PART II. CHAPTER XXIV. 



283 



Recent and Pliocene Plutonic Rocks. 



premature. The syenitic granite of Norway already alluded to 

 may be of the same age as the Silurian strata, which it traverses 

 and alters, or may belong to the Old Red sandstone period ; 

 whereas the granite of Dartmoor, although consisting of mica, 

 quartz, and felspar, is newer than the Coal. (See p. 499.) 



Test by included fragments — This criterion can rarely be 

 of much importance, because the fragments involved in granite 

 are usually so much altered, that they cannot be referred with 

 certainty to the rocks whence they were derived. In the White 

 Mountains in North America, according to Professor Hubbard, a 

 granite vein traversing granite, contains fragments of slate and 

 trap, which must have fallen into the fissure when the fused ma- 

 terials of the vein were injected from below,* and thus the granite 

 is shown to be newer than certain superficial and slaty trappean 

 formations. 



Recent and Pliocene plutonic rocks, why invisible. — The 

 explanation already given in the 8th and 9th, chapters of the 

 probable 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 after- 

 wards. It is not uncommon for lava streams to require more 

 than ten years to cool in the open air ; and where 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. f 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 sup- 

 plies of heat ; for we find that the lava in the crater of Strom- 

 boli, one of the Lipari islands, has been in a state of constant 

 ebullition for the last two thousand years ; and we must 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 perma- 

 nently in a state of liquefaction. If then it be a reasonable con- 

 jecture, that about 2000 volcanic eruptions occur in the course 

 of every century, either above the waters of the sea or beneath 



* Silliman's Journ. No. 69. p. 123. 



t See Principles of Geology, Index, "Jorullo." 



