UNSTRATIFIED OR IGNEOUS ROCKS. 213 



great masses, as if forced into cracks by heavy pressure 

 (Fig. 121, d). Their coarsely crystalline texture and 

 their mode of occurrence are well explained by supposing 

 that they have cooled at great depth in large masses, and 

 consequently slowly. When they appear at the surface, 

 therefore, they have been exposed by extensive erosion. 



Two Sub-Groups. All igneous rocks, whether plu- 

 tonic or volcanic, are divisible into two sub-groups, acidic 

 and basic. In the acidic, quartz and potash-feldspar 

 (orthoclase) predominate ; in the basic, hornblende or 

 augite and soda-lime feldspar (plagioclase) predominate. 

 The rocks of the former group are lighter colored and 

 less dense ; of the latter, are darker and heavier ; but the 

 two sub-groups run insensibly into each other. Among 

 the granitics, granite is the best type of the acidics ; and 

 diorite, and especially gabbro or diabase, of the basics. 



Intermediate /Series. 



Between the true plutonics and true volcanics there is 

 an intermediate series, called trappean or intrusives. If 

 the plutonics occur in masses beneath, the volcanics in 

 outpoured streams and sheets upon, these occur in sheets 

 intruded among, the strata, 

 especially of the older rocks. 

 They are finer-grained than 

 the plutonics and more crys- 

 talline than volcanics. The 

 reason, apparently, is that they 

 have cooled more rapidly than 

 the former, and less rapidly 

 than the latter. These are 

 also divisible into acidics and/ 



basics. Among the acidics Fm - ""rLm 1 porphyry 

 would come felsite and por- 

 phyry, and, among basics, diorite and diabase, for these 

 occur both massive and intrusive. 



