MARINE LITTORAL 667 



water layer, of about 125 fathoms, has a less salinity and density 

 than the deeper water, being largely renewed by the fresh water 

 of the drainage. The heavier lower water is derived from the 

 Mediterranean by way of the richly saline Marmora and ^gean 

 seas, requiring about 1,700 years for its renewal. Vertical cur- 

 rents are slight on account of the greater density of the deeper 

 water, and hence these lower waters have not sufficient oxygen to 

 support animal life. Sulphuretted hydrogen is separated out, prob- 

 ably through the agency of sulphobacteria, in the deeper water ; be- 

 ginning at 100 fathoms, with 33 c.c. of H^S per 100 liters of water 

 until, at 500 fathoms, 570 c.c. of H.S per 100 liters of the water are 

 separated. Below this the increase is less rapid. As the H2S in- 

 creases, the sulphates of the water decrease, and carbonates and 

 FeS are separated out. 



The sediments of the Black Sea comprise sandy detritus to a 

 depth of 20 fathoms, below which occurs gray, blue, sticky mud 

 rich in Modiola phascolina to the lOO-fathom line. In the great 

 depths occurs a very fine, sticky, black mud with rich separation of 

 FeS, abundant remains of planktonic diatoms, and shells of young 

 pelecypods, which have descended to this depth on completion of 

 their early pelagic existence (mero-planktonic stage). Besides the 

 black is a dark-blue mud with less FeS, but a richer separation of 

 CaCOg in minute grains, this lime often constituting thin banks, 

 while pelagic diatoms are likewise abundant. Clarke (21 : 201) has 

 suggested the possible origin of some of the Upper Devonic black 

 shales of New York (Portage) under similar conditions, but this is 

 a much mooted question. Pompeckj (71 : ^j et seq) has interpreted 

 the black Posidonia bronni shales of the Jurassic of Regens- 

 burg, Bavaria, as deposits of this type. It is, however, difficult to 

 understand how such a region can furnish conditions for the per- 

 fect preservation of the saurians of that period, so that in some 

 cases even the skin of the animals is intact. 



4. The Ocean Littoral, or the Neritic Zone. This is the most 

 widely distributed littoral belt at the present time. Its deposits vary 

 considerably, according to distance from shore, depth of water and 

 relation to currents. In general a seaward gradation in the texture 

 of the material from coarse to fine is observable, sands being found 

 near shore (though often extending out for great distances), while 

 muds are formed further offshore and in specially protected jilaces. 

 In the Arabian Sea muds are carried from 700 to 800 miles from 

 land, owing to the nature of the ocean currents. Calcareous accu- 

 mulations are typical of this region, especially those formed around 

 coral reefs. These have been fully described above. 



