198 THE PROBLEM OF THE METALLIFEROUS VEINS. 



cipitating action of some alkaline reagent, such as limestone, they 

 may even reach the ground waters, and their dissolved burdens may 

 be contributed to this reservoir, but the greater portion seems to be 

 deposited at the level of the ground water itself or at moderate 

 distances below it. Impressed by these phenomena, which present 

 a true cause of solution, and influenced by their familiar and every- 

 day character, we may build up on the basis of them a general con- 

 ception of the source of the metallic minerals dissolved in those 

 aqueous solutions which are recognized by all to be the agents for the 

 filling of the veins. 



Let us now focus attention on the ground water. This saturates 

 the rocks, fills the crevices, and forces the miner who sinks his shaft 

 to pump, much against his natural inclination. The vast majority 

 of mines are of no great depth, and the natural conclusion of our 

 earlier observers, based on this experience, has been that the ground 

 waters extend downward, saturating the strata of the earth to the 

 limit of possible cavities, distances which vary from 1,000 to more 

 than 30,000 feet. To this must be added another familiar phe- 

 nomenon. The interior temperature of the earth increases at a 

 fairly definite ratio of about 1° F. for each 60 to 100 feet of descent. 

 In round numbers, if we start with a place of the climatic conditions 

 of New York — that is, with a mean annual temperature of about 51° — 

 we should on descending 10,000 feet below the surface find a tem- 

 perature of about 212°, and if we go still deeper, it would be still 

 greater. Of course, under the burden of the overlying column of 

 water, the actual boiling points for the several depths would be 

 greater, and it is a question whether the increase of temperature 

 would overcome the increase of pressure and the consequent rise of 

 the boiling point so as to convert this water into steam, cause great 

 increase in its elasticity, decrease in its sjDecific gravity, and thereby 

 promote circulations. At all events, the rise in temperature would 

 cause expansion of the liquid, would disturb equilibrium, and to this 

 degree would promote circulations. 



There is one other possible motive power. The meteoric waters 

 enter the rocky strata of the globe at elevated points, sink down- 

 ward, meet the ground water at altitudes above the neighboring- 

 valleys, and establish thereby what we call '' head." In consequence 

 they often yield springs. If we imagine the head to be effective to 

 considerable depths we have again the deep-seated waters under 

 pressure, which after their long and devious journey through the 

 rocks may cause them to rise elsewhere as springs. The head may 

 in small degree be aided by the expansion of the uprising heated 

 column, whose specific gravity is thereby lowered as compared with 

 the descending colder column. 



