718 



SCIENCE. 



[N. S. Vol. II. No. 48. 



worked between Opliir and Auburn. Small 

 veins are occasionally worked near Clarks- 

 ville and in the vicinity of Pilot Hill. 



The central mass of granodiorite affords 

 excellent building stone. Limestones occur 

 chiefly as lenses in amphibolite at many 

 places along the eastern border. The soils 

 of the foothill region are residuarj^ in char- 

 acter, while the western part of the tract is 

 occupied by deep alluvial and sedimentary 

 soils. 



FOLIO 7, pike's peak, MONTANA, 1894. 



This folio consists of 4|^ pages of text, 

 signed by Whitman Cross, geologist, a topo- 

 graphic sheet (scale 1:125,000), a sheet of 

 areal geology, one of economic geologj% and 

 one of structure sections, followed by a 

 special description of the Cripple Creek 

 mining district, consisting of 1 page of text 

 on the mining geology by E. A. F. Penrose, 

 Jr., and a map (scale 1:25,000) showing the 

 economic geology of the district. 



Geography. — The district embraces an 

 area of 931 .5 square miles between the 

 meridians 105° and 105° 30' and parallels 

 38° 30' and 39°. In the eastern half of the 

 district lies the crest of the granitic Colo- 

 rado Eange, which extends from Manitou 

 Park through Pike's Peak to the southern 

 end of the range, where it sinks to the level 

 of the plains. The western portion of the 

 area is a plateau, of granite and volcanic 

 rocks, lying between 8,000 and 10,000 feet 

 in elevation, penetrated on the south by 

 deep canyons of streams tributary to the 

 Arkansas River and by the recess or bay of 

 Garden Park, nearly at the level of the 

 plains. The principal drainage of the dis- 

 trict is by tributaries of the Arkansas 

 River, which flows through the Royal Gorge 

 just beyond the southern boundary. The 

 remaining drainage is into the Platte River, 

 which cuts across the northwestern corner 

 of the area in a deep canyon. 



The Colorado Midland Railroad traverses 



the district from east to west near its 

 northern boundary. East of the center of 

 the area is the mining district of Cripple 

 Creek, reached by branch railroads from the 

 north and south. 



General Geology. — The granites of the 

 mountain and plateau regions are reddish 

 in color, coarse or fine grained, and similar 

 to those of many other regions in Colorado. 

 Of special interest is the observation, first 

 made by the survey corps, that these 

 granites contain many large and small frag- 

 ments of metamorphosed stratified rocks, 

 quartzites and schists belonging to the oldest 

 series of sedimentary beds, the Algonkian, 

 and hence the granites are not of Archean 

 age, as has previously been assumed. Most, 

 if not all, of the gneisses in this district have 

 been formed from the granites by a shearing 

 strain, as is very clearly demonstrated in 

 many places. 



The sedimentary formations of the area 

 and their characteristics of special interest 

 may be concisely referred to as follows : 



Algonkian. Nearly 4,000 feet of white 

 quartzite, in small part congiomeritic, is 

 shown in the huge inclusion in granite in 

 Wilson Park. These ancient strata are not 

 known in this region except as inclusions . 



Silurian. Three divisions of the Silurian 

 strata, each about 1 00 feet thick, have been 

 recognized in Garden Park, and named re- 

 spectively the Manitou limestone, Harding 

 sandstone and Fremont limestone. The 

 Harding sandstone contains the oldest fos- 

 sil fishes as yet known. Minor unconform- 

 ities sepai-ate these formations, and they are 

 not known in so good development else- 

 where. 



Carboniferous. Resting on the Silurian is 

 a thin limestone, called the Millsap, carry- 

 ing a few Carboniferous shells, and known 

 only in small remnants. The red sand- 

 stones and grits of Manitou and Garden 

 Parks, 1,000 feet in thickness, are consid- 

 ered as of Carboniferous age and named 



