SO GEOLLGY AND MINING INDUSTRY OF LEADVILLE. 



The evidence furnished by the deposits of this lake affords an interest- 

 ing confirmation of the deduction already made by geologists from the study 

 of the glacial drift in Europe and in the Eastern States, arid by Messrs. 

 King and Gilbert from their study of the lake deposits of the Basin regions 

 of Utah and Nevada; namely, that the Glacial period presented two maxima 

 of- cold, with an intervening warmer period during which the ice was 

 partially melted and vegetation flourished. The general character of the 

 stratified deposits of the Arkansas Lake shows that they must have been 

 carried down during a time of great floods and that they are formed largely 

 of rearranged moraine material. The thickness of these deposits proves the 

 the existence during a long period of a lake which during part of the year 

 was not frozen ; their position shows that the shores of the lake extended 

 several miles to the eastward of the Arkansas Valley. Finally, the facts 

 that these beds are deeply buried beneath surface accumulation of detrital 

 material and that the moraines of now extinct glaciers extend out beyond 

 the original shore-line of the lake and rest above its beds, prove that subse- 

 quent to the draining of the lake another set of glaciers, formed during a 

 later period of cold, covered the slopes of these mountains and carved out 

 to a greater depth the present valleys. 



Geological history. Although now so prominent a feature in the topogra- 

 phy of the Rocky Mountains, the Mosquito Range, from the sources of the 

 Arkansas River to the southern end of the main Arkansas \ 7 alley, is geolog- 

 ically a part of the Sawatch uplift. It was from the abrasion of the land 

 surfaces exposed in the Archean island which occupied the present position 

 of the Sawatch range that the sediments which constitute its stratified beds 

 were doubtless in a great measure formed. In the seas that surrounded this 

 island during Paleozoic and Mesozoic times was deposited a conformable 

 and, as far as present evidence shows, an almost continuous series of coarse 

 sandstones and conglomerates, alternating with dolomitic limestones and 

 calcareous and argillaceous shales. The geology of the Rocky Mountains 

 has not yet been studied in detail over a sufficiently extended area to afford 

 data for tracing the history of the elevations and subsidences to which the 

 region as a whole may have been subjected, or of the alternate recessions and 

 advances of ocean waters during this long lapse of time. The examination 



