302 GEOLOGY AXD MINING IKDUSTET OF LEADVILLE. 



have required about the same amount of force to bring the earlier intrusions 

 to their place of consolidation, 10,000 feet below the surface, that it did to 

 bring the later flows to the surface after the 10,000 feet of superincumbent 

 strata had been removed by erosion. 



In regard to the third alternative, it seems that a part at least of the 

 reason for the stoppage of the intrusive magma at its present position may 

 be found in the resistance to its passage offered by the sedimentary strata. 

 If it were a question only of the porphyry sheets above the Blue Limestone, 

 it might be assumed that the Weber Grits offered some special conditions 

 of impenetrability; but, in point of fact, although intrusive sheets are almost 

 always found at this horizon in the Mosquito region, they also occur at 

 many other horizons, both above and below. While it cannot be main- 

 tained, therefore, that any particular bed offered special resistance to the 

 passage of the fused mass, it is not only evident a priori, but supported by 

 observations of many of the transverse sheets and dike-like bodies, that a 

 continuous and unbroken horizontal rock stratum would offer more resist- 

 ance than one that was inclined, broken, or fissured. The molten rock-mass 

 would naturally seek joints or fault planes, or, in default of these, follow 

 the lines of least resistance along bedding planes. That the intrusive bodies 

 are not found following the planes of the great faults of this region would 

 in itself be a sufficient proof, were none other available, that these faults 

 are subsequent to the intrusion of the older igneous rocks. Taken as a 

 whole, it seems evident that the upward passage of a molten stream would 

 be much more impeded in a series of horizontal and comparatively unbro- 

 ken strata, such as are supposed to have existed here at the time of the 

 intrusion of the older rocks, than it would be after they were uptilted, flexed, 

 and dislocated, as they were at the time of the eruption of the younger 

 series. In the case of each of the three larger masses of true eruptive rocks 

 of the region, viz, those of Chalk Mountain, of Black Hill, and of Buffalo 

 Peaks, wherever the sedimentary strata are visible in close connection with 

 the eruptive mass they are seen to be standing at a very steep angle and 

 the eruptive mass has apparently flowed over their basset edges. 



internal structure From this point of view the division of the igneous 

 rocks of the region is also well marked, although it is in the nature of 



