Twenhofel — Notes on Black Shale in the Making. 273 



support of animal life. The deep water, fed only by the under- 

 current, which on account of the high specific gravity due to its 

 salinity, does not mix with the surface water, has insufficient O 

 for animal life. At a depth of about 100 fathoms the separa- 

 tion of H 2 S is observable. 



" The constant, specifically lighter surface lawyer over the 

 heavier, richly saline deep water, the lack of O and the separation 

 of H 2 S in the depths, thus condition in the Black Sea its peculiar 

 bionomic character, the absence of benthonic animals below the 

 100-fathom line. In the littoral and shallow water zone benthonic 

 life is present." 



"The sediments of the Black Sea are : (1) in the littoral zone 

 and to a depth of about 20 fathoms, accumulations of sandy detri- 

 tus ; (2) to the 100-fathom line, gray blue, sticky mud, from 

 35-100 fathoms, rich in Modiola phaseolina, etc.; (3) in the great 

 depths the bottom is covered with (a) very fine, sticky, black 

 mud with rich separation of FeS, abundant remains of plankton, 

 diatoms and with fragments of quite young lamellibranchs (early 

 stages and widely scattered plankton forms), (b) dark blue mud ; 

 FeS is here in less measure, but in richer quantity are separations 

 of minutely grained CaC0 3 , making thin banks ; skeletons of 

 pelagic diatoms are also abundant."* 



This hypothesis probably explains the origin of some black 

 shales, but it does not well accord with the fact that many 

 shales of this type bear evidence of deposition in the shallows, 

 and that many are confined to troughs of considerable length, 

 but little width. The abundant presence of widely distributed 

 pelagic animals shows that the troughs were connected with 

 the great oceans. Furthermore, the separation of iron sul- 

 phides is not an evidence of deep water, since Geikie states 

 that " Besides occurring in deep water iron-disulphide is met 

 with in many shallow seas, and on some coasts it cements sand, 

 gravel and shells into a coherent mass."f 



Schuchert is another student who has written on the origin 

 of this class of sediment. According to him : 



" It is probable . . . that black shales having wide distribution 

 were more often deposits in closed arms of the sea (cul de sacs), 

 or when of small areal extent, as the result of filling of holes in the 

 sea bottom. In all such places there is defective circulation and 

 lack of oxygen resulting in foul asphyxiating bottoms. 



"These are the 'halistas' of Walther and the 'dead grounds' 

 of Johnstone. To-day such are the Black Sea and the Bay of 

 Kiel ; where sulphur bacteria abound in great profusion. These 

 decompose the dead organisms that rain from the photic region 

 into such suffocating areas, or the carcasses which are drawn 



* Quoted by Clarke, loc. cit. 



■f Geikie, Text-book of Geology, vol. i, 582, 1903. 



