R. H. Rastall — Diferentiation and Ore-deposits. 295 



contemporaneously with their formation. But the question of the 

 quartz-veins in the stricter sense of the word requires a little more 

 detailed consideration. In the first place it should be remembered 

 that the processes here under review take place at temperatures 

 above the critical point of water, 365 degrees C, and therefore the 

 usual distinction between liquid and vapour is then non-existent. 

 For this reason the term fluid has been used throughout this 

 discussion. We do not know the temperatures at which the normal 

 quartz-felspar pegmatites actually solidify, but we do know that 

 they often contain ore-minerals. Some ore-minerals therefore form 

 at fairly high, temperatures. There is now abundant evidence from 

 observation that many true pegmatites pass by insensible transitions, 

 by diminution of felspar, mica, etc., and corresponding increase 

 of quartz, into quartz-veins in the ordinary sense of the term, either 

 with or without metallic minerals. In some cases a vein may be 

 a pegmatite within the granite, passing gradually into a quartz 

 vein in the sediments. It is clear, therefore, that there is no real 

 distinction between these two types. 



It is necessary next to inquire why some of the constituents of the 

 original magma should remain liquid so much longer, i.e. to a lower 

 temperature than the main mass. The answer to this question is 

 to be found in a consideration of the effect of the more volatile 

 substances contained in the original magma. As this gradually 

 solidifies it is obvious that all the substances of lowest freezing- 

 point must tend to concentrate in the last fluid residue. Among 

 these substances are to be reckoned compounds of the useful metals. 

 These probably exist in the magma as fluorides, borides, sulphides, 

 and hydrates. The presence of superheated water here is doubtless 

 of first-class importance, as shown by Arrhenius.^ For example, 

 tungsten fluoride is known to be a gas at the ordinary temperature, 

 while tin fluoride boils at 705 degrees C.^ As the temperature falls 

 these elements group themselves differently and eventually crystallize 

 out as metallic minerals, together with the silica of the residual 

 magma, which crystallizes as one of the two forms of quartz. The 

 last fraction of all is a pure, or nearly pure, solution of silica, which 

 forms the simple quartz-veins often found far from the margin of 

 any visible intrusion. It appears clear, therefore, that there is no 

 real line of demarcation between the so-called pegmatitic and 

 hydrothermal mineral deposits. 



When several metals are present in one individual magma, as is 

 usually the case, each will form a compound having its own solidifica- 

 tion temperature and therefore capable of travelling only as far as 

 the corresponding temperature extends into the surrounding rocks ; 

 we shall therefore have a series of ore-zones around the intrusion, 

 as is so well seen in Cornwall. 



^ " Zur Physik des Vulkanismus " : Geol. Foren. Forh., vol. xxii, 1900, 

 395. Abstract in Geol. Mag., 1907, p. 173. 

 ^ Eoscoe& Scborlemmer, Treatise on Chemistry,\o\.n, 1913, pp. 861, 1095. 



