60 OEIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



"critical state," ready to flash into the vaporous condition as soon 

 as the pressure is in any way relieved. Such vapours must at 

 once fill any fissures or " chimneys " which may be formed from 

 time to time in the circulation zone, and must there he speedily 

 condensed as they make their way outwards, depositing 

 successively their mineral contents ; and, finally must issue in 

 many cases at the surface as comparatively pure water. 



2. The extremely local limitations of many very important 

 mineral substances seem to indicate that beneath the solid crust 

 or " circulation-zone," there are not merely two distinct layers of 

 fused rock-substance — the one acidic, the other basic, as long 

 ago suggested by Durocher, but that there are many separate 

 areas — each characterised by the greater abundance of one or 

 more elements. And furthermore, as these are probably nowhere 

 within the depths here considered, in a state of true fusion, but 

 only in a pasty condition, any large withdrawal of matter, by 

 means of volcanoes or otherwise, could hardly fail to leave 

 cavities filled only with highly compressed vapours of the more 

 volatile components of the rock-mass, so affording very favour- 

 able conditions for marked differentiation. We have seen that 

 even in a small district like ours, the elements present are very 

 irregularly distributed, while some important compounds which 

 are elsewhere very common, such as chlorides, bromides, and 

 iodides sulphates, borates, and nitrates, are with us very rare 

 or altogether absent. The same is true of a whole series of 

 metals, and particularly of mercury and platinum, which, if 

 actually present, could hardly fail to be detected.* The com- 

 parative abundance of such substances as arsenic, tin, and boron 

 in the form of the characteristic fluo-borosilicate tourmaline — 

 substances which are almost entirely wanting in many very 

 important mining districts, is also particularly worthy of notice. 

 The last-mentioned substances occur in a sort of " granitic 

 penumbra" as it has been termed by St. Claire Deville, as if they 

 had separated from the main mass, and become concentrated on 

 its outer surface during the processes of solidification and cooling • 

 at the same time impregnating the killas for short distances 

 from the junctions. 



* The several times reported occurrences of quicksilver in the valley of the 

 Exe still need confirmation. 



