Ball — Pre- Cambrian Mocks of Georgetown, Col. 37 3 



sandstone occur on the North Fork of the South Platte one 

 mile above Shawnee and at Pine Post Office on the same stream, 

 both localities being south of the Georgetown quadrangle. 

 The sandstone is lithologically like certain facies of the lower 

 Wyoming of the foothills of the Front Range. It is possible 

 that in Mesozoic time the quadrangle or a portion of it was 

 submerged beneath the sea. In comparatively late, probably 

 Tertiary time, dikes, sheets and stocks of siliceous and inter- 

 mediate igneous rocks intrude the pre-Cambrian complex. 

 Within the quadrangle there is no evidence that these ever 

 reached the surface and formed flows, although they may have 

 done so. Certain of these rocks are somewhat like some of 

 the andesitic pebbles in the Denver formation* of the Denver 

 Basin and others are somewhat similar to dikes which cut the 

 Lower Wyoming formation near Boulder, Colorado. The 

 evolution of the land surface and the two periods of Pleistocene 

 alpine glaciatiou have already been mentioned. 



The stratigraphic succession of the formations of the George- 

 town quadrangle from the top down follows : — 



Recent. — Alluvium 



Alluvial fan deposits 

 Landslides 

 Talus 



Travertine. 

 Pleistocene. — High Basin debris sheets (sheets of rock debris at 

 the heads of non-glaciated streams ; indicating pre-glacial 

 downcutting and late glacial filling). 



Later glacial deposits, including lateral, terminal and 

 ground morainal deposits and overwash gravels. 



Gravels of terrace 25 feet above present stream channels. 

 Gravels of terrace 55 feet above present stream channels 

 (possibly pre-Glacial). 



Earlier glacial deposits, lateral and ground morainal 

 deposits, and terminal morainal deposits in the adjoining- 

 Central City (Colo.) quadrangle to the north. 



Gravels of terrace 180 feet above present stream channels. 

 Tertiary. — ? Intrusive igneous rocks of widely varying char- 

 acter in dikes, sheets and stocks. 

 Mesozoic. — ? Sandstone residuals. 



Pre- Cambrian. — Pegmatites and contemporaneous granite and 

 granite-porphyry. 



Silver Plume granite. 

 Rosalie granite. 



Quartz-monzonite. ) t-, i t ^ 



/{ . ■> • -,- ., y .Probably contemporaneous. 



(Quartz-bearing diorite. J J r 



Gneissoid granite. 



Quartz monzonite gneiss. 



Idaho Springs formation (biotite-sillimanite-schists, biotite- 



schist and quartz-gneiss with lenses of silicate rocks). 



* Cross. W., U. S. Geol. Survey. Monograph XXVII, pp. 315-6. 



