154 ON THE ORIGIN OP MOUNTAIN EANGES. 



Secondly, " the thickened area would sag downwards, and 

 the most thickened area would sink the most. Hence de- 

 pressions would arise on both sides of the ridge, and the 

 ocean, which covers tlie general surface, would bo deeper 

 than elsewhere along two channels parallel to, and at some 

 little distance from, the ridge. But should the ridge be 

 steeper on one side than on the otter, as seems inevitable, 

 the ocean would be deeper on the steeper side." This seems 

 in accordance with fact. As Dana puts it, " the highest 

 mountain-border faces the largest ocean." 



Lastly, on the principles of hydrostatic equilibrium, areas 

 of deposit, being constantly loaded with frosli sediment, will 

 sink ; while areas of denudation, being continually lightened 

 of their load, will tend to rise. And differential denudation 

 on opposite sides of a mountain axis will give tlio mountains 

 a tendency to tilt. 



Such is Mr. Fisher's hypothesis. I pass now to Mr. Mcllard 

 Reade's, merely interpolating an exclamation of I'cgretful 

 wonder that the newer writer should not have inti'oduced 

 into his volume a more detailed notice and criticism of the 

 views of his predecessor in this interesting field of research. 



Professor James Hall (1857) was apparently the first to 

 give prominence to the fact that the preliminaiy and pre- 

 paratory stage in mountain-building is the long-continued 

 accumulation of great thicknesses of sedimentary deposits. 

 Thus, whereas the total thickness of the Patoozoio strata in 

 the Appalachian chain is about 40,000 feet, the same for- 

 mation in tho Mississippi Valley, including the carboniferous 

 limestone, which is wanting in the east, attains a thickness 

 of scarcely 4,000 feet. At an earlier date (18;M) Babbage 

 had shown that the deposition of fresh layers of sediment 

 is necessarily accompanied by a rise of the isogeotherms, or 

 planes of equal temperature beneath the earth's surface. 



