538 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



along the upper surface of the limestone, and extending irregularly from 

 that surface downwai-ds, gives evidence also that the ore currents acted from 

 above downv/ards. Moreovei', iu the most important mine in the district, 

 the Robinson mine, the actual crack, either fault or jointing plane, through 

 which these currents ma}- have reached the limestone can be traced in the 

 roof of the main ore body and parallel with its longe st dimension. Along 

 this a certain amount of pyrite has been deposited in the micaceous sand- 

 stone which overlies the limestone. 



The beds in which these deposits are found belong to the Ujiper Coal 

 Measure formation, whose base has been arbitrarily taken at the Robinson 

 Limestone, so called from the mine of that name in which it was first ex- 

 amined. 



DEPOSITS IN THE ARCHEAN. 



On the opposite side of Tennessee Park from the El Capitan mine, 

 one of the prominent peaks of the Sawatch range is known as Homestake 

 T'eak, from the mine of that name, which occurs in the Archean rocks on its 

 slopes. It was discovered before the carbonate deposits of Leadville, and 

 is said to have produced considerable rich galena, carrying from 30 to 

 60 per cent, of lead and from 200 to 250 ounces of silver to the ton. In 

 it has been found a small amount of an arsenical nickel mineral, supposed 

 to be gersdorflfite. As the mine was not visited, nothing further can be 

 said as to the character of the deposit. 



In the Buckskin and Arkansas amphitheaters are many prospects and 

 a few small mines, on what are assumed by their owners to be true fissure 

 veins. None of the mines were being worked at time of visit, and they 

 were therefore mostly inaccessible. They were examined, however, when- 

 ever they came within the line of work, and it was found, so far as such 

 examination extended, that they were invariably a simple mineralization of 

 the country rock along some plane which admitted the percolation of water, 

 whether a jointing plane or the plane of a fault, and that the mineral 

 occurred as an impregnation or replacement of the country rock, the vein 

 materials being simply an alteration product of this rock, and not foreign 

 material filling pre-existing fissures, as is supposed to be the case in a true 

 fissure vein. 



