Jan., 1890 . 
CRYSTALLIZATION IN ROCKS. 
15 
silica with the bases alumina, oxide of iron, lime, magnesia, 
and the alkalis, and to show how the varying conditions of 
cooling have affected the resulting crystallization. 
In the first place we will consider the case of a perfectly 
melted mass, poured out as a stream of lava and losing heat 
by radiation and conduction. 
Fouque has shown experimentally that the behaviour of 
such a cooling mass will vary considerably, according to the 
temperature at which it has been maintained previous to 
eruption. He found that a mass which was quickly cooled 
from a temperature considerably above its fusing point 
solidified to a glass with no appearance of crystallization ; 
but if it has been kept for some time at a temperature which 
just does not permit of solidification, then on pretty rapid 
cooling crystals of one or more species are produced. This 
condition may be obtained naturally when the mass is 
injected among strata or in dykes—the cooling will then be 
so slow that the process of elaboration will go on and the 
various minerals will separate out. At the very edge of the 
dyke, however, there may be a layer so quickly cooled as to 
solidify at once to a glass. These experiments were all made 
on basic rocks as they are so much more fusible and generally 
manageable. In the course of gradual cooling, the first 
substance which separates in basic masses—by which term 
those containing not more than fifty per cent, of silica are 
meant—is magnetite. 
Following this the magnesian silicate, olivine, is always 
formed. It encloses the magnetite occasionally and portions 
of glass, but never the minerals which follow. The order of 
the crystallization of the remaining chief constituents of the 
basic rocks, augite and felspar, varies according to some 
unknown variations of condition. Sometimes the augite is 
present in well-defined crystalline grains, entangled as it were 
in a network of crystals of felspar ; at other times the augite 
forms broad plates with the felspar crystals embedded in them, 
or apparently indenting their edges. In the former case the 
augite has apparently solidified first, in the second case it is 
evident that it must at all events have finished crystallizing 
after the felspar. 
In the case of the felspars, we very often find that on 
examination in polarized light there is evidence that, though 
there is no visible separation, the chemical composition of the 
various zones, which have been successively added in the 
process of crystallizing, has been different. This is shown 
by the fact that the position of the optic axes is different in 
the different zones, so that they do not all become dark 
