372 Prof. J. Joly on the 



secure from impurities of this sort, would be seriously 

 affected. Consider the case of a mineral of early consoli- 

 dation such as biotite. It is held by many petrologists that 

 the substances first to crystallize are not necessarily those 

 whose molecules were first formed in the magma. Biotite or 

 hornblende may, indeed, crystallize in advance of felspar or 

 quartz, but they do so in presence of already formed 

 molecules of these bodies or of molecules which are fore- 

 runners of these bodies. If this were not the case the 

 adjustment of the alumina to the potash, soda, and lime 

 which appear in the felspars would be inexplicable *. On 

 this view a clear explanation is found of the heterogeneous 

 concentration of elements in bodies of early consolidation. 

 These minerals, in a sense, are residual, receiving those 

 elements which have been excluded from taking part in 

 earlier molecular grouping. The final result is a "forced 

 isomorphism/'' 



The same phenomena, on an intensified and more demon- 

 strable scale, appear in the formation of pegmatitic minerals. 

 Here very often it may be inferred that mother liquors 

 rich in the rarer elements and the products rejected by the 

 magma, generate on a large scale minerals which are quite 

 subordinate within the mass of the rock. Extruded gases, 

 under great pressure, also act under such conditions. In the 

 internal cavities and druses of granites, doubtless, all these 

 factors operate. Under such circumstances are generated 

 the beryls and zircons which find their way into museum 

 collections. 



In keeping with the conditions attending vein minerals 

 Strutt found that such minerals from the Cornish granite 

 contained more helium, relatively to the ridioactive elements 

 present, than did the granite itself, although the vein must 

 be younger than the rock containing it. The fact, also shown 

 by Strutt, that beryls often contain a quite unaccountable 

 quantity of helium, probably finds its explanation in the 

 original occlusion of this substance. 



Brogger, in writing of the syenitic pegmatites of Norway, 

 concludes that the minerals of the thorite-orangite group, 

 including urano-thorite, crystallized in the first phase of 

 vein-formation, " that of magmatic consolidation with the 

 cooperation of pneumatolytic processes,"" and that in the 

 second and principal phase of pneumatolytic activity galena 

 crystallized out. (Brogger, Die Mineralien der Syenitpeg- 

 matitgange, pt. 1, pp. 160, 164, and pt. 2, p. 10.) If the 



* Haiker, ' The Natural History of Igneous Hocks/ London, 1909, 

 p. 167. 



