TRAP ROCKS. 



129 



If we sufficiently keep in view that the crust of the globe with 

 which we are acquainted, does not exceed, in comparative thickness, 

 that of a wafer to an artificial globe three feet in diameter ; and that 

 a very large portion of the globe is now or has in ancient times been 

 rent and pierced through by active volcanoes, and that these volca- 

 canoes are not the seat of subterranean fire, but merely its chimneys, 

 we shall have no difficulty in admitting, that extensive parts of the 

 crust of the globe, far distant from any present volcanoes, may have 

 been softened by internal heat, and the more fusible beds partly crys- 

 tallized in sitUi under the pressure of the ocean. 



With respect to the overlying formations which pass by gradation 

 into primary rocks (as some porphyries allied to volcanic rocks pass 

 into granite), this fact, so far from proving that the porphyry was not 

 of igneous origin, tends strongly to confirm the hypothesis, which at- 

 tributes an igneous formation to granite itself.* It is granted by the 

 best observers, that a regular gradation maybe traced between gran- 

 ite and the more ancient volcanic rocks, and tliat there is likewise a 

 gradation between the products of ancient and recent volcanoes, of 

 which we shall afterwards treat more fully. It will be proper, before 

 we proceed, to state the mineral composition of trap rocks. Fel- 

 spar and hornblende, (see Chap. III.,) constitute the principal ingre- 

 dients of trap; in many trap rocks the mineral called augite is inter- 

 mixed with felspar : indeed, hornblende and augite resemble each 

 other so much in chemical composition, and, when uncrystallized, 

 in external character also, that they have till recently been confound- 

 ed together, and they often occur together in the same rock. These 

 compounds of felspar and hornblende, and felspar and augite, chiefly 

 form the different rocks called greenstone, sienitic greenstone, basalt, 

 clinkstone, pitchstone, wacke, and amygdaloid ; and also trap-por- 

 phyry, and pitchstone-porphyry. All these rocks may be regarded 

 as different modes and combinations of felspar with hornblende or 

 augite, differing chiefly in their internal structure. 



When hornblende and felspar are intermixed, and have a granitic 

 structure, they form what is generally called greenstone ; and if the 

 felspar be red, sienitic greenstone. When hornblende and felspar, 



* However highly and justly distinguished man)'" of the natural philosophers in 

 Prance may be, it cannot be denied that they adhere more closely to theories once 

 formed, and have a greater dread of thinking for themselves, than the philosophers 

 of other countries. In confirmation of this, I shall translate an extract from M. 

 Bonnard's Apergu GSngnostiqtie des Terrains. It is truly amusing to see the alarm 

 which he evinces, lest he should be compelled by stubborn facts to relinquish his 

 cherished theories. "Another species of difficulty should prevent every pru- 

 dent man {esprit sage) from attempting to explain the formation of these rocks of 

 trachyte by any hypothesis founded on volcanic action ; namely, the alarming ex- 

 tent of the consequences which may follow such an explication, relative to other 

 rock formations, hitherto regarded as having a very different origin." With great 

 respect for M. Bonnard, I would say. Let every esprit sage yield to the evidence 

 which Nature presents, and leave consequences and theories to take care of them- 

 selves. 



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