SOURCE OF THE METALS. 573 



feet of superincumbent rocks and an unknown amount of sea water. If 

 they had been deposited from hot ascending solutions, as the result of a relief 

 of pressure, it would naturally be expected that the bulk of the deposit would 

 have been found in the upper part of this mass of rocks, where the pressure 

 was least, instead of at its base. 



Secondly, as at the time of deposit the sedimentary beds in which they 

 occur were horizontal and relatively undisturbed, if the deposit had been 

 made from ascending currents it would naturally be expected that the proc- 

 ess of deposition should have acted from the lower surface of the beds up- 

 wards, instead of from the upper surface downwards, as is shown to have 

 been the case in the Blue Limestone, which carries the bulk of the ores. 



Thirdly, as far as present investigations have extended, there is a 

 noticeable absence, in the region of greatest ore development, of channels 

 extending downwards, through which the ascending solutions might have 

 come. The vast majority of eruptive bodies are in the form of nearly hori- 

 zontal sheets, parallel with the stratification. The few approximately verti- 

 cal bodies that have come under observation afford no evidence that their 

 walls form part of a channel through which the ore currents came up from 

 below. 



The above considerations seem sufficiently conclusive evidence against 

 adopting upward currents as the direct source of the ore deposits of Lead- 

 ville. The principal water channel at the time of deposition was evidently 

 the upper contact of the Blue Limestone with an overlying porphyry, and 

 from this surface they penetrated downwards into the mass of the lime- 

 stone. It may be assumed, therefore, that the currents were descending 

 under the influence of gravity, rather than ascending under the influence 

 of heat. 



It is well known that percolating waters circulate freely in every direc- 

 tion through massive or eruptive rocks, owing to the effect which cooling 

 and weathering have of splitting them into irregular blocks, while in sedi- 

 mentary rocks, however permeable, the bedding-planes are naturally the 

 easiest for them to follow. If, then, at the time of deposition the prevailing 

 direction of the ore currents had been downwards, it is easy to conceive 

 that they would have descended freely through the overlying porphyry 



