16 
CRYSTALLIZATION IN ROCKS. 
Jan., 1890 . 
together on rotating the section between crossed nicols prisms. 
The order which has been given is that of the most basic 
separating first, and this appears to be the almost invariable 
rule. It obtains also in the acid series in cases where the 
solidification has proceeded slowly, under great pressure and 
in presence of more or less water, as in the case of the 
granites. The general order here is mica (or hornblende), 
felspar, quartz, notwithstanding the very refractory character 
of the last named mineral. There are several circumstances, 
however, which prove that the solidification of granite is not 
due to such processes as we can imitate :— 
1 . —Neither mica, hornblende, orthoclase, or quartz have 
ever been formed artificially by cooling from dry 
fusion. 
2 . —The obvious presence of water in the cavities of the 
quartz proving a very great pressure. 
3. —Where granite has formed veins which have more 
nearly reached the surface, as in some cases where 
they are traced into quartz felsites, the character 
of the crystallization is quite altered. 
In all these cases we have only had under our notice rocks 
which have been subjected to only one tolerably uninterrupted 
process of cooling. In many cases, however, we find obvious 
differences between two sets of crystals in the same mass. 
We may have, for example, a mass of small, almost needle¬ 
like, crystals of felspar, with small augite grains forming a 
dense mass in which isolated crystals of felspar of much 
larger size are embedded, constituting the structure which is 
called porpliyritic. Here the larger crystals are plainly 
anterior to the smaller crystals of the so-called base, and 
have certainly formed under quite different conditions. The 
evidence points to the formation of these porpliyritic crystals 
while the mass was still subterranean, and, therefore, cooling 
with great slowness, which would favour the regularity of 
their growth. They were on eruption already contained in 
the fluid mass, which has solidified as a mass of small 
crystals on account of the relatively rapid loss of heat when 
it was exposed to surface cooling agencies. One proof of the 
existence of these crystals prior to eruption may be found in 
the often observed circumstance that when the mass forms a 
dyke, although the texture of the base frequently varies from 
almost microlithic at the sides to fairly coarse-grained in the 
centre of the dyke, the distribution of the larger crystals is 
usually quite uniform, as many being met with near the edge 
as further in. Here, again, it is often observed that the 
felspars of the first consolidation or generation, as they are 
