236 



ELEVATION AND SUBSIDENCE OF LAND. [Ch. XXXIII. 



temp 



or 800°, tills 



miglit produce an elevation of between 1^000 and 1^500 feet. 



t) 



same mass 



m and resume their original 

 might explain the gradual 



j)Osition. By such agency we 



rise of part of Scandinavia or the subsidence of Greenland. 



It is also possible that as the clay in Wedgwood's pyro- 

 meter contracts^ by giving off its water^ and then^ by inci- 

 pient vitrification j so^ large masses of argillaceous strata in 

 the earth's interior may shrink^ when subjected to heat and 

 chemical changes^ and allow the incumbent rocks to subside 

 gradually. 



Moreover^ if we suppose that lava cooling slowly at great 

 depths may be converted into various granitic rocks, we 

 obtain another source of depression ; for, according to the 

 experiments of Deville and the calculations of Bischoff, the 

 contraction of granite when passing from a melted or plastic 

 to a solid and crystalline state must be more than 10 per cent."^ 



Dr. Bischoff has also remarked, that wben the silicates 

 which enter so largely into the composition of the oldest rocks 

 — gneiss, mica-schist, clay-slate, and others — are percolated 

 by carbonic acid gas, which is of almost universal occurrence 

 at great depths, they must be continually decomposed. When 

 that happens, the carbonates formed by the new combinations 



rocks. 



must often aug-ment the volume 



sometimes 



rise to a mechanical force of expansion capable of uplifting 

 the incumbent crust of the earth, and the same force may act 

 laterally, so as to compress, dislocate, and tilt the strata on 

 each side of a mass in which the new chemical clianges are 

 developed . The same eminent German cliemist has attempted 



amount 



volume of the rocks. 



formed may 



I 



ise by adding to the 



If once some parts of the earth's crust are shattered, as in 

 regions of earthquakes, and reservoirs of melted stone and 



vapours have acquired force enough to uplift the 



heated 



* Bulletin do la Soc, G6ol. 2nd series, vol. iv. p. 1312. 



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The sud^^ 

 occasion^ ^ 



are CO 



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crevices 



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parts 

 springs 

 so that the 

 slowly depre 

 tlie geograp 

 areas of e^'^' 

 contiLrnoiis ' 



Balance o 

 Mstoricnl d 

 movement, ^ 

 or witliout 



(leTelo|., J i 



m 



tions pro due 

 of a few T. 1 



nation of li 



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forces ; tl, 

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