Ch. XXV.] 
PLTJTONJC ROCKS. 
353 
so that there are obviously two orders of mineral masses formed 
at the surface which have a distinct origin, the aqueous and 
the volcanic. 
No. 84. 
a, Formations called primary (stratified and unstratified). 
b, Aqueous formations. c, Volcanic rocks. 
Besides these, however, there is another class, which cannot 
be assimilated precisely to either of the preceding, and which is 
often seen underlying the sedimentary, or breaking up to the 
surface in the central parts of mountain-chains, constituting 
some of the highest lands, and, at the same time, passing down 
and forming the inferior parts of the crust of the earth. This 
class, usually termed ' primary,' is divisible into two groups, the 
tratified and the unstratified. The stratified consists of the 
ocks called gneiss, mica-schist, argillaceous-schist (or clay- 
slate), hornblende-schist, primary limestone, and some others. 
The unstratified, or Plutonic, is composed in great measure of 
granite, and rocks closely allied to granite. Both these groups 
agree in having, for the most part, a highly crystalline texture, 
and in not containing organic remains. 
Plutonic rocks. — The unstratified crystalline rocks have been 
very commonly called Plutonic, from the opinion that they 
were formed by igneous action at great depths, whereas the 
volcanic, although they also have risen up from below, have 
cooled from a melted state upon or near to the surface. The 
theory conveyed by the name Plutonic is, we believe, correct. 
Granite, porphyry, and other rocks of the same family, often 
occur in large amorphous masses, from which small veins and 
dikes are sent off, which traverse the stratified rocks called 
e primary,' precisely in the manner in which lava is seen in 
some places to penetrate the secondary strata. 
Granite Veins. — We find also one set of granite Veins 
intersecting another, and granitiform porphyries intruding 
Vou ill, 2 A 
