Ch.XXV.] PLUTONJC 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 

 stratified and the unstratified. The stratified consists of the 

 rocks 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 

 '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, arid granitiform porphyries intruding 

 VOL. Ill, 2 A 



