GEOGRAPHICAL SEDIMENTS 45 



(4) Globigerina ooze over diatom ooze ; and 



(5) Diatom ooze over blue mud. 



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

 areas of the océan floor. 



A final point for considération is the resemblance between 



OCEANIC DePOSITS AND GeOGRAPHICAL SeDIMENTS. 



Comparisons hâve been made between certain geological 

 sedimentary deposits and the varions types of marine sédi- 

 ments. Ail oceanographers agrée that it is only the littoral 

 and terrigenous deposits which are comparable to geological 

 sédiments of ail geological âges. The true deep-sea or 

 eupelagic deposits of marine and cosmic origin are generally 

 considered to be unrepresented in the geological strata. The 

 detailed considération of thèse comparisons will be found in 

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

 mentary strata which hâve been unquestionably formed by 

 déposition in marine areas — for instance, the Jurassic strata. 

 But thèse were not deposited or formed in abyssal régions. 

 The only modem sédiment 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 modem 

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

 view ; for instance, the chief gênera 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 glohulosa, 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 Océans. 



According to this theory the océan areas hâve been 

 permanent from remote geological epochs. The considération 

 of the varions théories — and views from one extrême to the 



