UNSTRATIFIED OR IGNEOUS ROCKS. 211 



ous regions (a, Fig. 121). 2. In vertical sheets intersect- 

 ing the stratified rocks or other igneous rocks, b. 3. In 

 streams or sheets overlying the stratified, or else between 

 the strata, c c'. 4. Sometimes as tortuous veins, d d, 

 connected with the great underlying masses. All of these 

 are connected with, and are extensions of, the great 

 underlying masses. 



Extent. As thus defined, igneous rocks occupy but a 

 small portion, certainly not more than one tenth, of the 

 land-surface. But beneath the stratified rocks they are 

 supposed to form the great mass of the earth. 



Classification of Igneous Rocks. Igneous rocks can 

 not be classified, like sedimentaries, by relative age. 

 They are best classified partly by texture and partly by 

 mode of occurrence. They thus fall into two strongly 

 contrasted groups, viz., plu tonics and volcanics, or gra- 

 nitics and true eruptives. The rocks of the one group 

 are very coarse-grained and wholly crystalline, of the 

 other, finer-grained or even glassy. The one occurs only 

 in great masses, either underlying the stratified rocks, or 

 appearing on the surface over wide areas, especially in the 

 axes of mountain-ranges ; the other, in sheets injected 

 among the strata, or as streams and sheets outpoured on 

 the surface. The granitics have not usually been erupted 

 at all, although they often form the reservoirs from which 

 eruptions have taken place. 



It is sometimes convenient to speak of an intermediate 

 group trappean. If so, then the three kinds correspond 

 to the three positions mentioned above. The granitic 

 (Fig. 121, a) occur beneath ; the trappean, b b, injected 

 among ; the volcanic, cc, outpoured upon, the stratified 

 rocks. 



I. THE MASSIVE OR GRANITIC GROUP. 



The rocks of this group occur in great masses, not in 

 sheets or streams. They are all very coarse-grained in 



