CONSTITUTION AND ORIGIN OF SPHERULITES. 439 
seems probable that the amount of water present in the magma 
may have much to do with this question. In the spherulites 
of Obsidian Cliff no amorphous matter is described by Mr. 
Iddings, but the fresh obsidian carries less than one per cent, 
of water, while the pitchstones of Silver Cliff contain four or 
five per cent. 
In conclusion, it may be well to briefly review the reasons 
for advancing a hypothesis which once more introduces an 
element of unknown character into the discussion as to the 
essential nature of spherulites. The idea that a radiate crys¬ 
tallization is the direct and primary cause of the peculiar 
forms assumed by spherulites cannot be accepted for all cases, 
because there are instances, mentioned above, in which the 
crystalline growth is very subordinate to an amorphous 
matter that in its development brings out the same charac¬ 
teristics of form, as opposed to the surrounding glass. In 
certain spherulites one finds indications that the form was 
outlined by the amorphous substance before crystallization 
began. Further, there are many minor peculiarities in the 
spherulites of Custer county which, in the author’s judgment, 
are best explained by the assumption of some force tending 
to produce the spherical form, other than that of radiate 
crystallization. 
If one accepts the idea of the formation of a colloid, pre¬ 
ceding the crystallization in the spherulitic area, for any 
case, one must necessarily extend the application to some 
instances of apparently holocrystalline spherulites, because 
it has been observed that evidence of the former amorphous 
material may be entirely destroyed by crystallization of 
tridymite, quartz, or feldspar in its place. The question is, 
how many holocrystalline spherulites are to be thus ex¬ 
plained ? 
The branching arborescent growths of feldspar are results 
of certain conditions. If those conditions are realized in the 
viscous glass, the growths in question will undoubtedly form. 
In the rocks studied by the writer it does not appear that 
they ever did form except in the complete spherulites. The 
