HEAT — VEINS. 343 



part of the limestone stratum where limited above and below by cherty 

 layers ; that the rock adjoining was largely converted into dolomite by mag- 

 nesian solutions, and that this " dolomization " was an early step in the 

 process, and aided in making the cavities ; that the ores often occur mixed 

 up with chert or sand that were set loose by the decomposition of the lime- 

 stone. / 



There are two theories of origin, one deriving the ore from above, the 

 other from, below. The former is favored and the latter opposed by the 

 absence of proof that the bodies of ore extend downward through the lime- 

 stone vein-like, and that igneous action was concerned. The theory of filling 

 from above encounters the objections that the ores of lead are not soluble, 

 and could not have been carried into the cavities in solution by sea water, 

 and that the gathering of galena from Archaean veins, once in the regions, 

 l)y abrading and transporting waters, is improbable, and does not account for 

 the presence of the eroding agents which made the cavities. 



The other theory, which was suggested by Percival, and is advocated by 

 Jenney (1893), makes the deposits similar in origin to the silver-lead deposits 

 of Leadville and other Rocky Mountain localities. But the objections to it 

 mentioned above exist; and so they do in the case of some Colorado ore 

 deposits, where igneous action below is nevertheless believed to be probable. 

 The making of the ore deposits is_generally referred to the close of Paleozoic 

 time, when the" Appalachians were made ; but Jenney supposes it to have 

 been at the close of__the Cretaceous period, simultaneous with that of most 

 Colorado deposits. 



In Derbyshire, England, the Subcarboniferous limestones contain similar 

 lead deposits, and along with the ores are Permian fossils, proving that 

 they originated not earlier than the Permian. 



The different modes of origin of ore-bearing deposits, above described, are the 

 ioUowing. In the deeper veins the earth's interior heat has been accessory to special 

 sources of heat. 



A. Heat from Crustal Movements, and not from Igneous Ejections ob 



Hot Springs. 



(1) Begular veins. — Mostly in metamorphic rocks. 



(2) Grouped interlaminar veins. — Generally short, as the smaller auriferous quartz 

 veins of gold regions, and some tin, copper and other veins. 



B. Heat from Igneous Ejections, Vapors, and Hot Springs. 



(3) Ore impregnating non-calcareous rocks. 



(4) Veins or groups of veins intersecting non-calcareous rocks. 



(5) The ores in veins intersecting calcareous rocks, and occupying cavities in them 

 made by their corrosion. Often combined in the same region with 3. 



Besides these there are, of uncertain origin : — 

 (6) Cavities s 

 Mississippi Valley 



J ^ . 



(6) Cavities supposed to be in part previously made limestone caverns, as those of the 



