1214 A TREATISE ON METAMORPHISM. 



No better illustration could be furnished of an impervious pitching arch 

 than that of the Elkhorn mine described by Weed. (See fig". 32.) Here the 

 ore is at the contact of limestone and an overlying argillaceous rock which 

 Weed calls hornstone. The deposit is altogether below the hornstone, and 

 extends down an irregular distance into the limestone which it replaces." 

 It can hardly be doubted that this deposit received its first concentration 

 by aseending waters. 



If the criterion that ores in troughs are deposited by descending 

 waters and ores in arches are deposited by ascending waters be applied to 

 the Leadville ore deposits, the conclusion is that the sulphides of Leadville 

 were originally deposited by ascending waters. These ores occur below a 

 relatively impervious porphyry in a much-broken limestone, mainly the 

 Blue limestone. 6 Later, when the second concentration by downward- 



^: 



Fig. 32.— Ore deposit in limestone beneatli impervious shale, Elkhorn mine, Montana. After Weed. 



moving waters occurred, the material which in many places was on denuded 

 anticlines was in part carried down the limbs of the folds under the 

 porphyry into the limestone. At this time, doubtless, also, the limestone 

 was largely dissolved, the ores were carried down, not only along the dip 

 but across the beds, thus producing the very great irregularities which are 

 characteristic of the deposits. If the above explanation be correct, the 

 Leadville ores present another case in which both ascending and descending 

 waters were concerned. 



« Weed, W. H. , Geology and ore deposits of the Elkhorn mining district, Jefferson County, Mon- 

 tana: Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2, 1901, pp. 472-483. 



b Emmons, S. P., Geology and mining industry of Leadville, Colo. : Mon. U. S. Geol. Survey, vol. 12, 

 1886, pp. 539-584. 



"Blow, A. A., Geology and ore deposits of Iron Hill, Leadville, Colo.: Trans. Am. Inst. Min. Eng., 

 vol. 18, 1890, pp. 173-181. 



