EECAPITULATION OF PALEOZOIC FORMATIONS. 147 



thickness eastward, and its change of character along the Front Range. Its 

 thickness in the Crested Butte quadrangle is from 50 to 350 feet; near Aspen 350 

 to 400 feet in the south and 200 feet or less in the north. On Eagle River it is 604 

 feet (Peale); at Red Cliff 235 feet (Tilden); in the Tenmile district 160 to 200 feet; 

 in the Leadville district 150 to 200 feet. On the east side of the Mosquito Range 

 it is only 40 feet; in Manitou Park, at different points, 40, 86, and 100 feet (Peale); 

 in Perry Park 105 feet (Peale); at Colorado Springs 50 feet (Peale); in the Pikes 

 Peak quadrangle, too thin to be mapped; at Canyon (and all but a few places along 

 the Colorado and Sangre de Cristo ranges) absent; near San Luis 160+ feet (Van 

 Diest) ; in the San Juan region to 200 feet. It seems to be generally present over 

 the Grand River, Elk Mountain, and South Park, and in the northern end of the 

 Sangre de Cristo areas. 



The Cambrian of the Front Range for the most part seems to consist of reddish 

 sandstones more or less comparable to the Black Hills Cambrian and to the upper 

 part of the Sawatch quartzite which rests iipon white quartzite in the Crested Butte 

 district and elsewhere. If the red sandstones of the Front Range do prove to be 

 the same as the reddish part of the Sawatch considerable evidence of an unconfor- 

 mity between its two members seems to be furnished by the fact that the red 

 member is lacking in some areas (e. g., on Eagle River, at Red Cliff, and locally 

 at Crested Butte), and the white quartzite is lacking in others (along the Front 

 Range). 



ORDOVICIAN. 



There is no evidence of the presence of Upper Silurian strata in Colorado. 

 The only Silurian known is that of the Ordovician, though man}^ of the older surveys 

 employed this tei-m to include the Cambrian also. In central Colorado this is a lime- 

 stone formation, and has received the name of the Yule limestone. The Silurian is 

 not a formation of great thickness anywhere in the State. Its outcrop is linear and 

 generally follows that of the Cambrian Sawatch quartzite. 



In the Crested Butte quadrangle, its typical area, the Yule limestone is assigned 

 an aggregate thickness of 350 to 450 feet. Three subdivisions are recognized, the 

 lower of which consists of quartzite, the middle of limestone, and the upper of marly 

 variegated beds. The quartzite, 75 to 100 feet thick, is generally white, often cal- 

 careous, and contains indistinct fossil remains. The limestones of the middle division 

 are 250 to 280 feet thick, light colored, thin bedded, frequentlj' siliceous, espe- 

 cially at the base, and carry grayish-white cherts. They contain characteristic 

 fossils, among which ma}^ be mentioned the fish scales abundantly found at this 

 horizon near Canyon. The upper division is 60 to 90 feet thick, and its horizon is 

 said to be wonderfully persistent in retaining the same lithologic characters. 



In the Aspen district, to the Silurian are referred some pure dolomites of light 

 graj'-blue color, hard and compact, with a fine frost}' luster. They are also some- 



