INTERMEDIATE SERIES. 



205 



if forced by pressure of superincumbent weight into small cracks of the 

 latter (Fig. 180, d, Fig. 182, A, and Fig. 183, A and B). But rocks 

 of more basic type, such as 

 diorite and diabase, probably 

 on account of greater fusibil- 

 ity, occur not only as Pluton- 

 ics in massive form, but also 

 as intrusives in dikes and in- 

 tercalary beds, like true vol- 

 canics. 



The rocks of the Plutonic 

 group are never found in 

 connection with scoriae, glass, ashes, or other evidences of rapid cooling 

 in contact with air. They have never been erupted on the surface. 

 They ivere cooled, and have solidified sloivly under pressure in great 



Fig. 182.— Diagram illustrating Mode of Occurrence of 

 Granite. 



Fig. 183.— Granite Veins. 



masses and at great depths. Hence, wheyi we find them at the surface, 

 they have been exposed by extensive erosion. They are either fused 

 masses solidified without eruption (a), or they are the solidified reser- 

 voirs (g, Fig. 180) from which eruptions have come. In either case, 

 they have themselves been cooled at great depths. 



Intermediate Series. 



Between the undoubted Plutonics, already described, and the un- 

 doubted volcanics, to be taken up hereafter, there is an intermediate 

 series of rocks, which are sometimes placed in one group, sometimes in 

 the other, and sometimes in a separate group, co-ordinate with the other 

 two, and called trappean or intrusive rocks. They occur mostly in the 

 older and middle rocks in the form of dikes, filling great fissures inter- 

 secting, or as intercalary beds between, the strata. If Plutonics are 

 in great masses beneath the strata, and volcanics are outpoured masses 



