524 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



mud were forming, but were allochthonous — that is, transported from 

 without. They were carried here as dead organisms or quickly expired, 

 and their remains were rapidly covered by the constantly accumulating 

 lime mud, probably stirred up at the time these bodies arrived, and 

 quickly settling again and so entombing the remains. It is precisely in 

 this manner that I assume that the Eurypterids and Ceratiocarids of 

 the Bertie water-lime were buried, the fine lime mud being brought in 

 by the rivers which swept the Eurypterid remains basin-ward. 



If the Eurypterids of the Bertie and of the Hudson shales can be 

 satisfactorily explained as river animals swept out to sea, or into basins 

 where conditions of preservation were especially favorable, nothing fur- 

 ther need be said about the Eurypterids of the other horizons, both in 

 America and Europe, for those admit of ready explanation on the hy- 

 pothesis of a river origin. The occurrence of these remains in the lagoons 

 behind the Gotlandian barrier reefs, associated with scorpions as well as 

 marine organisms; their presence in the evident delta type of deposit of 

 Wenlock age in Scotland and northern England, and their absence from 

 the purely marine Wenlock deposits of western England point equally 

 strongly with their occurrence in the Old Eed deposits to a river habitat. 

 That such a habitat was the unquestioned one in the Carbonic only adds 

 the final links to the long chain of evidence which to an unprejudiced 

 mind must clearly demonstrate the fluviatile and perhaps lacustrine 

 habitat of life of these remarkable arthropods. 



It remains now to be shown that the peculiar sediments which formed 

 the water-limes are in harmony with this theory. The remarkably fine 

 and uniform character of the lime mud composing these sediments has 

 led some to assign to them a chemical origin. Against such an origin, 

 however, all its lithic and structural characters protest. The material 

 is clearly a clastic lime mud deposited in quiet and shallow water unin- 

 fluenced by waves. This is shown by the fine stratification and by the 

 mud cracks and other evidences of shallow water. The lime mud was 

 clearly not derived from organic sources, since nowhere in the series is 

 there an accumulation of organic lime in the form of reefs or shell heaps 

 capable of furnishing this fine mud. It is, furthermore, a demonstrable 

 fact that life conditions had not yet reappeared in any marked degree 

 in the region of former salt deposition. Under these circumstances it 

 must be apparent that the older limestones of the Siluric land were the 

 only possible source of the lime mud, unless some unknown factor enters 

 into the calculations here. That these limestones, the Xiagaran and 

 underlying Trentonian, were perfectly able to furnish the lime can not 

 be questioned. Disintegration of the exposed surfaces during the arid 



