DIASTROPHISM AND THE FORMATIVE PROCESSES 139 



abysmal folds or blocks might well become continental occasionally, 

 while, in still other cases, oscillations between the two extremes 

 might arise. It does not militate, in any proper sense, against the 

 view of permanency of the continental and abysmal segments, that 

 there should be these diversities of local action on the hinge lines 

 of the great segments. 



Now there are not only simple hinge lines along the junctions 

 of continental and abysmal segments but there are complex hinge 

 areas, as for example areas in which the angles of a pair of conti- 

 nents and also the angles of a pair of abysmal segments approach 

 one another, as the angles of the two Americas and of the Atlantic 

 and Pacific basins, respectively, approach one another in the Antil- 

 lean region, and as similar approaches are made elsewhere. These 

 "four-corners" of the earth's segmentation are regions of exceptional 

 instability and are affected by unusual seismic and volcanic activi- 

 ties.' In these complex hinge areas it is natural that there should 

 be some exceptional behaviors of blocks and folds on the borders of 

 the four contesting segments, or between them. More than that, 

 these seem to be regions in which gigantic hooks, loops, and spits 

 were built out from the continents, because they are regions in which 

 conflicting shore drifts, as well as drifts of a deeper sort, actuated 

 by the profounder currents of the great water bodies, prevailed. 

 All the lands that lie between the massive portions of North America 

 and South America, including Florida, Mexico, the Gulf of Cali- 

 fornia, Central America, the Isthmus of Panama and the Antillean 

 ridges, bear the aspect of gigantic hooks, loops, and spits formed in 

 the progress of the ages between the massive nuclei of the two Ameri- 

 can continents. A similar aspect is borne by the hooks, loops, and 

 spurs that connect southeast Asia with Australia. Less notable 

 ridges and bridges of a similar kind appear in other junction areas. 



Now if these are in reality constructive features of this kind, 

 built out from the primitive continental segments upon the adjacent 

 borders of abysmal segments, they cease to be typical features of 

 either continental or abysmal type; they are rather conjoint pro- 

 ducts, with dynamic habits of their own, and they are to be inter- 

 preted on the basis of their own idiosyncracies. 



' Chamberlin and Salisbury, Geology (1904), pp. 573-75. 



