GEOLOGICAL STRUCTURE OF IRON HILL. 381 



ment of Iron and Carbonate faults, though much modified by later erosion. 

 The region, however, as distinguished from the other portions of Leadville, 

 has been scarcely affected by glacial action, California gulch, in which 

 erosion has been deepest, being, as has already been shown, essentially a 

 valley of erosion. The slopes of the hills are steep, but extremely regular, 

 and covered with an accumulation of "Slide," whose average depth may be 

 considered to be from six to ten feet. This slide is distinguished from 

 Wash by being not rounded, but angular and resulting from the disinte- 

 gration of rock in place. It consists mainly of the debris of White Por- 

 phyry, which forms the top rock of either hill. The porphyry weathers 

 into thin sherd-like fragments, which from their relative lightness are easily 

 carried down by rain or snow, and therefore cover the greater part of the 

 slopes of the hills, even where other rocks actually crop out. It is only 

 along the steep slopes of the V-shaped valley of California gulch that actual 

 outcrops of rock in place are found on either hill. 



Geological structure. The average strike of the formations on Iron Hill is 

 a little west of north, and the beds dip east at an angle of about 12 to 

 25, shallowing, however, to the eastward, and probably basining up toward 

 the Mike fault. The south face of the hill has, by the erosion of the deep 

 V-shaped valley of California gulch, been left so steep that its surface is but 

 thinly covered by detrital material, and east of the Iron fault, whose line is 

 marked by a slight depression down the slope, the outcrops of the succeeding 

 sedimentary beds can be readily traced, in the numerous prospect holes, from 

 the Lower Quartzite, immediately overlying the Archean, up to the main 

 body of White Porphyry, which forms the summit of the hill. 



The geological section represented on this slope is, then, in descending 

 order : 



1. Wbite Porphyry capping, iii which are included detached portions of the 

 Weber Shales, represented in the lines shaft by black shales and, along the outcrops 

 on the Liuie and Bull's Eye claims, by a greenish slate containing plentiful easts of 

 lAngula mytitoides. 



Feet. 



2. Blue Limestone 200 



3. Parting Quartzite (outcrop obscure) 20 



4. White or Silurian Limestone 140 



5. Lower or Cambrian Quartzite 160 



6. Archean gneiss (not exposed.) . , 



