GEOGRAPHICAL SEDIMENTS 45 



(4) Globigerina ooze over diatom ooze ; and 



(5) Diatom ooze over blue mud. 



On the whole the evidence is in favour of a subsidence of 

 areas of the ocean floor. 



A final point for consideration is the resemblance between 



OCEANIC DEPOSITS AND GEOGRAPHICAL SEDIMENTS. 



Comparisons have been made between certain geological 

 sedimentary deposits and the various types of marine sedi- 

 ments. All oceanographers agree that it is only the littoral 

 and terrigenous deposits which are comparable to geological 

 sediments of all geological ages. The true deep-sea or 

 eupelagic de-posits of marine and cosmic origin are generally 

 considered to be unrepresented in the geological strata. The 

 detailed consideration of these comparisons will be found in 

 textbooks of geology. There are, of course, numerous sedi- 

 mentary strata which have been unquestionably formed by 

 deposition in marine areas for instance, the Jurassic strata. 

 But these were not deposited or formed in abyssal regions. 

 The only modern sediment which can be compared with a 

 deep-sea deposit is the chalk. The comparison was made by 

 Huxley in 1858, who stated the globigerina ooze to be a modern 

 chalk. A detailed comparison, however, does not support this 

 view ; for instance, the chief genera of the foraminifera found 

 in the chalk are not pelagic, but bottom-living shallow-water 

 forms. The most abundant foraminifer of the chalk, Textu- 

 laria globulosa, is found in the Dee estuary near Chester. 



The scarcity, or, according to some authorities, the absence, 

 of abyssal or deep-sea deposits in the sedimentary continental 

 rocks has lead to a theory of the 



PERMANENCE OF THE OCEANS. 



According to this theory the ocean areas have been 

 permanent from remote geological epochs. The consideration 

 of the various theories and views from one extreme to the 



