284 LYELL'S ELEMENTS OF GEOLOGY. 



Relative Age and Position of Volcanic Rocks. 



them,* it will follow, that the quantity of plutonic rock generated, 

 or in progress during the recent epoch, must already have been 

 considerable. 



But as the plutonic rocks originate at some depth in the 

 earth's crust, they can only be rendered accessible to human 

 observation by subsequent upheaval and denudation. Between 

 the period when a plutonic rock crystallizes in the subterranean 

 regions, and the era of its protrusion at any single point of the 

 surface, one or two geological periods must usually intervene. 

 Hence, we must not expect to find the Recent or Newer Pliocene 

 granites laid open to view, unless we are prepared to assume that 

 sufficient time has elapsed since the commencement of the Newer 

 Pliocene period for great upheaval and denudation. A plutonic 

 rock, therefore, must, in general, be of considerable antiquity rela- 

 tively to the fossiliferous and volcanic formations, before it be- 

 comes extensively visible. As we know that the upheaval of land 

 has been sometiisaes accompanied in South America by yolcanic 

 eruptions and the emission of lava, we may conceive the more 

 ancient plutonic rocks to be forced upwards to the surface by 

 the newer rocks of the same class formed successively below, 

 — subterposition in the plutonic, like superposition in the sedi- 

 mentary rocks, being usually characteristic of a newer origin. 



In the accompanying diagram, Fig. 288., an attempt is made 

 to show the inverted order in which sedimentary and plutonic 

 formations may occur in the earth's crust. 



The oldest plutonic rock. No. L, has been upheaved at suc- 

 cessive periods until it has become exposed to view in a moun- 

 tain chain. This protrusion of No. L has been caused by the 

 igneous agency which produced the new plutonic rocks Nos. IL 

 in. and IV. Part of the primary fossihferous strata. No. 1., 

 has also been raised to the surface by the same gradual pro- 

 cess. It will be observed that the Recent strata No. 4., and the 

 Recent granite or plutonic rock No. IV., are the most remote 

 from each other in position, although of contemporaneous date. 

 According to this hypothesis, the convulsions of many periods 

 will be required before Recent granite will be upraised so as to 

 form the . highest ridges and central axes of mountain-chains. 

 During that time the Recent strata No. 4. might be covered by a 

 great many newer sedimentary formations. 



Tertiary plutonic rocks. — We have seen that great upheaving 

 . movements have been experienced in the region of the Andes, 

 during the Recent and Newer Pliocene periods. In some part, 

 therefore, of this chain, if any where, we may hope to discover 



* See Principles of Geology, Index, " Volcanic Eruptions." 



