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. 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 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 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. 
Brégger, in writing of the syenitic pegmatites of Norway, concludes 
that the minerals of the thorite-orangite group, including urano- 
1 Harker, The Natural History of Igneous Rocks, London, 1909, p. 167. “ 
