44 HISTORY OF THE OCEANS 



synclines should be looked at again in the light of our new knowl- 

 edge of present day marine sediments; we can no longer assume 

 that a coarse, ripple-marked, marine sand was formed in shallow 

 water near shore. 



If a shelf is to be converted into a mountain range, a mecha- 

 nism is required to lower the Moho from about 15 km to 35 km. 

 Two suggestions have been made. If the material below the ]\Ioho 

 has roughly the composition of achondritic meteorites, the Moho 

 could be lowered by differentiation of this material into basalt 

 and a more basic rock. If the Moho represents a phase change, 

 the alterations in pressure and temperature due to sedimentation 

 and sinking could drive it to a greater depth. The solution of 

 these problems is of great importance, but it is unlikely to be 

 achieved till we have independent evidence as to the material 

 beneath the Moho. 



Submarine Vuicanism 



In most places on land, extrusive rocks make a minor contribu- 

 tion to the materials visible at the surface, and volcanos and la\a 

 flows are relatively uncommon topographic forms. Terrestrial 

 scenery is predominately the result of folding, tilting, and erosion. 

 In the deep ocean the great majority of topographic features are 

 either piles of lava or fault scarps. The extrusion of lava is there- 

 fore one ot the key processes in the development of the ocean 

 floor. Unfortunately we know almost nothing about what is going 

 on. 



The hard rocks of the ocean are more difficult to collect than 

 are the sediments. Dredging collects material that is lying on the 

 sea floor and it is often difficult to tell whether it has been broken 

 from rock in situ, is a locally derived boulder, or is an erratic. 

 The usual opinion is that any unexpected rock, and particularly 

 any piece of granite, gneiss, or schist, is an erratic, though there 

 is often no direct evidence that this is so. A further difiiculty is 

 that the rocks are often so decomposed as to be difficult to identify. 



The principal requirement is a method of obtaining cores of 

 hard rock long enough to penetrate surface weathering and to 

 give some certainty that a specimen is derived from rock in situ, 



