
540 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
structure of granite itself, it appears now to be established that 
4 
granite has always consolidated under great pressure, in presence of — 
superheated water, with or without liquid carbon dioxide, fluorine, &e. 
(ante, pp. 295, 302), conditions which probably never obtained at the 
earth’s immediate surface, unless perhaps in those earliest ages when 
_the atmosphere was densely loaded with vapours and when the 
atmospheric pressure at the surface must have been enormous 
(p. 83). But whether the original crust was of a granitic or of a 
glassy character, no trace of it has ever been or is ever likely to 
be found. 
The presence of granite at the existing surface is, therefore, in 
all cases due to the removal by denudation of masses of rock under 
which it originally consolidated. The fact that, wherever extensive 
_ denudation of an ancient series of crystalline rocks has taken place, 
a subjacent granite nucleus is apt to appear, does not prove that 
rock to be of a primeval origin. It shows, however, that the lower 
portions of crystalline rocks very generally assume a granitic type, 
and it suggests that if at any part of the earth we could bore deep 
enough into the crust we should probably come to a granitic layer. 
That this layer, even if general round the globe, is not everywhere 
of the highest geological antiquity, or at least has consolidated at 
widely different periods, is abundantly clear from the fact that in 
many cases it can be proved to be of later date than fossiliferous 
formations the geological position of which is known; that is, the 
granitic layer has invaded these formations, rising up through 
them, and probably melting down portions of them in its progress. 
Granite invades and alters rocks of all ages up to late Mesozoic or 
Tertiary formations. Hence it does not belong exclusively to 
the earliest nor to any one geological period, but rather it has 
been extruded at various epochs, and may even be in course of 
extravasation now, wherever the conditions required for its pro- 
duction have existed. As a matter of fact granite occurs much 
more frequently in association with older, and therefore lower, than 
with newer and higher rocks. But a little reflection shows that 
this ought to be the case. Granite having a deep-seated origin 
must rise through the lower and more ancient masses before it can 
reach the overlying more recent formations. But many protrusions 
of granite would doubtless never ascend beyond the lower rocks. 
Subsequent denudation would be needed to reveal these protru- 
sions, aud this very process would remove the later formations and 
at the same time any portions of the granite which might have 
reached them. 
Granite frequently occurs in the central parts of mountain 
chains; sometimes it forms there a kind of core to the various 
gneisses, schists, and other crystalline rocks. More frequently it 
appears in large eruptive bosses, which traverse indifferently the 
~ rocks on the line of which they rise, and commonly send out abun- 
dant veins into them. Sometimes it even overlies schistose and 
