CROP? 



DENVER. 437 



is the light pinkish rhyolitie toff intercalated in the Tertiary strata 

 south of Denver. The fine flagstones used in the sidewalks of Denver 



come from the red beds of the Trias or from the Dakota Cretaceous. 



The geological formation immediately underlying and surrounding 

 the city is the Denver beds, 7:l a fresh-water lake deposit whose sand- 

 stones and conglomerates are characteristically, and in the lower 

 portion almost exclusively, made up of volcanic rocks, representing 

 many varieties of andesite. The Monument Creek beds rest uncon- 

 formable upon the Denver strata in the highlands to the southeast. 



Below the Denver beds occurs another freshwater lake deposit, the 

 Arapahoe beds, whose most prominent member is a conglomerate free 

 from volcanic materials, but containing pebbles of sedimentary rocks 

 recognized as belonging to various horizons from the Laramie down to 

 the red sandstones of the Trias. Below the Arapahoe beds come the 

 normal Laramie Cretaceous clays and coal-bearing sandstones. Uncon- 

 formities of deposition occur between the Laramie and Arapahoe, and 

 between the latter and the Denver beds. 81 



Both the Arapahoe and the Denver beds contain numerous fragments 

 of I> i nosaurian remains, the majority belonging to the recently dis- 

 covered family of the Ceratopsidce. The fossil flora of the Denver 

 beds is very rich, but paleobotanies have not as yet differentiated it 

 from that of the coal-bearing Laramie of this region. The molluscan 

 fauna is small, but also shows a close relationship between the Denver 

 and the Laramie. For these reasons paleontologists have held that 

 the two lake deposits in question belong to the Laramie Cretaceous, 

 but the stratigrapher finds, in the evidence of enormous erosion preced- 

 ing the Arapahoe epoch and of a long period of eruptive activity before 

 the DenvCr epoch, grounds for holding that these formations should he 

 distinguished from the Laramie proper, and that they are either of 

 earliest Eocene age or of a Cretaceous period succeeding the Laramie. 



The Denver strata are exposed in the hanks of the Platte river, ami 

 in many of its smaller tributaries; bat a good idea of the characteris 

 tics of the series can only be obtained by examining the sections shown 

 on the line of folding near the mountains. The rusty brown sand- 

 stones appearing on the plains are often indurated by a large amount 

 of zeolitic cement, and the soils resulting from the decomposition of 

 the eruptive materials are quite fertile. 



A few years ago artesian water of remarkable purity was struck in 

 Denver while boring for coal. The water was found to come from sev- 

 eral horizons between 175 feet and 1,200 feet in depth, mainly in the 

 Arapahoe beds. As the supply basin was a small one the large num- 

 ber of wells which were sunk soon decreased the pressure, and to-day 

 Dearly all the wells in use are pumping wells. 



The formations constituting the plains about Denver are chiefly a 



