62 ELEMENTARY GEOLOGY 
them, cool much more slowly. These intrusive or 
plutonic rocks can cool only as fast as their heat can 
be carried away through the crust which surrounds 
them. Rocks conduct heat slowly,’ and so we may say 
that these intruded masses are surrounded by a blanket, 
which prevents their rapid cooling. It may take 
thousands of years for a deeply buried lava intrusion 
rd 
b 
< 
c 
° 
, 
VOLCANIC NECK Ax" 
Fria. 21. 
To illustrate intrusion of rocks and rate of cooling. Left-hand figure, a voleano 
recently in eruption. Right-hand figure, the same several centuries later. 
to solidify ; and this allows the minerals to grow to a 
good size. So as a general rule (though there are some 
exceptions) the surface flows are fine grained or glassy, 
and the intruded rocks of coarser texture. 
Let us suppose that Vesuvius is in eruption for the 
last time (Fig. 21). From the tube of the volcano a lava 
flow is running down the mountain side and cooling 
rapidly. In the tube, down to the reservoir which is fur- 
nishing the lava, is molten rock ; and perhaps from this 
1 To illustrate the.slow conduction of heat by rocks, it may be pointed out 
that the heat of the summer sun does not produce any effect on the tem- 
perature of the earth at a depth of fifty to sixty feet, and its effect is almost 
limited to the upper ten feet. 
