266 W. Cross — Denver Tertiary Formation. 



of the Denver beds was first observed in the dark conglomer- 

 ates of Table Mountain, which contain only pebbles of ande- 

 sitic rocks, some porous and some compact. The sandy matrix 

 is merely finer material of the same character and the cement- 

 ing substance is usually some zeolite (heulandite, stilbite, chab- 

 azite, etc ) or yellow calcite. Microscopical examination of 

 the sands and of the sand grains in the clays reveals only par- 

 ticles which, so far as they can be identified, are like the con- 

 stituents of the andesites seen in pebbles. Augite grains or 

 rough crystals containing glass-inclusions and other character- 

 istic interpositions ; plagioelase, hornblende and biotite, with 

 the peculiarities noticed in these minerals as components of the 

 andesites ; dull reddish brown grains with imbedded crystals, 

 which clearly represent particles of the andesite rocks more 

 or less decomposed, — all these are found, the only recognizable 

 mineral constituents of Table Mountain sandstones and clays-. 



At the base of the main Green Mountain exposure which 

 has been mentioned is a dark conglomerate, about 25 feet in 

 thickness, dipping 45° eastward. This bed is probably at the 

 horizon — also exhibiting a marked conglomerate — a few feet 

 below the basalt cap of Table Mountain. While tracing out 

 and studying this horizon in Green Mountain three Archaean peb- 

 bles were seen, — all others being eruptive and apparently all 

 andesites. The average diameter of the j)ebbles in this bed is 

 from two to three inches. 



Above this conglomerate appear again fine grained sands and 

 clays in an estimated thickness of 285 feet, in which there is 

 no 'strongly developed pebble bed, though sand layers greatly 

 predominate over clays. This series is like the lower one f 

 almost exclusively made up of andesitic debris. 



The fine-grained sediments are abruptly succeeded by a 

 series of very coarse conglomerates or bowlder beds of an esti- 

 mated thickness of 525 feet. In the first of these coarser beds 

 the pebbles range in size from a diameter of two feet down- 

 ward, and while eruptive rocks largely predominate there are 

 many of granite or of gneiss and a few of red and white sand- 

 stone. The gravelly matrix is largely made up of angular 

 quartz and feldspar grains. A return to fine-grained sediments 

 is quickly followed by bowlder beds which continue to the top 

 of Green Mountain, the average size of the bowlders gradually 

 increasing upward in the series while the • amount of eruptive 

 material rapidly decreases and becomes at last quite subordin- 

 ate. Bowlders of various sedimentary rocks are numerous in 

 these beds, the characteristic Dakota conglomerate being per- 

 haps most prominent among them. 



The Denver strata of the plains belong to horizons repre- 

 sented in Table Mountain, i. e., to the lower third of the For- 



