152 GEOLOGY AND MINING INDUSTRY OF LEADV1LLE. 



at all into the Archean keystone of the arch ; whereas the erosion of the 

 adjoining canons, Big Sacramento on the north and Horseshoe on the south, 

 has cut into this body to the depth in one case of about five hundred and 

 in the other of nearly one thousand feet. The sandstone of the Weber Grits 

 formation overlying the Blue Limestone sweeps up on the ridges between 

 Little and Big Sacramento gulches for a considerable distance above their 

 junction, as is shown by numerous prospect holes. The continuity of the 

 intervening belt of Sacramento Porphyry cannot be definitely proved, 

 owing to considerable spaces where the outcrops are masked by surface 

 accumulations, but is reasonably probable. 



Spring valley. The region between Little Sacramento and Horseshoe 

 gulches is split by a little longitudinal valley, called Spring Valley, into two 

 low ridges, either of which is capped by Blue Limestone. Their general 

 form can be seen in outline on the Sacramento arch sketch, Plate XV. 



On the eastern slope of the northern of these two ridges is the Sacra- 

 mento mine, which has obtained rich silver ores from the Blue Limestone. 

 At the mine itself the overlying porphyry has been eroded off; but exten- 

 sive outcrops, covering a very considerable superficial area, are found to the 

 east, and are well shown in the steep rocky ravine which carries the drain- 

 age of Spring Valley into the main Sacramento gulch. The same body of 

 porphyry is found on the southern ridge, where it rapidly thins out, over- 

 lapping a similar tongue of White Porphyry ; a portion of the Weber 

 Grits formation is included between the two. It is evident that this body 

 Of porphyry was once a continuation of the main body of Sacramento 

 Porphyry, although it occupies a lower horizon and necessitates the supposi- 

 tion that in separating out at a certain horizon a portion of the main lacco- 

 lite body has cut down to a lower horizon. Improbable as this may seem, 

 it can be pi'actically proved to have occurred on the south of the Twelve- 

 Mile amphitheater, as shown in Section H, Atlas Sheet IX. Moreover, 

 the thickest portion of this body is opposite the thickest portion of the main 

 body. 



Horseshoe gulch. Perhaps the most complete and instructive series of 

 sections, and certainly those which have the most direct bearing on the 

 geology of the immediate vicinity of Leadville, are afforded by the erosion 



