352 THE CAMBRIAN. [bull. 81. 



into the Silurian limestone. 1 He describes the strata in the Mosquito 

 range referred provisionally to the Cambrian as follows : 



The beds assigned provisorily to this horizon * * * are prevailingly of 

 quartzite. To them, therefore, the local name of Lower Quartzite has been given. 

 There average thickness is about 150 feet to 200 feet, of which the lower 100 feet are 

 composed of finely and rather thinly bedded white saccharoidalquartzites, while the 

 upper 50 feet are shaly in character and more or less argillaceous and calcareous, 

 passing by almost imperceptible transition into the siliceous limestone of the Silurian 

 formation above. 



At the very base of the series, at the contact with the underlying Archean, wherever 

 this could be observed, is found a persistent bed of fiue grained conglomerate, from 

 a few inches to a foot in thickness, made up of rounded and finely polished grains of 

 bluish translucent quartz, generally not larger than a pea in size. Above this is a 

 white quartzite of remarkably uniform and persistent character, always very readily 

 distinguishable as a white band in the numerous sections offered by the canon walls 

 of the range. Its thickness, when measured on the west side of theraDge or near the 

 Sa watch Island, is, as mentioned above, 100 feet of purely siliceous beds. On the east 

 side of the range the thickness seems somewhat to diminish, and in places was found 

 to be only 40 feet. * * * 



Owing to their similar lithological character and to the general absence of fossil 

 evidence it is difficult to establish a hard and fast line between this and the succeed- 

 ing formation above. In practice the line has been drawn at the top of the shaly 

 beds, and the commencement of the beds of more massive limestone. The transition 

 beds consist essentially of alternating bands of calcareous quartzite and shales. The 

 name sandy limestones is often applied to them for the reason that on weathered 

 surfaces of the cliff faces they appear like sandstones, the carbonate of lime having 

 been entirely washed out, and only the fine quartz grains left on the thin surface 

 crust.' 2 



As already stated, the only fossil remains found in this series occur 

 in a bed of greenish chloritic shales on the east flank of Quandary Peak, 

 about a mile above the Monte Cristo mine. They belong to the genus 

 Dikelocephalus, and closely resemble Dikelocephalus minnesotensis of 

 the Potsdam formation of Wisconsin. This band of shales was not 

 found in stratigraphic relation to the quartzite of the limestone of the 

 sections already mentioned, but it occurs above a heavy white quartz- 

 ite and beneath a bed of white marbleized limestone; and, from aual- 

 ogy with other sections, Mr. Emmons thinks it safe to assume that it 

 occurs above the main body of quartzite and near the base of the trans- 

 ition series. 3 



For the purpose of comparison, a section was measured by Mr. Whit-, 



man Cross, in William's Canon, near Manitou Springs. Of this section 



Mr. Emmons says : 



The purely siliceous beds at the base are much thinner than in the Mosquito Range,- 

 the greatest thickness found being 50 feet. They are succeeded by calcareous sand- 

 stones and shales of variegated colors, red prevailing, which pass up into white or 

 drab limestones, sometimes containing chert secretions and alternating with shaly 

 beds, with an aggregate thickness of about 200 feet. These beds may be considered as 

 the equivalents of the lower quartzite and white limestone of the Mosquito Range. 4 



1 Geology and mining industry of Leadville, Colorado, U. S, Geol, Surv., Monograph, vol. 12, 1886, 

 p. 60. 

 *0p.cit.,pp.58,59. »Op.cit.,p,60. ♦0p.cit.,p.62, 



