296 GEOLOGY AND MINING INDUSTRY OF LBADVILLB. 



remarkable. They vary in thickness from a foot or two up to over a thou- 

 sand feet. In Mosquito gulch sheets of porphyrite averaging 20 feet in 

 thickness can be traced continuously on the canon walls for several miles 

 without showing any vent, and the sheet of White Porphyry which covers 

 the Blue Limestone is shown by its outcrops to have been practically con- 

 tinuous over the area of the southern half of the Mosquito map. The only 

 direct evidence of a channel or vent leading to this sheet of porphyry from 

 below is at White Ridge, the point where it occurs in maximum thickness, 

 whence it might be assumed to have spread out from this point as a center 

 of eruption. In this case it would have spread ten miles from its center, 

 gradually thinning out from 1,500 feet over the vent and 500 feet within a 

 mile or two of it to 20 feet at the farthest point observed. The recon- 

 structed form of this body, as shown in Section F, corresponds to that of 

 the dome-shaped bodies in the Henry Mountains, described by G. K. Gil- 

 bert 1 under the name of laccolites. Indeed, it is evident that the manner 

 of eruption of all the older igneous rocks of this region was analogous to 

 that of the Henry Mountain rocks, although the amount of plication, dis- 

 location, and subsequent erosion to which these have been subjected ren- 

 ders it more difficult to reconstruct accurately their original form, and it is 

 probable that if they were restored they would be found to want the regu- 

 lar, symmetrical shapes he describes. The large dome-shaped bodies are 

 rare, but relatively thick sheets, one above another, are often very numer- 

 ous ; for instance, in the Ten-Mile district, just beyond the northern limits 

 of the map, the outcrops of seventeen were observed in a single transverse 

 section across an estimated thickness of less than 15,000 feet of sediment- 

 ary strata. 



Dikes. Normal dikes in the sedimentary strata were rarely observed, 

 and wherever the sheets cross the strata transversely it is usually at a very 

 low angle. In the Archean formations, on the other hand, the older rocks 

 were almost invariably found in the form of narrow and rather irregular 

 dikes, as a rule not over fifty feet in thickness, the principal eruptions being 

 two large bodies of diorite, whose outlines were not very accurately deter- 

 mined. 



1 Geology of the Henry Mountains Washington, 1877. 



