ON THE EFFECTS OF LONG-CONTINUED HEAT. 179 



version by these means " into alternate and irregularly penetrating beds of 

 white ferruginous, and coloured ferruginous, fumerole clay, the deposits 

 being disclosed to a considerable depth, and exhibiting in the clearest man- 

 ner the phenomena of alternating colours." " One is astonished," he re- 

 marks, " at observing the great similarity between the external phenomena 

 of these metamorphic deposits of clay still in the act of being formed, and 

 certain structures of the Kenper formation. Thousands of years hence the 

 geologist who explores these regions when the last traces of the now active 

 fumeroles have vanished, and the clay formations have become consolidated 

 into marl-like rocks by the silica with which they are saturated, may suppose, 

 from the differently stratified petrographic and chemical character of these 

 beds, that he is looking at flceiz strata formed by deposition from water." 

 " At the surface, especially, where the deposition is favoured by slow eva- 

 poration, innumerable crystals of gypsum, often an inch in diameter, may 

 frequently be observed loosely surrounded by an argillaceous mass. At the 

 mountain ledge of the Namarfeyall, and at Krisuvik, this gypsum is found 

 to penetrate the argillaceous masses in connected strata and floor-like depo- 

 sits, which not unfrequently project as small rocks where the lower soil has 

 been carried away by the action of the water. These deposits are sometimes 

 sparry, corresponding in their exterior very perfectly with the strata of gyp- 

 sum so frequently met with in the marl and clay formations of the Trias." 



The great disturbances and fractures, the trappean rocks, and the frag- 

 ments of porphyritic, conglomerates, at. the bases of these formations, tend to 

 confirm the opinion of Bunsen, that they have had a metamorphic origin, an 

 origin very probably common to other beds, whether consisting of marl, shale, 

 or sand. AH the sand-beds now forming are clue to the disintegration and 

 detritus of ancient, sandstones, a process, which continued through a great 

 lapse of time, has but coated some portions of the sea-side with unconsoli- 

 dated sand. In the soundings of the Atlantic depths, the microscope.according 

 to Maury, has failed to detect a single particle of sand or gravel. For the 

 origin and consolidation of the inferior grits and shales we must look to ac- 

 tions, mechanical and chemical, more potent than those which the present 

 tranquil course of nature presents. In examining the carboniferous sand- 

 stones of the Blue Mountains in New South Wales, with their shales and coal- 

 beds, more than 12,000 feet in thickness, Darwin was "surprised at obser- 

 ving, that though they were evidently of mechanical origin, all the grains of 

 quartz in some specimens were so perfectly crystallized that they evidently 

 had not in their present form been aggregated in a preceding rock ; " and 

 he quotes Wm. Smith as having long since made the same remark on the 

 millstone grit of England. If any one, in fact, will observe with a lens the 

 surfaces of the quartz pebbles included in that grit, he will find on most of 

 them numerous unabraded facets, which bear evidence of a quartz-crystalline 

 action having pervaded the rock whilst its consolidation was going on. 



There can be no better proof of widely-spread chemical action due to 

 heat than the frequent presence of crystallized silica in every part of the 

 stratified rocks. 



The deeper we descend in the strata, the more plentiful are the veins and 

 beds of quartz, and the more manifest the signs of metamorphic action. 

 Von Buch was the first to explain, on the principle of metamorphism, 

 the change of calcareous rocks, in contact with pyroxenic porphyries, into 

 dolomites; and in 1835 the same principle was extended by Fournet to 

 the metallization of rocks by contact with quartziferous porphyries, and to 

 their felspathication and silicification by the contact of granite. " Since 

 the theory of a central fire," he observed, " has been confirmed by modern 

 researches, all the great questions in the history of the globe appear suscep- 



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