124 WALKS AND TALKS. 



a width varying from a few inches to several feet, and with 

 its contents often arranged in layers upon each of the two 

 walls, in the same order from the wall. Each of these layers 

 is called a comb, and the whole is styled the gangue. The 

 metalliferous layer is the ore. Many of the most important 

 Old World mines are based on true veins. Many, also, in 

 America; but in many of the most celebrated mines, the 

 mode of occurrence of the ore is different. 



In the Eureka mining district, in Central Nevada, we have 

 a regular succession of strata consisting of limestones, shales, 

 and quartzites, ranging from the Cambrian to the lower car- 

 boniferous, but mingled with porphyritic eruptions and all 

 shattered by a process of faulting. The silver-bearing lead 

 one is found imbedded in the lower Carboniferous Limestone, 

 within masses of hydrous iron oxide. The deposits are dis- 

 covered and worked out by a regular system of mining 

 through shafts and galleries; though, in the works of the 

 Richmond company, these formalities are discarded, and the 

 deposits are reached and worked out by the shortest cuts. It 

 can scarcely be said that the ore occurs here in veins. It lies 

 in masses having cavities above. Its origin is from below ; 

 but the stratified rocks have not served as its source. But 

 the quartz-porphyry of the region, by leaching with hot alka- 

 line waters, may have afforded the ore ; and this is thought 

 by Curtis to have been its probable source. 



The silver-bearing galena of Leadville, in Colorado, ac- 

 cording to Emmons, has a similar mode of occurrence. The 

 deposit of the silver-bearing minerals took place in the lowest 

 member of the Carboniferous System. They were derived 

 from circulating waters, which obtained them in passing 

 through eruptive rocks. How introduced into the eruptive 

 rocks is a matter for speculation. 



In the lead-producing region of Wisconsin, Town, and Illinois, 

 the galena and blende occur as a lining on the walls of cavities 

 or caverns in a magnesian limestone of upper Cambrian age. 

 In Missouri, similar cavities in the Lower Magnesian Limestone, 

 of lower Cambrian age, are found lined with galena and quartz. 



