Page:The Habitat of the Eurypterida.djvu/104

 siliceous sediments could come, and the characteristics of the sediments just noted clearly point to that continent as the source.

It will probably be readily accepted that the Normanskill and Schenectady are of terrigenous origin, especially since they are several thousand feet thick, but the point which is difficult of determination, is the origin of the fauna of these formations. That the sediments were fluviatile does not at all imply that the organisms in those sediments were also fluviatile. Indeed, it is usually argued that the presence of graptolites and "sea-weeds" in the same beds with the eurypterids is ample proof that all these types of life were marine and that they lived in the littoral zone in the sandy and muddy facies. First, in regard to the "sea-weed" Sphenophycus latifolius, there is no reason that I know of why such plant remains could not have been washed in from the land or might not have been living in the rivers, and have thus been swept into the sea. Secondly, it is evident that the presence of graptolites does not indicate deep sea conditions of quiet sedimentation as so often stated. Certainly, there is nothing incompatible with the assumption that the graptolites were spread out on mud flats, or river flood-plains as modern hydroids are, when washed in by the sea. At least the possibility must be granted that the pelagic graptolites would after death be more likely to float near or on the surface of the water until thoroughly decayed and disintegrated, rather than sink to the bottom, and be buried by sediments. In such a case, their only chance for preservation would be through stranding upon some surface where they could be quickly entombed by layers of mud or sand. This line of argument has only just been propounded by Professor Grabau, and while heretical it yet explains many curious occurrences. Thus, although no definite statement can be made at present regarding the precise habitat of the graptolites, we may consider that it is reasonable to assume that their remains are chiefly found in formations accumulating near land especially on delta surfaces. If the graptolite-bearing beds are thin we may suppose that they were formed by frequent inundations from the sea, but when 1500 feet thick, as is the case of the Schenectady beds throughout the entire thickness of which graptolites occur at intervals, then the only interpretation is that the beds were a series of flood-plain and delta deposits, mostly above sea-level, and that the graptolites were stranded on the low-lying land areas by periodic incursions