392 
THE GEOI^OGIsr. 
The best examination would be if we could jienetrate, by means of 
boring or by shafts, to a very great depth in some ore-districts, in 
order to reach the ore-bearing eruptive rocks, or a more ricloly 
metalliferous portion of the same. The stock-formed massive-rocks 
already metalliferous at the sui-face— for example, the Zumstockwerke 
or the Braunsdorfer ore-bearing porphyi'ics — might be best adapted for 
such an investigation. The problem wotdd be worthy of a country, 
the mining-operations of which are amongst the most important 
branches of its industiy. Certainly, on the other hand, the consider- 
ation may arise, that the slower cooling at the greater depths may 
have accelerated still more the sinking down of the metallic particles. 
Let us now throw back a general glance. Our theory explains 
very well why the newer eraptive rocks (trachyte, phonolite, basalt, 
lavas) are mvxch more rarely accompanied by ore-veins than the 
older ones, and especially why ore-veins seldom occur in the newer 
sedimentary fonnations, even where these, as in the Alps, are often 
broken and penetrated by eruptive formations. It explains it, since 
according to it the progi'essive cooling of the earth as a whole must 
have caused the zone of the deposition (determined by the tempera- 
ture) of the most abundant and notable constituents of ore-veins to 
have sunk deeper below the surface of the earth. If, besides, the 
whole phenomena of ore-vein-formation in volcanic districts still 
continue, they can, judging by the analogy of the old ore-vein- 
foi-mations, only be going on at a considerable depth beneath the 
surface. Only some constituents are projected to the sm-face in 
recent volcanic fissures, such as Silica, Calc-spar, and Oxide of Iron ; 
these in ore-veins partly extend tlrroughout all periods, and partly 
occur only as the most recent or uppermost members (as gozzans), 
being, consequently, in the latter case deposited at a relatively lower 
temperatm'e. 
The ironstone-veins, filled partly by means of sublimation (as spe- 
cular iron-ore), and partly by means of infiltration (as hydi'ated 
oxide of iron), are the only ore-veins which we can at present, in a 
measure, see originate. They are precisely, also, of all ore-veins the 
most frequently combined with the newer eruptive rocks, for ex- 
ample, the basalt fonmation. 
Tin-ore and platina seem hardly ever to occur in those veins which 
