290 NEW ZEALAND NATURE-STUDY BOOK 
It is a remarkable fact verified by observations all over the - 
world that below the limit of the influence of ordinary 
seasonal changes the temperature rises, as we proceed 
downwards, at the rate of about 1° Fahr. for every 50 or 60 
feet of descent. This at a depth of 20 miles would give a 
temperature of 1760° Fahr. and at 50 miles one of 4600° 
Fahr., or higher than the fusing-point of platinum. | 
That rocks buried beneath the surface-crust must be at 
a high temperature there can be no doubt, though it does 
not follow that they are in a liquid or molten condition ; 
for the pressure under which they exist is probably, if not 
certainly, sufficient to raise the point of fusion beyond even 
the high temperature attained. 
From time to time, owing to earth movements and other 
causes, ways of communication are opened up between these 
underlying rocks and the surface. This has a twofold 
effect: it reduces the pressure and in some cases causes 
the superheated rock at once to become molten ; and it 
furnishes a vent through which the molten material is 
forced to the surface. 
GRANITE. 
Granite is a crystalline compound of the minerals 
quartz, felspar, and mica. Typical specimens showing 
coarsely erystalline structure should be obtained—a face of 
one of these should be well polished—the others should 
show in places freshly fractured surfaces. 
Observe the general shape and colour of the specimen 
—also the colour of the substances composing it. Notice 
that the particles of which it is made are firmly welded 
together and cannot be separated—that they form in fact a 
compact mass. 
Examine the polished surface and make out as well as 
you can the form of the particles—notice that these are of 
