FORCES AND PROCESSES IN OCEAN BASINS 47 



remains represented in the shield areas, appear to be absent from 

 the oceans. It is obviously of importance to determine what 

 happens to a continental or an oceanic structural feature when it 

 meets the continental edge. 



In many places, particularly in North America, the fold moun- 

 tains run parallel to the edge of the continent, but in Europe there 

 are several places where folding runs almost at right angles to 

 the edge. There is no topographic evidence for a continuation of 

 any of these features beyond the continental edge, and the same 

 is true of the Appalachian folding where it meets the sea to the 

 north of Newfoundland. It is possible that there are submarine 

 extensions of these lines of folding which are buried beneath the 

 sediments of the continental slope and abyssal plains. If so it 

 should be possible to discover them by magnetic and gravity 

 surveys. A start has been made by M. N. Hill and T. D. Allan, 

 w^ho are making a magnetic survey off the end of the Brittany 

 peninsular. The results of this and similar investigations are of 

 crucial importance since it is difficult to conceive that a continent 

 could be folded along an axis at right angles to the coast without 

 disturbing the sea floor on the seaward extension of the line of 

 folding. It is of course geometrically possible to imagine that the 

 north-south movement involved in the folding terminates on a 

 north-south transcurrent fault hidden beneath the continental 

 shelf, but the sinuous form of the west coast of Europe does not 

 suggest such a feature. 



The structural lines of the North Atlantic are, on the whole, 

 parallel to the continental edge (apart from the rather vague 

 ridge running from the Azores toward the south coast of Spain). 

 In the Pacific, however, there is a series of parallel fracture zones, 

 some running for over a thousand miles across the ocean floor and 

 approaching the west coast of the United States and Mexico 

 (Menard and Fisher, 1958). It is extremely striking that these 

 great features are not a continuation of any comparable features 

 on land. The Mendocino escarpment and the Murray fracture 

 zone may be connected in some way with the San Andreas fault 

 but they are certainly not continuations of it. It has been sug- 

 gested (Menard and Fisher, 1958) that the Clipperton fracture 



