Ch. XXXIV.] PLUTONIC ROCKS. 575 



ebullition for the last two thousand years ; and we may suppose this 

 fluid mass to communicate with some caldron or reservoir of fused 

 matter below. In the Isle of Bourbon, also, where there has been an 

 emission of lava once in every two years for a long period, the lava 

 below can scarcely fail to have been permanently in a state of liquefac- 

 tion. If then it be a reasonable conjecture, that about 2000 volcanic 

 eruptions occur in the course of every century, either above the waters 

 of the sea or beneath 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 relatively 

 to the fossiliferous and volcanic formations, before it becomes extensively 

 visible. As we know that the upheaval of land has been sometimes 

 accompanied in South America by volcanic 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 suc- 

 cessively below, — subterposition in the plutonic, like superposition in the 

 sedimentary rocks, being usually characteristic of a newer origin. 



In the accompanying diagram (fig. 697), 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. I., has been upheaved at successive 

 periods until it has become exposed to view in a mountain-chain. This 

 protrusion of No. I. has been caused by the igneous agency which pro- 

 duced the newer plutonic rocks Nos. II., III., and IV. Part of the 

 primary fossiliferous strata, No. 1, have also been raised to the surface 

 by the same gradual process. 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, or granite of the human period, 

 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. 



Eocene granite and plutonic rocks. — In a former part of this volume 

 (p. 230), the great nummulitic formation of the Alps and Pyrenees was 



* " Principles," Index, " Volcanic Eruptions." 



