CHEMISTRY OF THE EARTH. 203 



r.re Avantinft" it is sometimes difficult to distioguisli in haiul-si)eeimeus 

 between iudigenous, exotic, and endogenous granites. 



§4G. The deposition of tbese mineral species from solution has doubt- 

 less taken place nnder considerable degrees of heat and pressure, which 

 could only exist at great depths in the earth's crust. Waters charged 

 with mineral elements by percolation throngh deeply-saated strata rise 

 through tissnres in these and deposit along the channels their dissolved 

 matters, a process not so much the result of cooling as of that decrease 

 of solvent power which must follow the diminution of pressure in accord- 

 ance with Sorby's conclusions.* 



§47. As pointed out in §17, the tirst precipitates from the water of the 

 primeval sea must have contained oxidized compounds of most of the 

 heavy metals. These early deposits, by mechanical division or by solu- 

 tion, became subsequently diffused, and entered into the composition of 

 later sedimentary strata. Removed from these by watery solution, the 

 metallic compounds have been, in different ages, brought to the surface 

 to be deposited in some cases as oxides or carbonates, or reduced by the 

 action of organic matters to the state of sulphurets or native metals, and 

 mingled with the contemporaneous sediments in beds or in disseminated 

 grains. During the subsequent alteration of the strata, these metallic 

 matters, being taken into solution, have been re-deposited hi lissures in 

 the metalliferous strata, forming veins, or, ascending to higher beds, have 

 given rise to metal-bearing veins in strata not themselves metalliferous. 

 The metals of the sedimentary' rocks are now, however, for the greater 

 part, in the form of insoluble sulphurets, so that we have only traces of 

 them in a few mineral springs, which serve to give a faint notion of the 

 agencies at one time at work in the sediments and waters of the earth's 

 crust. Like the iron, (§ 19) these metals have been in great part with- 

 drawn from the terrestrial circulation. The frequent occurrence of these 

 metals in waters which are alkaline from the presence of carbonate of 

 soda, is significant, when taken in connection with the metalliferous 

 character of certain dolomites, which probably owe their origin to the 

 action of similar alkaline springs upon basins of sea- water, (§-0.) The 

 intervention of intense heat and fusion or sublimation to explain the 

 origin of metallic ores is uncalled for. The solvent ])owers of water 

 and of various saline, alkaline, and sulphuretted solutions at high tem- 

 peratures, in connection with the notions above enunciated, will suffice to 

 form the basis of a rational theory of metallic deposits.! 



§ 48. The consideration of the nature and origin of endogenous rocks 

 has led to a digression to discuss the theory of metalliferous veins, which 

 the plan of this essay did not permit us to treat before. We now resume 

 the line of inquiry followed from § 30 to § 43, and proceed to consider the 

 phenomena of volanoes and earthquakes in accordance with the notions 

 already put forward. 



Violent movements of the earth's crust are confined to certain regions 

 of the globe, which are at the same time characterized b3' volcanic 



* Of this a remarkable example was afforded in 1SG6 at Goderich, iu Ontario, where, 

 at a depth of 1,000 feet, a bed of rock-salt was met, from which for a time a satnrated 

 or rather supersaturated brine was obtained. As an evidence of this, I saw a cube of 

 pure salt, one-fourth of an incli in diameter, which had formed upon and around a pro- 

 jecting point of an iron valve in the pump, above the surface of the ground. The liquid 

 beneath a pressure of 1,000 feet of brine, equal to about 1,200 feet of water, or 36 at- 

 mospheres, having taken up more salt than it could hold at the ordinary pressure, de- 

 posited a portion of it as it reached the surface, and actually obstructed thereby the 

 action of the pump. Alter a few months of pumping, however, the well ceased to afford 

 a fully saturated brine. 



t American Journal of Science, [2,] xxxi, 405, and xl, 213. 



