


218 STRUCTURAL AND FIELD GEOLOGY 
contain quartz. In other places, where the surrounding rocks are not 
calcareous, the same granite is transformed into rocks of a still more 
basic character, such as norites and peridotites. Numerous xenoliths 
are scattered through the granite—all being metamorphosed and often 
passing by insensible gradations into the igneous mass that surrounds 
them. 
It is believed that water has played an important réle in thermal 
metamorphism. Deep-seated magmas probably contain large supplies 
of water and other vapours and gasses dissolved in them, the presence 
of which must increase the liquidity of the molten masses. Indeed, 
direct evidence of the existence of this contained water is supplied by 
volcanic phenomena, vast volumes of steam and vapours issuing from 
craters and escaping from lavas. Unaltered sedimentary rocks also con- 
tain considerable stores of water, for all are more or less porous, and are 
thus capable of retaining a larger or smaller proportion of interstitial 
moisture. In addition to this supply we must take note of the fact that 
many of the mineral constituents of rocks contain water in chemical 
combination. It is not surprising, therefore, that the more important 
metamorphic changes effected by a batholith are just such as should 
have been produced by steam permeating the rocks under great pressure 
and at a very high temperature. The steam has simply acted as a solvent 
menstruum, and has tended to produce a more or less perfect crystallisa- 
tion or recrystallisation of the constituents of the rocks affected, leaving 
the chemical composition practically unchanged. 
It is only generally true, however, that metamorphism has left the 
chemical composition of rocks unchanged. Not infrequently, silica has 
been introduced in abundance from batholiths, so as to permeate the 
contiguous rocks and to fill up cracks and fissures. The rocks of the 
metamorphic zone are thus frequently more or less abundantly traversed 
by smaller and larger veins of quartz, which in places may extend out- 
wards almost to the very margin of the zone, but they rarely go beyond 
it. In some cases, these quartz-veins are accompanied by new minerals, 
the composition of which shows that they could not have been derived 
from the alteration of the surrounding rocks. Among the most interesting 
examples are the tin-bearing veins which are associated with intrusive 
masses of granite and other acid eruptives, and the apatite-veins which 
are more particularly connected with batholiths of gabbro. In the 
formation of the cassiterite-veins, various volatile fluorides, boron- 
compounds, etc., have taken part ; for the tin-ore is usually accompanied 
by fluor-spar, schorl, etc. According to Professor Vogt, the contents 
of such veins were extracted from the granite before the plutonic mass 
had fully congealed. This is proved by the fact that the same series 
of elements which characterise the cassiterite-veins occur also in the 
pegmatite-veins of granite. In the case of the apatite-veins, analogous 
phenomena occur, the elements they contain being the same as those 
met with in gabbro. Thus, while potassium and lithium minerals are 
characteristic of tin-veins, magnesium and calcium-sodium minerals 
are notable constituents of apatite-veins. ‘In both classes of veins,” 
