272 Twenhofel — Notes on Black Shale in the Making. 



Art. XXL — Notes on Black Shale in the Making ; by 

 W. H. Twenhofel. 



Much has been written relating to the origin of black shales, 

 bat, judging from the divergence of published opinion, no 

 hypothesis has gained a general acceptance. Since this type 

 of deposit occurs at many levels in the geologic column, has 

 often a wide horizontal distribution and is frequently of great 

 thickness, has a fairly definitely assured economic future and 

 has been given an important correlative value, it has furnished 

 the theme of a considerable amount of discussion. It is in the 

 hope of throwing light on the problem of origin that the writer 

 wishes to place on record some observations of black shales in 

 the making and to draw some conclusions from the observa- 

 tions. The locality studied is in Russia on the east shores of 

 the Baltic, and the notes were made during the summer of 

 1914 while the writer was a member of the Shaler Memorial 

 Expedition to the Baltic Provinces. Before presenting these, 

 it is thought desirable to give a brief review of the more 

 important modes of origin which have been suggested. Also, 

 as a preliminary for discussion, a brief summary of the general 

 characters of black shales will be given. 



Hypotheses of Origin of Black Shales. 



Doctor John M. Clarke's studies of the Naples and G-enesee 

 shales of New York led him to the conclusion that the former 

 and the black bands in the latter " are to be probably con- 

 sidered as pointing to deposition in deep water."* He was 

 largely led to this conclusion by the observations of Andrussow 

 on the conditions of the water, character of the sedimentation 

 and bionomic conditions existing in the Black Sea.f The 

 latter author, in brief, makes the following statement : 



"The Black Sea has a superficial water layer of about 125 

 fathoms, of less salinity and density than the water in the depths. 

 The yearly increment of surface water is due in great part to the 

 ingress of fresh water. The heavier deep water is derived from 

 a lower current coming from the Mediterranean by way of the 

 richly saline Marmora and iEgean seas, and requires about 1700 

 years for its removal. In consequence of the salinity and density 

 of the deep water, the Black Sea shows only slight evidence of 

 vertical currents. It is apparent only to a depth of 125 fathoms, 

 a,nd only to this depth, therefore, is there sufficient O for the 



* Clarke, Bull. 6, New York State Museum, 1913, 199-201. 



f Andrussow, Guide des excursions du 7 Congres geolog. internat.. No. 27. 



