748 DYNAMICAL GEOLOGY. 



Pacific islands prove that the bottom has its ranges of southeast and 

 northwest elevations and depressions, crossing the ocean ; and this 

 would occasion the unequal tension required. 



Between the directions of the structure-lines and the directions of 

 the acting force, as determined by the oceanic and continental areas, 

 the origin of the prevalent trends and of their frequent curving courses 

 may therefore be explained. 



4. America simple in evolution, because of its situation between the 

 great oceans. — From the above, we perceive why it is that North 

 America should illustrate most simply and perfectly the laws of the 

 earth's genesis. Unlike the other continents, it is bounded on all 

 sides by oceanic basins ; on one side, the North Atlantic with a north- 

 east trend, on the other, the greater Pacific with a northwest trend. 

 The conditions under which the lateral pressure acted were therefore 

 the simplest possible ; and the evolution was therefore regular as well 

 as systematic. Europe has Africa on the south, and Asia on the east ; 

 and hence the complexity in its feature lines. Yet, even amid that 

 complexity, results according with the general principles here ex- 

 plained may be made out. 



3. Special Development of Mountain Chains. 



1. A Geosynclinal, or downward bend of the Crust, the first step 

 in ordinary Mountain-making. — In the making of the Appalachians, 

 there was first, under the lateral pressure, a slowly progressing sub- 

 sidence ; it began in, or before, the Primordial period, the com- 

 mencing era of the Silurian, and continued in progress until the 

 Carboniferous age closed. As the trough deepened, deposits of sedi- 

 ment, and sometimes of limestone, were made, that kept the surface 

 of the region near the water level ; and, when the trough reached 

 its maximum, there were 40,000 feet in thickness of stratified rock 

 in it (p. 380), and this, therefore, was the depth of the trough. The 

 Green Mountains began in a similar subsidence, and at the same 

 time ; and the trough was kept full with deposits as it progressed ; 

 but it reached its maximum, or the era of catastrophe, at the close of 

 the Lower Silurian. Such facts are in the history of many, if not all, 

 mountains. 



The bearing of the great subsidence of the Appalachian region during the Paleozoic, 

 under the action of lateral pressure, and of the consequent formation there of a very 

 thick accumulation of sedimentary beds, on the origin of the mountain -range, was 

 dwelt upon by the author, in an address before the American Association in 1856. (Am. 

 Jour. Sci.,II. xxii. 1856.) 



In 1859, the general statement was made by Hall (Report on the Palaeontology of 

 New York, vol. 3, Introduction), that the formation of all mountains commenced with 

 a slowly progressing subsidence of the region, and, pari passu, a thick accumulation of 



