24 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



culminating points are the San Juan Mountains, may very likely conceal 

 the remains of a former land mass of equal, if not greater, dimensions than 

 this. The present Archean areas to the south, in the Wet Mountain and 

 Sangre de Cristo Ranges, may also, in part at least, have been land masses 

 at those times. Moreover, the not infrequent occurrence of Cretaceous 

 beds lying directly upon the Archean at points far away from any well- 

 defined ancient shore line, suggest elevations and subsidences of which the 

 geological studies thus far made in Colorado furnish no record. The areas 

 already mentioned were, however, the most important elevations, since they 

 are the only ones of which it may now be said with tolerable certainty that 

 they have been permanent land surfaces through the long cycles that have 

 elapsed since the commencement of the Paleozoic era. Their considera- 

 tion, therefore, is all that is necessary for the purposes of the present study. 



Mountain structure. It is no longer assumed, as it was in the early days 

 of geology, that the elevation of mountains is the result of a vertically 

 acting force or a direct upthrust from below. On the contrary, the gen- 

 erally received contraction theory, which is the one that best accords with 

 all observed facts of geological structure, supposes that it is horizontally 

 acting forces that have uplifted them. According to this theory, during 

 the secular cooling of the earth from a molten mass, a solid crust was first 

 formed on its exterior. As cooling and consequent contraction of the whole 

 mass went on, this first-formed crust, in order to adapt itself to the reduced 

 volume of its nucleus, also contracted ; but, as it was more or less rigid, this 

 contraction resulted in the formation of wrinkles or ridges on its surface, 

 which there is considerable evidence to show occupied essentially the same 

 lines that the present mountain systems of the world do. Whatever the 

 determining cause that originally fixed these lines, the earth's crust along 

 them would have been compressed, plicated, and probably fractured, and, 

 in subsequent dynamic movements resulting from continued contraction, 

 they would have constituted lines of weakness along which the effects of 

 these movements would have found most ready expression. 



Whether the consolidation of the entire earth- mass is already com- 

 pleted, or whether there still remains a molten nucleus towards its centre, 

 is a purely speculative question, upon which geologists are not yet in entire 



