1895. OCEANIC DEPOSITS ANCIENT AND MODERN. 273 



grains, for the distribution of which off-shore currents are indispen- 

 sable. At the present day, the coarse arenaceous Foraminifera are 

 found at depths rarely exceeding 400 fathoms, and the terrigenous 

 deposits bordering true continental areas (as, for example, from the 

 north of France to the south of Africa, or throughout the whole 

 South American coast-line) rarely extend to a distance from the 

 shore-line of inore than 200 miles. The depression having apparently 

 been very gradual in Cretaceous times, the coarsely arenaceous species 

 might have occurred in shallower water than at the present day if 

 the currents were weaker, or in deeper water if currents were 

 stronger. For our purpose, it is only necessary to prove that the 

 width of the border of terrigenous deposits was not less at that time 

 than it is at present. 



The deep-sea researches have shown that at points where the 

 continental areas meet the oceanic, there is to be found a band 

 composed of terrigenous deposits varying but little in breadth, and 

 rarely extending more than 100 miles from the present coasts. This 

 appears to be of almost universal occurrence, it having been traced 

 from the north of France to the south of Africa, and it differs but 

 little in width round Australia and South America. The evidence 

 to hand points rather to this band having been wider in Cretaceous 

 times, seeing that the Gault, occurring as a sandy deposit in the 

 Blackdown Hills in Devonshire, is not represented by limestones till 

 170 miles from that locality, viz., in the north of Norfolk. Similarly, 

 the Upper Greensand belt has a width of over 150 miles ere it becomes 

 merged in the limestones of the Hunstanton district. Further, if 

 we compare these with the width of the terrigenous belt in the Gulf 

 of Mexico, the resemblance between the past and present conditions 

 is very close. If, then, it be granted that the currents in those times 

 did not greatly differ in strength from those of the present day — 

 and, in my view, specific identity should point to similarity of 

 condition — is there any marked parallelism in the sequence of these 

 protozoan forms ? That is to say, is there any resemblance to be 

 observed between the upward stratigraphical sequence in foramini- 

 feral distribution, and the variation due to bathymetric change in 

 those species existing at the present day ? My own studies lead me 

 to the conclusion that such a resemblance not only exists, but is of a 

 most striking character. 



In the Chalk Marl and Lower Grey Chalk, the Textularian and 



Tritaxian fauna is very abundant ; but in the upper strata of the 



Lower Chalk a marked change takes place, only certain species — 



Textulavia aggliitinans and Haplophragmia — of finer texture being 



obtained, the Bulimines now playing the most important and 



prominent part. In the lower zones of the Middle Chalk all the more 



coarsely-tested forms have entirely disappeared, and only the minutest 



BuHmines — a most delicate little Textularia, and the ammonitoid- 



looking shells of Ammodiscns incertns — survive to represent the 



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