54 GEOLOGY OF THE DENVER BASIN. 



sandstone is heavily cross-bedded, and in sonic layers conglomeritic, peb- 

 bles and matrix consisting chiefly of quartz, with occasional admixtures 

 of other ddbris from the crystalline rocks, and some brown arenaceous or 

 cherty limestone. 



The upper half of the sandstone is also conglomeritic at the base, but 

 becomes fine-grained above. It consists of quartz of great purity, and 

 affords nearly the entire amount of silica used in the pottery and fire-brick 

 establishments a1 Golden. 'Twenty feet from the summit are numerous 

 small, brown or reddish nodules of cemented sand one-eighth to one inch 

 Or more in diameter, many of which first weather in relief and then roll 

 out in halls: some of the larger, on being broken, are found hollow. The 

 upper 6 feet of the Creamy sandstone locally becomes conglomeritic and 

 calcareous, easily disintegrating and leaving a surface strewn with pebbles, 

 all more or less angular. In this layer small geodes, lined with calcite 

 crystals, also occur. The Creamy sandstone as a whole is remarkably 

 uniform in texture and appearance. In the region of the South Boulder 



Teaks, however, where disturbed by faults Or tolds, the lied becomes a, 



tolerably hard quartzite, fractured and slickensided, but its leading features 

 are still maintained and its identity is easily established. 



The upper division. The loWCl' half of this division COUs'lsTS of bright, 



brick-red, arenaceous shales and sandstones, with important intercalations 

 of limestone. The limestones occur within 75 feet of the base, usually 

 three or four beds from 6 to 18 inches thick in the lower 15 feet, and 50 

 feet higher up a bed 5 feet thick, a red. sandy shale intervening. The 

 upper bed is overlain by a succession of thin, wafer-like layers of white 

 limestone and red mud, in all ."> or 10 feet; these present in crOSS-section 

 a wavy structure, with sharp contrast of color and texture, the surface 

 weathering in delicate corrugations. Close examination occasionally 

 reveals this structure in the limestone itself, the clay bands, however, 

 being absent. Chert concretions, of purple color, which weather in con- 

 centric circles and ultimately develop circular holes in the backs of the 

 layers, are present, a- are also vugs filled with calcite crystals. Minute 

 grains of an undetermined black mineral also occur quite generally at this 

 horizon. 



