UNCONFORMABLE ROCKS. 



49 



them. If a thick stream of lava, as frequently happens, were to flow 

 over a range of conformable rocks, filling up the cavities and inequal- 

 ities of the surface, — when it became hard by cooling, it would form 

 a bed of superincumbent unconformable rock. Such instances are 

 common in volcanic countries. Very extensive ranges of rocks and 

 mountains occur, in this position, in various parts of the world, not 

 only covering the primary, but the secondary rocks. These will be 

 hereafter described, under the name of porphyry, sienite, and ba- 

 salt. They, frequently, assume the columnar structure, and, some- 

 times, form vast ranges of natural pillars ; as at StafTa, one of the He- 

 brides, on the north coast of Ireland, in Iceland, Sicily, and many 

 volcanic countries. 



Having described the position of both stratified and unstratified 

 unconformable rocks, it may be proper to state, that the latter rocks 

 occur, covering primary, transition, secondary, and tertiary strata : 

 many of those which cover the secondary and tertiary seem, evident- 

 ly, to have been the products of subterranean fire ; and even those 

 which cover the primary and transition rocks bear a close affinity to 

 volcanic rocks. If we admit, that our loftiest ranges of mountains 

 were elevated by the expansive force of central fires, this power, 

 acting upon an extensive portion of the globe, might be, ages, in up- 

 heaving the incumbent surface, which would continue to rise, until 

 vast fissures were made, through which the subterranean melted 

 matter would be thrown over the mountains and plains then existing, 

 and form the superincumbent rocks of basalt, porphyry, and sien- 

 ite, that seem to be so nearly allied to volcanic products. While 

 one part of the surface was rising, another part would sink, and 

 form a new bed, into which the waters of the ocean would gradual- 

 ly retire. 



According to Humboldt, the extraordinary eruptions by which 

 new islands have been formed since the period of authentic history, 

 have been preceded by a swelling of the softened crust of the globe. 

 At Kamenoi, the new island made its appearance above the sea, 

 twenty-six days before the smoke was visible. Every thing indi- 

 cates that the physical changes of which tradition has preserved the 

 remembrance, exhibit but a feeble image of those gigantic catastro- 

 phes which have given mountains their present form, changed the 

 position of the rocky strata, and buried sea-shells on the summit of 

 the higher Alps. It was undoubtedly in those remote times which 

 preceded the existence of the human race, that the raised crust of 

 the globe produced those domes of trappean porphyry, those hills of 

 isolated basalt in vast elevated plains, those solid nuclei covered with 

 the modern lavas of the Peak of Teneriffe, of Etna, and Cotopaxi." 

 —Humboldt. 



To these great catastrophes, and to vast inundations, and, in some 

 cases, to submarine currents, must we ascribe many inequalities of the 

 earth's surface, the fracture of strata, and the transport of the brok- 



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