306 Henry C. Stetson 



comes from within the orthogeosynchnes themselves, from fluctuating upwarps of 

 their floors, from volcanoes and igneous intrusions, and from the islands themselves. 

 The volcanic rocks within the sedimentary series seem to be restricted to what are 

 termed the eugeosynclines, while the miogeosynchnes contain only sediments. This 

 is in direct contrast to the views of those who consider the hypothetical continent of 

 Appalachia lying somewhere near the present 100 fathom Une the sole source. The 

 Dutch East Indies have been cited as a modern large-scale parallel to the paleo- 

 geography along the eastern seaboard and along the Gulf during the Palaeozoic. 



Temporarily at least, orogenic activity and vulcanism has ceased along the eastern 

 and Gulf coasts of the United States ; and the parallel geosynclinal belts, the Appala- 

 chian, the Ouachita and the Witchita systems, have long been quiescent. The net 

 result since Cretaceous time has been the construction of two " paraliageosynchnes " 

 according to Kay's terminology. These are hnear troughs of undeformed sediments 

 lying along the outer margins of the geosynchnal belts, and their seaward hmits form 

 the present continental slope. Obviously their strata, which in the case of the Atlantic 

 and Gulf are thickening seaward at the present shorehne, must thin as the outer 

 margins are reached, but there are few data on this point. 



Continental terraces like these obviously need an extensive hinterland to supply the 

 sediments if they are to reach any sizable proportions, and furthermore the drainage 

 must be into the ocean. These conditions are not always met, which may be the expla- 

 nation for our inability to find comparable structures built during the Palaeozoic. 

 KuENEN (1950) has speculated on what could have happened to them, reasoning that, 

 because of the much greater interval of time that was available for their building, 

 such structures would not be easy to hide. It well may be, however, that under mobile 

 geosynclinal conditions they never existed. 



Specifically, in the case which we are considering here, the two terraces are now 

 building on what are probably the bevelled surfaces of the rocks forming the outer 

 parts of the Palaeozoic eugeosyncHnes peneplaned after orogenic and plutonic activity 

 had ceased. They serve as models of the simple forms which will be constructed by 

 the processes of marine transportation and deposition when these operations are not 

 interfered with by activity in the mobile belts. 



The sediments, topography and structure of but very few submerged continental 

 areas are well enough known to warrant critical comment. Aside from the two 

 under discussion, we have only the areas off" Cahfornia, Norway, the West Indies, 

 and Indonesia, for which adequate bathymetric charts exist. Some oceanic areas 

 have been charted in considerable detail, but they have no bearing on this problem. 

 The East Indies have already been mentioned as a modern illustration of deposition 

 in eugeosynclines and miogeosynchnes. The West Indies present a somewhat analo- 

 gous case, but very little is known about the sediments. 



Off" California the sedimentary platforms have been extensively faulted and tecto- 

 nically broken up into numerous basins and ridges. Occasional granitic outcrops 

 have been found in the wafls of a few of the minor, nearshore submarine canyons, 

 but extensive rock dredging shows that they are only small intrusions and that the 

 continental terrace, hke that in the Atlantic, is composed of Cretaceous and Tertiary 

 sediments. Nothing is known about the basement. The continental slope along the 

 entire margin is in many cases controlled by long fault scarps of considerable dis- 

 placement (Shepard and Emery, 1941). The same is true of the walls of the deep 



