172 Notices of Memoirs — Hatton, on Mountains. 



I might also adduce the Appalachian mountains in America as a 

 beautiful illustration of the theory, every elevation that has taken 

 place distinctly following the deposition of limestone, and occurring 

 only where the limestone was deposited, except, perhaps, the last 

 elevation after the Carboniferous period, which at present I pannot 

 account for — for, according to American geologists, the Carboniferous 

 Limestone never overlaid these mountains. But, besides the- deposi- 

 tion of matter, any other cause that changed the temperature of the 

 surface, would also produce alterations of level — -a rise in the tem- 

 perature being followed by elevation, and a fall by subsidence owing 

 to the cooling and contracting rocks forming part of the surface of a 

 sphere. In this way, the G-nlf Stream, by raising the temperature 

 of Norway and Sweden, is causing them to rise ; while the cold 

 Arctic current, sweeping down through Baffins Bay and striking 

 against Greenland, is causing that country to sink. 



Turning now to the second cause of the subversion of equilibrium, 

 viz. change in the incidence of pressure, we find our data not so 

 satisfactory, for we have no means of estimating the rigidity of the 

 crust, and therefore of the weight it would bear before beginning to 

 move, but nearly all geologists agree that most thick formations of 

 sandstones and clays have been deposited upon a sinking area, and 

 that in a large number of cases the subsidence has been approxi- 

 mately equal in rapidity to deposition. Now the chances are, of 

 course, enormously against subsidence being equal to deposition, 

 unless one is caused by the other, and when many cases of the kind 

 are brought forward, our former suspicion becomes almost a cer- 

 tainty. This, of course, only applies to clays and sandstones, for I 

 have already shown that limestones, as soon as they begin to expand, 

 by heat, rise up and relieve the pressure ; but with unconsolidated 

 beds, like clay or sand, the horizontal thrust could never get power- 

 ful enough to overcome the rigidity of the crust, and consequently 

 they could never rise. 



As large deposits of limestone, therefore, elevate areas, so large 

 deposits of argillaceous strata depress them. But while argillaceous 

 sti'ata are being formed they will not only be compressed by the 

 sinking of the spherical surface on which they rest, but they will also 

 at the same time be expanded by heat, and these two, together, will 

 throw the beds into folds or contortions. 



If we suppose that the subsidence is equal to the thickness of the 

 formation, which is the most reasonable supposition that we can 

 make, we can calculate the amount of compression due to sinking, 

 and to expansion from heat, due to different thicknesses. Some of 

 these are given in the following table, the upper line of which 

 represents the thickness of the formation in feet, and the lower the 

 proportionate compression : — 



Thickness ... 

 Compression 



5,000 feet 



10,000 feet 



20,000 feet 



25,000 feet 



