

Parr VIL Szor.i.§1.] METAMORPHIC GRANITE, 545 
The mineralogical composition of granite formed by the meta- 
morphism of other and specially sedimentary rocks must necessarily 
_ vary with that of the masses out of which it has arisen. In 
some cases there is a regular gradation from true granite outward 
into the schistose and gneissose masses, of which instructive ex- 
_ amples occur in the Scottish Highlands (Part VIII.), and in northern 
New York and New England.* But such a transition need not 
always occur, for if the granite was subject to unequal pressure 
(which it assuredly would in most cases be), it would in its soft, 
pasty condition undoubtedly be squeezed into any rents made in 
the surrounding rocks, and would thus imitate a truly eruptive 
mass, which in actual fact it would then be. When granite rises 
through unaltered or only locally altered strata, it may fairly be 
termed igneous and intrusive. When, on the other hand, it is 
intimately associated with extensive masses of schist and gneiss, 
many of which can only be distinguished from it by their foliated 
structure, its metamorphic origin may at least be strongly suspected. 
Fundamentally, indeed, eruptive and metamorphic granite seem to 
be due only to different modifications of the same subterranean 
processes. A mass of originally sedimentary rocks may be depressed 
to a depth of several thousand feet within the earth’s crust, where, 
subjected to vast pressure and considerable heat in presence of 
_ interstitial water or steam, it may be metamorphosed into crystalline 
schist. A portion of this mass, undergoing extreme alteration, may 
so completely lose all trace of its original fissile structure as to become 
amorphous crystalline granite, some of which may even be thrust as 
veins into the less highly changed parts above and around. One 
stage further would bring before us a connection opened between the 
earth’s surface and such a deep-seated granitic mass, and the con- 
sequent ascent and outburst of acid lavas and their fragmental 
accompaniments (p. 544). * 
Diorite, &e.—On a smaller scale usually than granite, other 
_ erystalline rocks assume the condition of amorphous bosses. Diorite, 
syenite, quartz-porphyry, and members of the basalt family have often 
been erupted in irregular masses, partly along fissures, partly along 
the bedding, but often involving and apparently melting up portions 
of the rocks through which they have made their way. Such bosses 
have frequently tortuous boundary-lines, since they send out veins 
into or cut capriciously across the surrounding rocks. In Wales, as 
shown by the maps and sections of the Geological Survey, the Lower 
Silurian formations are pierced by huge bosses of different crystalline 
rocks, mostly included under the old term “ greenstone,” which, after 
runping for some way with the strike of the strata, turn round and 
break across it, or branch and traverse a considerable thickness of 
stratified rock. In central Scotland numerous masses of dolerite and 
quartziferous diabase have been intruded among the Lower Carboni- 
ferous formations. One horizon on which they are particularly 
1 Dana, Amer, Jour, Sez. xx. (1880), p. 194. 2 See Dana, op. cif. 
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