294 R. H. Rastall — Dij^erentiation and Ore-deposits. 



the splitting is a matter of minor importance ; the facts of nature 

 and the distribution of the varying rock-types are in themselves 

 sufficient proof that something of the sort must have occurred, 

 however it was brought about. It is unnecessary here to consider in 

 detail the means by which the differential fractions have become 

 isolated in separate basins within the earth's crust, or poured out 

 as extrusions at the surface (lava-flows). This part of the subject 

 has been discussed in detail by many winters. What we are mainly 

 concerned with now is the ultimate fate of the metallic constituents 

 that were dissolved in the differentiated partial magmas before 

 solidification began. 



Let us first consider the case of a granite magma, such as the 

 granites of Cornwall, into which has been carried during partition 

 a considerable proportion of tin, tungsten, copper, arsenic, lead, 

 zinc, and other metals, together with fluorine, boron, andw^ater, in 

 addition to the constituents of the silicates, felspar, and mica, together 

 with an excess of silica. The micas crystallize first, followed by the 

 felspars, in the order of their freezing-points ; these are followed by 

 quartz, the metallic constituents, together with the fluorine, boron, 

 and water, all tending to concentrate in the last unfrozen residue, 

 presumably in the middle and lower parts of the mass, which are the 

 hottest. All these probably remain unsolidified to a comparatively 

 low temperature, perhaps below a visible red heat, although it must 

 be remembered that pressure raises freezing-points. However, the 

 already solidified mass is still cooling and therefore shrinking in 

 volume. Consequently cracks must be developed to relieve the 

 strain. These cracks apparently often extended radially into the 

 surrounding rocks. Along these cracks the still fluid material from 

 the interior must shoot out, under high pressure due to superheated 

 water, filling them with the residual magmatic material, and in the 

 process also bringing about far-reaching chemical changes in the 

 wall-rock of the fissures. Thus we account for the pegmatites, 

 aplites, and greisens, so common in and around many highly 

 mineralized granites. In some cases also the escape of fluids from 

 the cooling magma has been facilitated by earth movements, inducing 

 fissure systems dependent on the direction of these movements, 

 rather than merely radial. Sometimes the active chemical agents 

 derived from the magma bring about the most far-reaching changes, 

 such as tourmalinization, in the granite itself, the extreme case 

 being schorl-rock, where everything has been converted to quartz 

 and tourmaline ; in other instances topaz is very abundant. 



But the point of immediate interest is the fate of the metallic 

 compounds in the magma. We know from incontrovertible evidence 

 that the deposition of the ores belongs to the later stages of magmatic 

 cooling. There is no need to labour this point. It may be taken 

 as demonstrated that the deposition of the ores is subsequent to 

 the crystallization of the main mass, and many of them are 

 found in the pegmatites and greisens, having been deposited 



