G. F. Becker—Teuture of Massive Rocks. 51 
simultaneous development of different crystalline grains, each 
of which, as it increases in size, interferes with the growth of 
others. A porphyry, on the other hand, is produced when only 
one or two minerals, crystallize from a homogeneous fluid in 
advance of the remainder and individual crystals of these favor- 
ed species are afforded an opportunity to reach a comparatively 
large size and a good crystallographic development. Porphy- 
ritic structure is thus certainly due to what chemists would call 
“fractional” crystallization. Granting that a given homoge- 
neous fluid may be cooled so slowly and under such pressures 
as to yield a holocrystalline porphyritic product, it certainly 
does not follow as a matter of logic, that if it were cooled still 
more slowly and under a still greater pressure, the various ulti- 
mate compounds would crystallize simultaneously. Neither 
does this conclusion follow as a matter of chemical or physical 
theory. The solidification of minerals must proceed, as I have 
shown in a former paper,* in obedience to the principle that 
the particular compound, the formation of which in the solid 
state liberates heat at the highest rate, will be the first to sep- 
arate. Now it is certain that, in general, the formation of dif- 
ferent solid compounds is attended by the liberation of heat at 
different rates; and the more slowly the conditions change the 
more perfect will be the divisions between the intervals of time 
during which different compounds will be precipitated. If a 
chemist desires to separate salts by partial crystallization, he 
will cool or evaporate his solutions as slowly as possible. It 
would thus appear both from experiment and from theory that 
if a homogeneous eruptive magma were cooled very slowly in- 
deed, one or two minerals would be most likely to crystallize 
at first, or early in the process, and to attain good crystallo- 
graphic development; that if these minerals constitute a fairly 
large portion of the mass, they would eventually build up a 
framework full of interstices; that as the solidification proceed- 
ed beyond this point and other compounds crystallized out, 
these could no longer develop symmetrically but must be 
crippled by the interference of earlier crystals and by mutual 
opposition. In short one would expect to obtain a holocrys- 
talline porphyry as the product of a homogeneous fluid glass 
slowly cooled. 
Geological observations bearing upon this subject are readily 
accessible. Kvery microscopical ‘lithologist knows that some 
minerals show the clearest evidence of early separation from 
eruptive magmas, being imbedded in other crystals; and that 
such minerals, for example apatites, are very usually well-de- 
veloped crystals; indeed irregular crystalline grains of apatites 
are very rare in lavas. Hruptive magmas do then separate out 
* A new Law of Thermochemistry, this Journal, 1886, vol. xxxi, p. 120. 
