284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



in a general way consider that the same causes which lead to the 

 segregation of uranium or thorium most probably led to the concen- 

 tration of other substances. This at least is probable where, as in 

 the case of zircon, none of the substances dealt with are essential 

 parts of the molecule of the mineral. The magma or menstruum 

 from which the parent radioactive substances are derived may be 

 very rich in helium or lead, and the amounts of these constituents 

 which enter into the mineral may be considerable. It follows that 

 the absolute value of the helium or lead ratio involves the events 

 attending the genesis of the mineral. It is even quite probable that 

 substances crystallized out within a plutonic mass, and which, at 

 first sight, might be thought secure from impurities of this sort, 

 would be seriously affected. Consider the case of a mineral of early 

 consolidation 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 feldspar or quartz, but they 

 do so in presence of already formed molecules of these bodies or of 

 molecules which are forerunners of these bodies. If this were not 

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

 lime which appear in the feldspars would be inexplicable. 1 On this 

 view a clear explanation is found of the heterogeneous concentra- 

 tion of elements in bodies of early consolidation. These minerals, 

 in a sense, are residual, receiving those elements which Jiave been 

 excluded from taking part in earlier molecular grouping. The final 

 result is a "forced isomorphism." 



The same phenomena, on an intensified and more demonstrable 

 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 radioactive elements present, than did the 

 granite itself, although the vein must be younger than the rock con- 

 taining it. The fact, also shown by Strutt, that beryls often contain 

 a quite unaccountable quantity of helium, probably finds its expla- 

 nation 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- 



i Harker, The Natural History of Igneous Rocks, London, 1909, p. 167. 



