OEIGIN AND DEVELOPMENT OF ORE- DEPOSITS. 175 



By dissolving away rock-substance from below by waters 

 subsequently rising to tbe surface, it has not only brought up 

 large quantities of mineral matter, and deposited it more within 

 the reach of man ; but is must have produced chambers within 

 the earth's crust, some of which have subsequently served as 

 depositing chambers to be filled with mineral substances of value; 

 while others, by collapse have occasioned many of the minor 

 irregularities of the strata, and, subsequently thereby, many of 

 the minor surface sculpturings. Such effects are well-known in 

 regions where the strata contain beds of rock-salt and gypsum, 

 and the same must be the case with beds of limestone and 

 dolomite, though the action will of course be much slower.*" 



We have seen that to some extent all strata may be regarded 

 as soluble, especially in waters which are heated and already 

 charged with active reagents of various kinds ; but, except in 

 such cases as those cited above violent deformation is not likely 

 to result from the solvent action of the circulating waters — since it 

 is so very slow — and when even large masses have been removed 

 there has been usually plenty of time for the cavities so 

 produced to become refilled with material brought from a 

 distance. 



The great local abundance of certain veinstones as will be 

 seen hereafter is apparently connected with original differences of 

 composition of the parent rocks situated comparatively near the 

 earth's surface. But the occasional local abundance alone or in 

 combination with metals of such metalloids as arsenic, sulphur, 

 and tellurium has probably more connexion with deeper seated 

 sources. The same may be said of the locally abundant fluor 

 spar of many copper, lead, and zinc mines, and of the still more 

 locally abundant boron and fluorine-bearing silicates, schorl, 

 topaz, &c., so generally associated with deposits of tin and 

 kaolin. 



The waters circulating in canals and open fissures seem also 

 in some instances to carry matters which are not dissolved but 

 in a finely divided state of suspension ; and the deposition of such 



*The altered coral reef at Newbam, near Truro, now only containing traces 

 of lime, must once Lave been mainly carbonate of lime, and was probably as 

 many fathoms tbick as it is now inches. See "Eecent Analyses," Journ, E.I.O, 



XXIII. 



