362 THE GEOLOGIST. 



one great period. Here also tliey only in general penetrate the sedi- 

 mentary formations as far as the Magnesian-limestone, into which 

 certainly ore-veins have more frequently penetrated than have 

 granite, porphyry, or gTeenstone. 



Now if we examine these old crystalline massive rocks somewhat 

 more closely, we find that they contain, tolerably often, the elements 

 of ore-veins as accessory mixtures, or chemically combined with 

 their essential constituents. These (metal- contents) usually increase 

 with the diminution of the volume with which the rocks project on the 

 earth's surface. Where the latter occur in great masses they are then 

 mostly very free from metallic particles, or only contain them on 

 their contact edges and in their small stock- or vein-formed ramifi- 

 cations. The following may serve as examples : the Zinnstockwerke 

 of Altenberg, Zinnwald, Geier, &c. ; the ore-containing greenstones 

 of Schwarzenberg ; many magnetic-iron stocks, and ore-containing 

 porphyry- veins. Indeed, the outer solid crust of massive rocks seems 

 often to be richer in metals than the regions lying beneath it ; and 

 from the destruction of such a crust may have been derived the rich 

 stream- works, which are or have been found in districts where in the 

 contiguous rock scarcely a trace of tin-ore can be discovered, as 

 for example in the granite country at Weissenstadt in the Fich- 

 telgebirge. 



Can we not, under these circumstances, suppose the massive rocks 

 to be the original bearers of the metal contents of ore-veins ? It is 

 a supposition which can be brought into the most beautiful harmony 

 with the theory of the cooling of terrestrial bodies. Of course, pro- 

 visionally, it can be nothing but a hypothesis. But let us endeavour 

 to build further on this hypothesis, and by means of it to explain 

 certain facts. Of course we must guard ourselves, in doing so, from 

 subordinating and accommodating the facts to the hypothesis ; the 

 hypothesis must rather be modified to suit the facts, and if not it 

 must be abandoned. 



Let us likewise assume, also provisionally, an original distribution 

 of the elements of ore- veins in the crystalline massive rocks, that is, 

 in the eruptive portions of the original fluid nucleus, and let us see 

 how this assumption suits the facts. Where these rocks occurred in 

 great masses, they cooled very slowly, with the exception of their 



