274 Twenhofel — Notes on Black Shale in the Making. 



there by the slow undertow from the higher ground. These bac- 

 teria in the transforming process deposit in the cells sulphur that 

 ultimately combines with the iron that is present and replaces the 

 calcareous skeletons of invertebrates by iron pyrite."* 



This explanation, though not widely different from that of 

 Clarke, does not postulate inclosed or relic seas with renewed 

 surface waters, nor waters of great depth ; but rather shallow 

 marine conditions and areas devoid of tidal and oceanic cur- 

 rents other than the undertow from higher to lower places. 

 Schuchert makes the additional suggestion that sargasso seas 

 may be places where black shales are depositing ; but as such 

 deep oceanic areas appear, so far as vegetal accumulations are 

 concerned, to have little basis in fact,f and certainly little 

 application in general stratigraphy, hypotheses looking to such 

 sources must await further investigation of their deposits. 



As graptolites are among the most characteristic fossils of 

 early Paleozoic black shales, it is but natural that Kuedemann 

 should be led to a consideration of the origin of the latter. 

 He takes exception to the hypothesis of an inclosed basin. He 

 states : 



" The black carbonaceous graptolite shales do not indicate con- 

 ditions of a nearly inclosed basin, such as is now exampled by 

 the Black Sea, for in the latter life exists only near the surface, 

 and the Axonophora, at least, quite surely lived in the more 

 quiet depths, nor would in such a basin be found the great mass 

 of floating seaweed to support the Axonolipa. Many different 

 graptolite zones occur, as a rule, in a small thickness of rock, but 

 sometimes they are also imbedded in coarser sediments. The 

 most essential requisite for the formation of black, fine-grained 

 graptolite shales is, therefore, not the depth, but the tranquillity 

 of the water. The graptolite shales, therefore, indicate a zone 

 between the agitated water, where coarser sediments are deposited, 

 and the dead or currentless water of the deeper sea. Their longi- 

 tudinal distribution, then, also indicates the direction of a coast 

 line, which has to be sought on the farther side of a parallel band 

 of coarser littoral sediments, and two such flanking littoral bands 

 may be looked for in narrow channels like the Levis channel."J 



Deep-sea dredging apparently supports Ruedemann's conten- 

 tion, for " During the course of the voyage of the Challenger, 

 the approach to the land could always be foretold from the 

 character of the bottom even at distances of 150 and 200 miles. 

 The deposits were found to consist of blue and green muds 

 derived from the degradation of the older ciwstalline rocks. 

 The blue or dark slate-coloured mud takes its colour from decay- 



* Schuchert, Pop. Sci. Monthly, 598, June, 1910. 



f Stevenson, Science, Dec. 9, 1910, 841. 



iEuedemann, Bull. Geol. Soc. Am., vol. xxii, 234, 1911. 



