GABBROS OF EAST SOOKE AND ROCKY POINT. 
37 
pyroxene crystals forming at the same time by the more rapid 
sinking of the latter, and, as they themselves slowly sank, they 
became aggregated together into small masses. Further sinking 
would probably have resulted in aggregation into larger masses, 
and two or three such masses are found ; but for the most part the 
process was brought to an end by the increasing viscosity of the 
cooling magma. 
Bowen's hypothesis also explains easily the formation of the 
rounded bodies of very coarsely crystalline “ pegmatite ” de- 
scribed on page 7. They may be supposed to represent large, 
loose masses of earlier-formed crystals, which have been con- 
solidated by a movement of the magma, with squeezing off of the 
interstitial liquor. 
KATE OF COOLING OF THE INTRUSIVE. 
Bowen has shown that by internal evidence, chiefly the 
zonary banding of mix-crystal minerals such as feldspars, an 
approximate estimate may be made of the rate at which a magma 
has cooled and whether or not it underwent much supercooling 
before crystallization commenced. A very rapid loss of heat is 
apt tot result in great undercooling of the liquid mass before 
crystallization commences ; crystallization once initiated, an 
almost instantaneous separating of mineral takes place sufficient 
to bring the liquid once more into stable equilibrium with the 
solid phase. The whole amount of each mineral so separating 
will have the same composition. Very slow cooling of the liquid 
magma, with sinking of crystals prevented, would favour resorp- 
tion of the earlier-formed crystals and thus produce a similar 
rock composed of minerals of uniform composition. Such a 
condition, however, is almost impossible to attain in nature. An 
intermediate rate of cooling produces zoning in the mix-crystals 
present, there being a special rate which will produce maximal 
zoning. The feldspars seem to be particularly liable to zoning, 
probably because on account of their low specific gravity they 
sink very slowly through the magma and hence are not so apt to 
be removed from the scene of their formation before outer 
layers can be deposited on them. 
