276 Twenhofel — Notes on Black Shale in the Making. 



fossils in hundreds of feet of shale — is usually in the basal foot or 

 two. Although there is no appreciable macroscopic difference 

 between the shale without marine fossils and the matrix of thin 

 bands crowded with them, it yet seems probable that the extinc- 

 tion of marine life is in many cases due to increased fouling of 

 the water by decaying vegetable matter. Marine life could have 

 existed only as long as the upper layers remained uncontami- 

 nated."* 



Ulricli further suggests " that their origin is in some manner 

 connected with cool temperatures. It is not that glacial cli- 

 mates prevailed at such time, but only that the average, or at 

 least occasional, temperature on the lands adjacent to the con- 

 tinental seas was too low to encourage the development of 

 normal littoral faunas. In other words, that the climates 

 prevailing at times and places of black shale deposition in con- 

 tinental seas were cool enough to render their shores inhospita- 

 ble to contemporaneous littoral and benthonic life" (loc. cit.). 

 He finds this last suggestion favored by evidence of cool cli- 

 mates having existed during several of the times of black shale 

 formation, but he further admits that the explanation is not so 

 satisfactory as one could wish. To any one familiar with the 

 littoral and benthonic life of northern latitudes the suggestion 

 altogether fails, since on some of the most bleak and inhospita- 

 ble shores of North America there is no paucity of either plant 

 or animal life in the shallow water zone. 



Ulrich's final conclusion is that " great depths and inclosed 

 conditions are seldom if ever essential factors in the origin of 

 black shales," and that none of the black shale deposits in 

 America is comparable in the matter of depth and inclosure of 

 waters in which they were laid down to the black muds in the 

 Black Sea today." and " probably the real cause, if there is 

 any that operated alike in all cases, remains to be discovered." 



These different views cannot be made to harmonize and 

 probably there is no need that they should, in that, in all 

 probability, black shale may have originated in each of the 

 different ways postulated ; but it does not follow that in these 

 ways only has black shale been formed. 



General Summary of Black Shale Characters. 



A general summary of the characters of this type of sedi- 

 ment gives : It appears in many cases to be confined to 

 troughs of great length and slight width ; it is rich in carbon 

 or hydrocarbons ; has the fossils preserved by carbon, pyrite or 

 marcasite; and, in general, shows evidence of quiet deposition, 

 but intraformational conglomerates are not infrequent. Up to 

 the time of the extinction of the group, the graptolites con- 

 tributed a considerable part of the organic remains now pre- 

 *Ulrich, Bull. Geol. Soc. Am., vol. xxii, 358, 1911. 



