OSCILLATORY CHARACTER OF CONTINENTAL SEAS 359 



ages they were deposited in shallow or perhaps comparatively deep chan- 

 nels with evidently perfect circulation as well as in broad shallow pans in 

 which, except at times when they were abundantly peopled by certain 

 kinds of marine animals, circulation may have been very sluggish and 

 imperfect. The vertical distribution of the marine organisms in the last 

 suggests that the wide seas which filled the shallow 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 gen- 

 erally 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 fossils in hundreds of feet of shale — is usually in the 

 basal foot or two. Although there is no appreciable macroscopic differ- 

 ence between the shale without marine fossils and the matrix of the thin 

 bands crowded with them, it yet seems probable that the extinction of 

 marine life is in many cases due to increased fouling of the water by de- 

 caying vegetable matter. Marine life could have existed in these waters 

 only as long as the upper layers remained uncontaminated. 



Vegetable fouling of at least the post-Silurian instances of inland black 

 shale seas — whether acquatic or terrestrial, or both, in origin is not of 

 immediate importance — almost certainly prevailed. Similar fouling in 

 the earlier cases is less probable but not impossible. That it did occur 

 as a rule is assumed without argument except to say that something of 

 the kind is required in accounting for the high percentage of car- 

 bonaceous matter in these shales. This condition also explains the com- 

 mon absence of marine shells better than does the suggested dissolution 

 of the calcareous shells in the deep, acidulated waters. But it does not 

 account for the absence of littoral faunas along the shores, since here the 

 waters were subjected to sufficient agitation to keep them fit for the 

 existence of such life. The fine conglomerates and beach sands which 

 occur at the base of the overlapping Chattanooga shale in central Ten- 

 nessee and in northern Arkansas clearly prove the presence, at least 

 locally, of a littoral zone that should have been admirably adapted to near 

 shore faunas. But excepting the hard jaws of errant annelids and an 

 occasional fish bone, there is no sign of contemporaneous strictly ben- 

 thonic life. 



Cool climates a possible cause. — Depending on such evidence, T have 

 been led to the conviction that great depths and inclosed conditions 

 are seldom if ever essential factors in the origin of black shales. In 

 casting about for a more generally applicable explanation the thought 



