358 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



from the Gulf of Mexico and spread in shallow continental basins to cen- 

 tral Oklahoma on the west and northern Ohio in the opposite direction, 

 or in the case of the similarly distributed Pottsvillian black shales, which 

 are widely distributed in the Mississippi Valley. 



Black shale faunas. — In all cases of typical black shales the faunas are 

 strikingly uniform. Littoral and benthonic faunas, of the usual warm 

 water facies, are practically absent. Of brachiopods we see only the phos- 

 phatic inarticulate shells, such as Lingula and Discina, ubiquitous and 

 evidently hardy types that seem to be less susceptible to changes in tem- 

 perature and character of bottom and water than most other marine or- 

 ganisms. The mollusca are all thin-shelled, and as a rule depauperate, 

 and among them the pelecypods are mostly byssiferous forms that are as 

 likely to attach themselves to floating as to fixed objects. In short the 

 general aspect and composition of black, non-calcareous shale faunas is 

 very different from that of the limestones and calcareous shales, whether 

 blue, green, or black, that are found in the same areas. The latter faunas 

 comprise chiefly species dependent on mild temperatures, shallow depths, 

 and favorable bottom conditions and shorelines for their existence and 

 migration; the former, on the contrary, include few or no species so 

 limited. 



The graptolitiferous black shales of the Levis, Athens, and Ouachita 

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

 else in America, prove as certainly as anything may be established by 

 faunal evidence that inclosed and stagnant conditions are not essential in 

 black shale deposition. That most graptolites were pelagic in habitat and 

 passed from one ocean 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. Conse- 

 quently, 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 possible 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 



