74 HISTORY OF THE OCEAXS 



foram in i feral limestone pre\'iously discussed, indicates that the 

 simplest supposition is that the high-velocity lower layers in the 

 Pacific are composed of limestone lithified from oozes deposited 

 during former times of calcareous deposition and now covered by 

 unlithified calcareous ooze and red clay. 



All previously noted alternative explanations for a thin veneer 

 of sediment o^■er high-velocity layers are possible and should be 

 accorded proper consideration. Certainly in many areas, especially 

 those adjacent to islands and seamounts, the best explanation for 

 the high-velocity second layer is undoubtedly afforded by layers 

 of lava. The section is undoubtedly complicated in many areas by 

 fractured zones, pyroclastics, dikes, and sills; and metamorphism 

 of sediments probably plays a part. 



I should like to propose an alternative pre\'iously discussed. As 

 a background, imagine the situation which might arise in an ex- 

 tremely ancient ocean basin which dates back at least into the 

 Paleozoic or pre- Paleozoic. In an area where calcareous deposition 

 is now taking place, the original deposits could have been clay, 

 now lithified into clay shale (Fig. 10). During the Cretaceous, 

 planktonic Foraminifera evolved and, together with coccolith 

 ooze, began to form calcareous deposits which have been con- 

 tinuous to the present day. At depths of several hundred meters 

 in the sediment this material, through pressure-induced chemical 

 changes, has lithified into limestone of high velocity. The underly- 

 ing clay shale would, of course, not be detectable by refraction 

 surveys because it has a lower xelocity than the material above. 



In an area in which clay is now being deposited, as in the north- 

 east Pacific, the first deposits could again have been clay, now 

 lithified into clay shale. Above this material, during the Creta- 

 ceous, calcareous deposition began, but ceased sometime during 

 the Tertiary under the influence of cooling bottom waters, as 

 warm isotherms moved south. Geologic history afiirms that these 

 climatic changes actually occurred. Red clay deposition then con- 

 tinued until the present. At depths of several hundred meters in 

 the sediment the calcareous material lithified into limestone 

 (Fig. 10) ._ 



There is no reason to suppose that the material lithified at the 



