CHAPTER II. 



GENERAL GEOLOGY OF THE MOSQUITO RANGE. 



ROCKY MOUNTAINS IN COLOKAUO. 



The simplest expression of the geological structure of the Rocky 

 Mountains in Colorado is that of two approximately parallel uplifts or series 

 of ridges of Archean I'ocks, upon whose flanks rest at varying angles a 

 conformable series of sedimentaiy formations extending in age from the 

 earliest Cambrian to the latest Cretaceous epochs, the latter being locally 

 overlaid by unconformable Tertiary beds. 



The eastern uplift is generally known as the Colorado or Front Range 

 and the western as the Park Range, the series of depressions or mountain 

 valleys between them having received the name of parks. 



The most prominent fact thus far recognized in the geological history 

 of this region is that a great physical break or non-couformity in the strata 

 is found between the Cretaceous and Tertiary formations; in other words,, 

 that at this ])eriod occurred the great dynamic movement which uplifted 

 the Rocky Mountain region essentially into its present position. As the 

 beds of the Paleozoic and Musozoic systems have been thus far found to 

 be practically conformable throughout the region, it may be assumed that 

 no important dynamic movement took place during these eras, and that 

 deposition went on continuously, except when continental elevations of the 

 whole region may have caused a temporary recession of the waters of the 

 ocean for a limited period, and thus produced a gap or gaps in the geolog- 

 ical series without causing any variation in angle of deposition in the at 

 present successive beds. 



