3G2 
THE GEOLOGIST. 
one great period. Here also they only in general penetrate tlie sedi- 
mentary formations as far as the Magnesian-limestone, into which 
certainly ore-veins have more frequently penetrated than have 
granite, porphyry, or greenstone. 
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 dimimition 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 metalhc 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 cmst 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 
Avith the theory of the cooling of terrestrial bodies. Of course, pro- 
visionally, it can be nothing but a hyjjothesis. 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 
