192 GEOLOGY AlW MINIl!^G INDUSTRY OF LEADVILLE. 



ciated with this is a porphyry difiFerent from those hitlierto described, and 

 characterized by small feldspar crystals of a deep purplish-red color. This 

 and a decomposed green mineral, which are its only porphyritic compo- 

 nents, lie in a light-green groundmass. Under the microscope the green 

 mineral is seen to be altered to chlorite, so that its original condition cannot 

 be determined, though it was probably biotite. The coloring matter of the 

 feldspar is a reddish substance in small flakes, possibly oxide of iron. 

 From the limestone outcrop eastward to the Mosquito fault the ridge is 

 made up of coarse white sandstones, having a gentle easterly dip. Beyond 

 the fault line the steeper slopes of the range are made up of fine-grained 

 granite, which resembles an eruptive granite. In this about half way up 

 the slope is an irregular dike of porphyrite. 



At the mouth of English gulch, just north of the Dome, are several 

 bodies of porphyr}^, and the structure of the sedimentary beds is extremely 

 irregular, the dip being rather to the northeast or away from the porphj'ry 

 mass, while on the ridge between English and French gulches the beds dip 

 to the west and southwest, giving further evidence of the synclinal fold 

 shown in Section B. 



In the lower portion of French gulch the south and west dips still 

 continue, and several small bodies of limestone are found between beds of 

 quartzite or altered sandstone. About a mile up the gulch, at the Mount- 

 ain Lion claim, is a body of diorite of blue-gray color and largely impreg- 

 nated with pyrites. It has a thoroughh- granitic texture and shows macro- 

 s:opic crystals of feldspar, hornblende, biotite, and quartz. At the head of 

 tlie gulch easterly dips again come in; but these change again to the west 

 before the fault line at the foot of Mount Arkansas is reached, showing a 

 second syncline, which may be a continuation of the syncline adjoining 

 the fault that is so well developed at the north edge of tl^e map, though 

 the general strike of that fold would carrj- it to the west of this. On the 

 divide between the head of French gulch and the Arkansas was observed 

 a body of Lincoln Porphyry, opened by a prospect hole to a depth of twenty 

 feet or more, which is so thoroughly disintegrated that when cut down by 

 a pick it crumbles in the hand. 



