176 GEOLOGY OF THE DENVEB BASIN. 



Division d, 525 feet. — This stands in contrast to lower divisions both in 

 structure, being composed of coarse conglomerate beds, and in material, 

 through the appearance and general increase in importance of Archean 

 debris with the eruptives. 



The actual base of 1) i> not shown li\ the outcrops, although it is 

 approximately indicated by the exposures in the small ravine just south of 

 the section. It is in all probability a sharp line. The series begins with a 

 conglomerate which is less compacl and firm than that of 15. At the base 

 the pebbles are not very large, hut they vary in character, showing for the 

 first time a decided admixture of Archean with the andesite. There are 

 also distinct clay bowlders mingled with the others in these lower horizons. 

 Above the lirst 25 feet of conglomerate there comes a gap which evidently 

 represents a return to clay or tine, sandy deposits. Above this gap 



come still coarser conglomerates with a greater admixture of Archean than 



before. Bowlders here vary from 1 to 2 feet in diameter for the large 

 ones, and from that limit downward. The bowlders he in a matrix of 

 coarse, crumbling sand in which angular Archean particles are predominant. 

 Cohering outcrops are naturally rare, and smooth, steep slopes covered 

 with round pebbles and bowlders are usually presented, but a few strokes 

 of the pick reveal beneath the surface a mass of varying-sized bowlders 

 embedded in crumbling gravel. The relative proportion of Archean 

 and eruptive materials in these strata is hard to determine without close 

 scrutiny. The andesites are much decomposed as a rule, and on exposed 

 surfaces crumble away quickly, while the Archean remains comparatively 

 fresh, and is thus more prominent. In the lower 100 feet of D the 

 eruptive material is nearl\ equal to the Archean in quantity. Large 

 bowlders increase in number upward in the series. 



At about 100 feet above the base of 1> there appears on the line of 

 this section a stratum with an easterly dip of 18 which contains so much 

 iron oxide as its cement that it is hard and forms a projecting outcrop. At 

 about the same horizon, too, there is a decided gain in the amount of 

 Archean relative to the eruptive bowlders. The iron is noticeable through 

 about lo feet of strata, though present in large quantities only in the 

 stratum at the base, about 1 foot in thickness. 



