Twenhofel^-Notes on Black Shale in the Making. 275 



ing organic matter and sulphide of iron, frequently giving off 

 the odour of sulphuretted hydrogen, and assuming a brown or 

 red hue at the surface, owing to oxidation."* 



It is not, however, clear that the facts of stratigraphy are in 

 general agreement with the views expressed by Ruedemann, 

 since there is little universal evidence given by the graptolifer- 

 ous shales that they were deposited in waters so deep as exists 

 150 to 200 miles from the shore, although some of them may 

 have so originated. 



Among the latest papers dealing with black shale deposition 

 is that of Ulrich. The following extensive quotation from his 

 writings is given because it is about the best published pre- 

 sentation of the characters of black shales. Ulrich expresses 

 his views as follows : 



"The graptoliferous black shales of the Levis and Ouachita 

 troughs, in which there are thicker beds of such shale than any- 

 where else in America, prove as certainly as anything that inclosed 

 and stagnant conditions are not essential to black shale deposi- 

 tion. That most graptolites were pelagic in habit and passed 

 from one open basin into another solely by means of marine cur- 

 rents is universally accepted. They could not have entered a 

 continental basin except a marine current carried them in, and 

 there is no normal possibility of their transportation to the head 

 of a narrow bay. Consequently, when it is established that the 

 deposits in question are confined to narrow strips hundreds of 

 miles in length, it is at the same time proved that they were laid 

 down in channels open at both ends so as to give free passage and 

 egress to the graptolite-bearing currents. Marine thoroughfares 

 like these surely can not be called inclosed nor does it seem possi- 

 ble that they could have become stagnant. And the not infre- 

 quent occurrence of intraformational conglomerates in these 

 graptolite shales is almost conclusive proof that the channels were 

 not of unusual depth. 



" Obviously, black shale deposition took place under varying 

 conditions of depth and degrees of inclosure. We find similar 

 black muds forming today in the stagnant depths of an isolated 

 Black Sea, and in Paleozoic ages they were deposited in shallow 

 or perhaps comparatively deep channels with evidently perfect 

 circulation as well as broad shallow pans in which, except at 

 times w r hen they were abundantly peopled by certain kinds of 

 marine organisms, circulation may have been very sluggish and 

 imperfect. The vertical distribution of marine organisms in the 

 last suggests that the wide seas which filled the interior basins 

 with black shale may well have been stagnant during most of the 

 time in which such deposits were being laid down. Marine 

 faunas are never found generally distributed through the mass of 

 these black shales. They occur only in occasional thin seams, in 

 which, however, their remains are likely to be very numerous, 

 and the best of these — indeed it may be the only zone with such 



* Geikie, Text Book of Geology, vol. i, 582, 1903. 



