392 THE GEOLOGIST. 



The best examination would be if we could penetrate, by means of 

 boring or by shafts, to a very great depth in some ore-districts, in 

 order to reach the ore-bearing emptive rocks, or a more lichly 

 metalliferous portion of the same. The stock-formed massive-rocks 

 already metalliferous at the sm-face — for example, the Zinnstockwerke 

 or the Braunsdorfer ore-bearing porphpies — might be best adapted for 

 such an investigation. The problem would be worthy of a country, 

 the mining-operations of which are amongst the most important 

 branches of its industry. 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 eruptive rocks (trachyte, phonolite, basalt, 

 lavas) are much more rarely accompanied by ore-veins than the 

 older ones, and especially why ore-veins seldom occur in the newer 

 sedimentary foimations, even where these, as in the Alps, are often 

 broken and penetrated by eruptive formations. It explains it, since 

 according to it the progressive 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- 

 formations, only be going on at a considerable depth beneath the 

 surface. Only some constituents are projected to the surface in 

 recent volcanic fissures, such as Silica, Calc-spar, and Oxide of Iron ; 

 these in ore-veins partly extend throughout all periods, and partly 

 occm" only as the most recent or uppermost members (as gozzans), 

 being, consequently, in the latter case deposited at a relatively lower 

 temperature. 



The ironstone-veins, filled partly by means of sublimation (as spe- 

 cular ii'on-ore), and partly by means of infiltration (as hydrated 

 oxide of iron), are the only ore-veins which we can at present, in a 

 measm-e, see originate. They are precisely, also, of all ore-veins the 

 most frequently combined with the newer eruptive rocks, for ex- 

 ample, the basalt formation. 



Tin-ore and platina seem hardly ever to occur in those veins which 



