150 ROLLIN T. CHAM BERLIN 



The differentiation of Colorado's mountains into the Rocky 

 System proper and the subordinate scattered groups mostly associ- 

 ated with Tertiary vulcanism is apparent on either the topographic 

 or the geologic map of the state.^ The Rocky Mountain Y has 

 a pre- Cambrian granite backbone throughout nearly the whole of 

 its extent. In fact, the pre- Cambrian areas of Colorado, except 

 for a large area of igneous and metamorphic rocks south of the 

 Gunnison River, delineate very well the pattern of the Rocky 

 Mountains. The southward converging of these folded ranges of 

 old rocks, which taper into a single mountain ridge, appeared 

 significant and possibly suggestive of the type of deformation 

 which has taken place. It seemed to imply some sort of wedge 

 dynamics below. 



HISTORY OF THE MAIN RANGE 



The present Rockies are not the first mountain system developed 

 within the area of Colorado. After a flooding of the region by 

 early Pennsylvanian seas there arose, before the close of the 

 Pennsylvanian period, an ancestral Rocky Mountain range. - 

 The elevation of these ancient Rockies has been assigned by Lee 

 ''to the period of diastrophism usually called the Appalachian 

 Revolution which wrought world-wide changes in climate, 

 geography, and biology. "•^ But Lee recognizes that these ancient 

 Rockies were probably upHfted before the Permian, because 

 sediments derived from the erosion of this ancestral range have 

 formed the Permian beds of the neighborhood. Still further, if 

 the present tendency to place the Fountain sandstone and its 

 equivalents within the Pennsylvanian'' be correct, this growth of 

 early Rockies must have taken place during the Pennsylvanian, 

 as that period is now defined, since the sediments which made the 

 Fountain sandstone came from the destruction of the newly 



' The Topographic Map of Colorado. R. D. George, 1913. 



^ Wilhs T. Lee, "Early Mesozoic Physiography of the Southern Rocky Moun- 

 tains," Smithsonian Misc. Coll., LXIX, No. 4 (1918), pp. 5-7. 



3 Ibid., p. 6. 



■t R. M. Butters, "Permian or ' Permo-Carbonif erous ' of the Eastern Foothills 

 of the Rocky Mountains in Colorado," Colo. Geol. Survey, Bull. 5 (1913), Part 2, 

 pp. 65-94. 



