facts just considered the sudden and almost 

 complete change in constitution which is met with in the 

 Denver beds is certainly worth more than a passing notice. 

 Instead of minerals and rocks derived either directly or indi- 

 rectly from Archaean sources there is found material resulting 

 from the degradation of a great series of eruptive rocks. It 

 is a fact of much significance too, that for 900 feet of thick- 

 ness the Denver beds are very fine grained, having been slowly 

 deposited under conditions which have usually brought quartz 

 into relative prominence. 



The andesites represented by pebbles and bowlders in the 

 Denver strata are of many different types and the name must 

 be used in its widest sense to cover them. Hornblende- or 

 augite-andesites of rather basic composition are the most com- 

 mon types, but more acid varieties, carrying quartz or tridy- 

 mite, with biotite, are numerous, while at the opposite extreme 

 are hypersthene-bearing andesites of characteristic features. 

 Only andesites have thus far been identified. 



There is a variation in texture and structure shown by these 

 andesites which is almost as marked as that in composition : — 

 some are vesicular, many are porphyritic, and others are com- 

 pact and fine-grained. In the development and in the mutual 

 relations of the mineral constituents of these andesites one ac- 

 customed to studying the microscopical physiography of erup- 

 tive rocks will see abundant proof of their extrusive character. 

 Some of the denser beds of Table Mountain seem composed of 

 volcanic ashes or of small angular grains belonging to a single 

 rock type. Such material may be called tufa, but the sand- 

 stones containing worn particles of various kinds are largely 

 predominant. 



It has already been stated that there is some quartz in the 

 sandy strata of the plains and that great bowlders of Archaean 

 and of sedimentary rocks are found in the upper division of the 

 Denver beds. 



The sources of materials. — The nature of the rock and min- 



