556 
ELEMENTS OF GEOLOGY. 
water are always descending through fissured and porous 
rocks, and must at length find their way into the regions of 
subterranean heat; and secondly, because in a state of com¬ 
bination water enters largely into the composition of some 
of the most common minerals, especially those of the alumi¬ 
nous class. But the existence of water under great pressure 
affords no argument against our attributing an excessively 
high temperature to the mass with which it is mixed up. 
Bunsen, indeed, imagines that in Iceland water attains a 
white heat at a very moderate depth. To what extent some 
of the metamorphic rocks containing the same minerals as 
the granites may have been formed by hydrothermal action 
without the intervention of intense heat comparable to that 
brought into play in a volcanic eruption, will be considered 
when we treat of the metamorphic rocks in the thirty-third 
chapter. 
PorphyrAtic Granite, —This name has-been sometimes giv¬ 
en to that variety in which large crystals of common feld¬ 
spar, sometimes more than three inches in length, are scat¬ 
tered through an ordinary base of granite. An example of 
this texture may be seen in the granite of the Land’s End, 
in Cornwall (Fig. 609). The two larger prismatic crystals in 
Fig. 609. 
Porph 5 ^ntic granite. Land’s End, Cornwall. 
this drawing represent feldspar, smaller crystals of which are 
also seen, similar in form, scattered through the base. In this 
base also appear black specks of mica, the crystals of which 
have a more or less perfect hexagonal outline. The remain¬ 
der of the mass is quartz, the translucency of which is strong¬ 
ly contrasted to the opaqueness of the white feldspar and 
black mica. But neither the transparency of the quartz nor 
the silvery lustre of the mica can be expressed in the en¬ 
graving. 
The uniform mineral character of large masses of granite 
seems to indicate that large quantities of the component ele- 
