probably not very much more than 100 m.y. old, the age of the 

 oldest rocks yet dredged — and the theory of continental drift, 

 already strongly argued on palaeomagnetic grounds, begins to look 

 more plausible. Only drilling through the sedimentary layers at 

 several points in the deep ocean can finally solve this problem. 



NEW YORK 1959 TO MOSCOW 1966 



During the seven years between the First International 

 Oceanographic Congress in New York in 1959 and the second in 

 Moscow in 1966 the subject came of age. Existing techniques were 

 used especially to study the structure of the atypical areas, particu- 

 larly the mid-ocean ridges. New techniques included the develop- 

 ment of safe and reasonably cheap devices for continuous reflection 

 profiling (sparkers and air guns) with a resulting huge increase 

 in knowledge of structure within the sediments and of the relief 

 of the top of layer 2, the volcanic layer (Fig. 3). Also, it became 

 possible to measure gravity continuously from a surface ship. 

 Towards the end of the period the first deep offshore holes were 

 drilled to explore the geological structure of the B'ake Plateau, 

 the deepest in more than 500 fm. of water. The Blake Plateau, an 

 area of deep continental shelf separated from the rest of the shelf 

 off Florida by a 400 fm. scarp, was found to have been formed not 

 by faulting but by the non-deposition of the Tertiary strata due to 

 the erosional effects of the Gulf Stream; the Cretaceous rocks 

 beneath the scarp pass under it without disruption. 



Other advances during these seven years included the discovery 

 by Raff and Mason of the great pattern of magnetic anomaly 

 lineations running north-south off the west coast of the U.S.A. and 

 the recognition by Vacquier and others (see Bullard and Mason 

 in Hill, 1963) that this pattern has been displaced sideways by 

 distances as much as 600 miles along great transcurrent faults like 

 the Mendocino Escarpment which has been traced out almost one 

 third of the way across the Pacific. The publication of the third 

 volume of The Sea (Hill, 1963) summarising almost all our know- 

 ledge of the sea floor up to that time, of Menard's (1964) Marine 

 Geology of the Pacific, and of the Royal Society's discuss-'on 

 meetings on Continental Drift (Blackett, Bullard and Runcorn, 

 1965) and on the geological results of the International Indian 

 Ocean Expedition (Hill, 1966) also marked the consolidation of 

 knowledge. 



THE STATE OF THE ART 



It became clear, during the years between the two International 

 Oceanographic Congresses^ that the upper mantle is the key to the 

 geological process that affects the crust of the earth. At present 



3^ 



