182 KEMPS ORE DEPOSITS. 



various igneous rocks. Considerable carbonate of iron is asso- 

 ciated with them, and a variety of rare minerals, including vanadi- 

 nite, descloizite, etc., occur. The limestones are probably Subcar- 

 boniferous. There are other districts in the territory of minor 

 importance. 1 



COLORADO. 



2.08.05. Example 30. Leadville. Bodies of oxidized lead- 

 silver ores, passing in depth into sulphides, deposited in much 

 faulted Carboniferous limestone, in connection with dikes and 

 sheets of porphyry. Leadville is situated in a valley which is 

 formed by the head waters of the Arkansas River. The valley 

 runs north and south, being confined below by the closing in of 

 the hills at the town of Granite. It is about twenty miles lon^ 

 and sixteen broad, and even to superficial observation is seen to be 

 the dried bottom of a former lake. The mountains on the east 

 form the Mosquito range, a part of the great Park range, while 

 those on the west are the Sawatch, and constitute the Continental 

 Divide at this point. Leadville itself is on the easterly side, upon 

 some foothills of the Mosquito range. The eastern slope of the 

 Mosquito range rises quiter gradually from the South Park to a 

 general height of 13,000 feet. The range then forms a very ab- 

 rupt crest, with steep slopes looking westward, which are due to a 

 series of north and south faults whose easterly sides have been 

 heaved.upward as much as 7500 feet. The faults pass into anti- 

 clines along their strike. The Mosquito range consists of crystal- 

 line Archaean rocks, foliated granites, gneisses, and amphibolites 

 and of over 5000 feet of Paleozoic sediments and igneous rocks. 

 The former include Cambrian quartzite, 150 to 200 feet; Silurian 

 white limestone, 160 feet, and quartzite, 40 feet ; Carboniferous 

 blue limestone, 200 feet (the chief ore-bearing stratum) ; Weber 

 shales and sandstones, 2000 feet; and Upper Carboniferous lime- 

 stones, 1000 to 1500 feet. The igneous rocks are generally por- 

 phyries. The sedimentary rocks were laid down in Paleozoic time 

 on the shores of the Archaean Sawatch island, and were penetrated 

 by the igneous rocks, probably at the close of the Cretaceous. 

 They were all upheaved, folded, and faulted in the general eleva- 

 tion of the Rocky Mountains, about the beginning of the Tertiary 

 period. The intrusion of the igneous rocks was the prime mover 



1 Rep. Director of the Mint, 1882, pp. 341, 376. B. Silliman, " Mineral 

 Regions of New Mexico," M. E., X. 224. 



