392 DYNAMICAL GEOLOGY. 



2. The mountain chains and volcanoes of the continents mostly confined to 

 their borders. — The facts on these points are briefly mentioned on page 32 

 and beyond. The situation of the chains on the continental borders, so 

 well exhibited in ISTorth America, and the position of the greater mountain- 

 mass of this continent, greater by 25 times, on the borders of the larger 

 ocean, have manifestly a cause that is in some way connected with the mutual 

 relations of the border region and the oceanic basin adjoining. The author 

 has explained these features (1847, 1873) on the view (1) that the lateral 

 pressure at work was lateral thrust chiefly from the oceanic direction against 

 the continental borders (the landward side of the border region being the 

 side of least pressure or greatest resistance); and (2) that since the oceanic 

 area was depressed below the level of the continental, the thrust was in a 

 small degree obliquely upward. If the crust in which the strain exists has 

 only five miles of depth, there is still stronger reason in favor of this expla- 

 nation, and for accepting it also as accounting for the making of the greater 

 mountain-mass on the side of the widest ocean ; for width of ocean, not 

 depth, is the important element. The view explains equally the abundance 

 of border volcanoes. 



3. Great mountain uplifts in the later part of geological time and also 

 great igneous ejections. — The fact that the highest and broadest of moun- 

 tains and the chief part of the mass of the continents were lifted above the 

 ocean mostly after the Cretaceous period is one of the most marvelous in 

 geological history. 



After the crust had become stiffened by the thickening, plication, and 

 solidification, and partly the crystallization, of the strata of the supercrust, 

 the chief movement in mountain regions, caused by the ever-continuing 

 lateral pressure, was an upward one, and then mountain chains received 

 through epeirogenic movements their great heights. Under the same cir- 

 cumstances, moreover, igneous ejections and volcanoes reached their maxi- 

 mum at the close of the Cretaceous and during the Tertiary. 



In correspondence with the great continental geanticlines of the Tertiary 

 and later time, there should have been oceanic geosynclines, for the material 

 constituting the rising mass could have had no other source than the crustal 

 mass beneath the oceans. On this point there is the great fact of the sub- 

 sidence over the central Pacific, described on page 349, of which the coral 

 islands are a monumental record. Its area was hardly less than 6000 miles 

 in length, and the breadth, reckoning only from the Hawaiian to the Friendly 

 Islands, over 2500 miles. Such a subsidence fully meets the demands of 

 the Pacific-border geanticline of ISTorth America. It suggests, also, that the 

 other great mountain-masses, uplifted during the Tertiary and Quaternary, 

 among them the lofty Andes and the still loftier Himalayas, derived a 

 supply of material by a like method from beneath the oceans. Under this 

 compensating relation, the two great movements become one epeirogenic 

 event, and, therefore, the combined result of one comprehensive cause. 



4. North America a type-continent. — Among the continents, North 



