574: RECENT AND PLIOCENE [Ch. XXXIV. 



there are many varieties, such as Syenite, Talcose granite, and others 

 One of these varieties is sometimes found exclusively prevailing through- 

 out an extensive region, where it preserves a homogeneous character ; so 

 that having ascertained its relative age in one place, we can easily recog- 

 nize its identity in others, and thus determine from a single section the 

 chronological relations of large mountain masses. Having observed, for 

 example, that the syenitic granite of Norway, in which the mineral 

 called zircon abounds, has altered the Silurian strata wherever it is in 

 contact, we do not hesitate to refer other masses of the same zircon- 

 syenite in the south of Norway to the same era. 



Some have imagined that the age of different granites might, to a 

 great extent, be determined by their mineral characters alone ; syenite, 

 for instance, or granite with hornblende, being more modern \han com- 

 mon or micaceous granite. But modern investigations have proved these 

 generalizations to have been premature. The syenitic granite of Nor- 

 way 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. 580.) 



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 Ame- 

 rica, 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 materials of the vein were injected from below,* 

 and thus the granite is shown to be newer than certain superficial slaty 

 and trappean formations. 



Recent and Pliocene plutonic rocks, icliy invisible. — The explanation 

 already given in the 29 th and in the last chapter, of the probable rela- 

 tion 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 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 accu- 

 mulated in some places to the height of 550 feet, was found to retain a 

 high temperature half a century after the eruption.'j' 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 extremely gradual. Sometimes, in- 

 deed, this process may be retarded for an indefinite period, by the acces- 

 sion 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 



» Silliman's Journ., No. 69, p. 123. f See " Principles," Index, "Jorullo." 



