CAUSE OF UPHEAVAL AND DEPRESSION. 171 



De la Beche and many of the founders of geology either arrived 

 independently at the same idea, or adopted Prevost's views. They 

 thus eliminated the idea that upheavals were due to direct elevation 

 from beneath ; and affirmed that they result from lateral pressures, 

 acting against each other in nearly horizontal directions, and trans- 

 verse to the lines of elevation. The Kev. Osmond Fisher proved 

 that the mechanical force thus originated was ample to account for 

 the elevation of the highest mountains. 1 And this method of upheaval 

 accounts for the diminished density of the earth which has sometimes 

 been detected beneath great mountain-chains. 



Observations which Illustrate the Theory. Such changes in 

 the earth's crust leave effects which are easily recognised; and we 

 find indubitable evidences of changes of level now in progress in 

 great masses of land. The depression of Greenland is well attested 

 by the removal of settlements farther inland, and by the sinking down 

 and submersion of islands off the coast. 2 A corresponding elevation 

 of the north of Norway and Sweden, originally observed by Von Buch, 

 was demonstrated by Lyell. 3 On our own coasts sunken forests, in 

 many places, testify to depression ; and the existence of raised sea- 

 beaches proves recent upheaval. And as we go backward in time, 

 the whole series of strata, with their alternations of deep-sea and 

 shallow-sea deposits, with fresh-water beds and old-land surfaces, 

 superimposed in the same region, afford incontestable evidence that 

 the permutations of level of the earth's surface in past ages were pre- 

 cisely such as are now in progress in almost every sea ; while the very 

 existence of continental lands, chiefly composed of marine strata, 

 demonstrates upheaval on a grand scale. 



If, further, we examine the geological structure of almost any land, 

 its strata are seen to be almost invariably thrown into undulations and 

 grand upward and downward folds, such as have already been indicated 

 in the structure of our own islands, and may be seen on a large scale 

 in a geological map of Europe. And whenever we approach regions 

 which have been greatly elevated, the intensity of the plication of the 

 rocks always bears some relation to the height above the sea which is 

 attained. Not rarely we find the oldest rocks the most disturbed, 

 because they have been longest exposed to disturbing forces, but in 

 many cases newer rocks are almost as much changed. 



Mr. Robert Mallet's Theory of Volcanic Heat. It is well known 

 that all movement or pressure which meets with resistance becomes 

 transformed into heat. 4 And Mr. Robert Mallet, F.RS., 5 developed 

 the remarkable conception that the heat which produces volcanic 

 energy is developed locally within the solid crust by transforming 

 the mechanical work of compression into heat; while these compres- 



1 Trans. Camb. Phil. Soc., vol. xi. pt. 3. 



2 Arctic Papers of the Royal Geographical Society, Proc. Geol. Soc., vol. ii. p. 

 208. Quart. Jour. Geol. Soc., vol. xxvi. p. 690. 



3 Phil. Trans. Royal Soc. 1835. See also ''Lyell's Principles of Geology." 



4 Tyndall, " Heat as a Mode of Motion." 



5 Mallet, " On Volcanic Energy," Phil. Trans. Royal Soc., vol. clxiii. p. 

 147- 



