FRAGMENTATION— BAIJRELL 287 



characteristic of the ocean basins. The process left jjreat angular 

 segments of the original lighter crust as continental platforms stand- 

 ing in relief between the coalescent basins. The waters gathered into 

 the basins and the continents emerged. 



Most American geologists hold strictly to Dana's theory of the 

 permanency of the continents and ocean basins, whereas European 

 workers in general stand by the oldei- view that ocean basins are 

 broken-down portions of the granitic shell. We may also include 

 this grander process of crustal change under the term of continental 

 fragmentation. Great intoi'continental troughs, sucli as the Red Sea 

 and the Caspian, are thought to have been niade in later geologic 

 times by fracture of their margins and subsidence of their floors. The 

 writer accepts the European vicAv, since, in spite of its difficulties, 

 it yet accounts for many geological relationships. If continental 

 fragmentation is real, it has a strong bearing upon the general prob- 

 lem of the origin of ocean basins, for the progress of fragmentation 

 is in reality a continuation of the formative process. Through frag- 

 mentation the margins of the continents break down into the oceanic 

 depths and enlarge them, and at the same time diminish the areas 

 of the land. 



INDICATIONS OF CONTINENTAL FRAGMENTATION 



The ocean basins, including slopes and deeps, occupy two-thirds of 

 the surface of the earth. In form they are made up of coalescent, 

 rudely circular or polygonal segments of the crust. The great Pacific 

 Basin covers nearly one-half of the globe, is fairly regular in outline, 

 and the structure of the mountain systems which face it is related to 

 its margins. Along the western coasts of North and South America 

 the parallelism is most conspicuous. Eastward from Asia and Aus- 

 tralia, on the other hand, the mountain systems advance into the 

 Pacific as chains of islands convex toward the ocean. On both sides 

 of the Pacific the structure shows the mountains to be much younger 

 in origin than the ocean basin. This is what Suess has named the 

 Pacific type of continental margin. Fragmentation on a large scale, 

 if it ever occurred there, must have been antecedent to the develop- 

 ment of the present mountain systems. 



The margins of the Atlantic and Indian Oceans show, however, a 

 very different relation to the continental structure. Here the conti- 

 nents protrude into the ocean in sharp southward-pointing wedges, 

 and they face the sea as table-lands of ancient rocks. The sedi- 

 mentary systems are more numerous in the continental interiors than 

 on these margins, and the ancient rock structures are in general cut 

 across by the shorelines, showing that they are older than the present 

 margins of the ocean basins. This type of coast Suess calls the 



