Ch. XXXIV.] PLUTONIC KOCKS. 719 



relation of the plutonic to the volcanic formations, will naturally lead 

 the reader to infer, that rocks of the one class can never be produced 

 at or near the surface without some members of the other being 

 formed below simultaneously, or soon afterwards. It is not uncom- 

 mon for lava-streams to require more than ten years to cool in the 

 open air ; and when they are of great depth, a much longer period. 

 The melted matter poured from Jorullo, in Mexico, in the year 1759, 

 which accumulated in some places to the height of 550 feet, was 

 found to retain a high temperature half a century after the eruption.* 

 We may conceive, therefore, that great masses of subterranean lava 

 may remain in a red-hot or incandescent state in the volcanic foci 

 for immense periods, and the process of refrigeration may be ex- 

 tremely gradual. Sometimes, indeed, this process may be retarded 

 for an indefinite period, by the accession of fresh supplies of heat ; 

 for we find that the lava in the crater of Stromboli, one of the Lipari 

 Islands, has been in a state of constant 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 liquefaction. 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 be- 

 neath them,f it will follow, that the quantity of plutonic rock gen- 

 erated, 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 even the Pliocene granites laid open to view, unless we 

 are prepared to assume that sufficient time has elapsed since the com- 

 mencement of the Pliocene period for great upheaval and denudation. 

 A plutonic rock, therefore, must, in general, be of considerable an- 

 tiquity 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 erup- 

 tions 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 sedimentary rocks, being usually 

 characteristic of a newer origin. 



* See " Principles," Index, " Jorullo." 

 f Ibid., " Volcanic Eruptions." 



