34 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



been formed prior to the dynamic ruoveaients, and that they are therefore 

 older than the Mosquito Range itself. Tiiese deposits were formed by the 

 action of percolating- AA'atcrs, which, having taken up certain ore materials 

 in their passage through neighboring- rocks, deposited them in a more con- 

 centrated form in their present position. This process may have taken place 

 Avhile the sedimentary beds were still covered by the waters of the ocean, 

 and the waters therefore have been derived from it; whether this was actu- 

 ally the case or not cannot be known until the age of the eruptive rocks is 

 more exactly determined. However, as it is already known by the estua- 

 rine character of its fauna that the latest Cretaceous formation must have 

 been deposited in an already shallowing ocean, it seems probable that the 

 area occupied b}' the Mosquito Range may have ah-ead}' emerged from the 

 ocean at this time. 



Structural results of the dynamic movements. Before proceeding tO a detailed 



geological descri|)tion of the region included in the Mosquito map (Atlas 

 Sheets VI and VII), which represents the results of the dynamic movements 

 and of subsequent erosion, it may be well to give a brief summary thei-eof, 

 thus reversing the natural order, for the benefit of those readers who may 

 not have time or inclination to follow all the details of Chapter IV. 



The average or major strike of the sedimentary beds and of the a.xes of 

 the principal folds is northwest magnetic, or N. 30° W., but in some cases a 

 strike due north and south is observed. In the.se two directions are seen 

 the influence of the shore lines of the Sawatch ishmd, against which the 

 sedimentary strata were compressed; for, while this area lies mainly along 

 the eastern .sliore line which has a north and south direction, in the north- 

 ern part the beds had already commenced to sweep round to the westwai'd 

 along the northern shore line of the island. To the south of this area the 

 crest of the Mosquito Range itself marks the eastern limit of Paleozoic 

 beds, while from South Peak, near Weston's pass, northward this limit bends 

 to the northwest toward the mouth of the east fork of the Arkansas. 

 Beyond this line to the west everything is Archean; to the east of it Archean 

 exposures are found onlv where denudation has removed their previous cov- 

 ering of Paleozoic and later beds; it may be assumed, therefore, to repre- 

 sent approximately the original shore line of the Paleozoic ocean. 



