MINERAL DEPOSITION. 33 



of contraction in a north and soutli direction, whose effects can now be seen 

 along the eastern foot-hills in gentle lateral folds, their axes ai)])roxiniately 

 at right angles to the ti-end of the range, and whose presence is indicated 

 by a sudden bend or curve in the line of sedimentary outcrop, where at one 

 point, owing to a local synclinal, the beds have been more or less preserved 

 from erosion, and again where, owing to the crossing or coincidence of 

 ci'ests of the folds, like those of waves crossing each other, is found an 

 otherwise unexplainable steepening in the dip of the strata. 



It must be borne in mind that, while this great dynamic movement is 

 defined as occupying a certain lapse of geological time and its principal 

 effects were brought about within that time, it is not to be regarded as a 

 sudden convulsion, like that of an earthquake, though such disturbances 

 may have occasionally occurred. On the contrary, it must be conceived 

 to have been rather a slow and gradual movement, extending over a period 

 of time of which human experience can form no adequate conception. 

 Moreover, as will be shown in the detailed study of the region, it can be 

 proved that in a modified degree this movement has been continued into so 

 recent a period as that following the Glacial epoch, and may very probably 

 be going on at the present day, although, owing to the great area involved, 

 it has been impossible to obtain any demonstrable proof of its actual exist- 

 ence. 



Mineral deposition. — It was during the period which intervened between the 

 intrusion of the eruptive rocks and the dynamic movements which uplifted 

 the Mosquito Range that the original deposition of metallic minerals in the 

 Leadville region took place. These original deposits were probably in the 

 form of metallic sulphides, though as now found tliey are largely oxidized 

 compounds, and therefore the result of a secondary chemical action; although 

 during this secondary action they may have been to a slight degree removed 

 from their original position, their relation as a whole to the inclosing rocks 

 nuist remain essentially the saiiie. 'I'heir manner of occurrence and the 

 probability that they were derived, in great part at least, from the eruptive 

 rocks themselves prove that they must be of later formation than the latter, 

 while the fact that they have been folded and faulted together with the 

 inclosing rocks, both eruptive and sedimentary, shows that they must have 



MON XII 3 



