Page:The Habitat of the Eurypterida.djvu/88

 recognized in ancient deltas by a lithological and faunal interfingering. The silts brought down by the river will contain the remains of the river fauna, while the submarine deposits, be they sandstones, shales or limestones will contain a marine fauna. The two types of deposits as well as the two types of faunas, though they interfinger, will be of a distinct and recognizable character as a rule, and the nature of the faunas, in regard to numbers of individuals and species, will be recognized by the characteristics listed on page 77 above. The deposits on the subaërial portions of the delta and laterally in the flood-plain areas will consist of fine silts. Along the river banks the coarsest of the silts will be deposited and with them the heavier organic remains if there are any, such as the shells of molluscs. But with the finer silts periodically spread out at times of flood far to either side of the river, will be carried only the lightest materials, probably only plant remains and the exoskeletons of the various fluviatile crustaceous animals. Such organic remains may be carried out in great numbers, and if quickly buried will be excellently preserved in the fine muds. On the other hand, if they are carried a long distance, dropped and exposed to the air, and later perhaps picked up by some distributary and carried on again, the process being often repeated, they may be broken up, and when they finally come to rest and are buried, not a single complete organism will remain. Indeed, if in their final resting place they are exposed to the air for a long time and the mud on which they lie becomes sun-cracked, the fragments, drying up, may be blown for great distances, perhaps far inland or perhaps out to sea, coming to rest at last in regions far removed from those in which the organisms had lived and there amidst a strange fauna the remains may be entombed. In the sediments such a history could be read, if in shales or waterlimes the only organic remains were those of light specific gravity. Of the invertebrates there would probably be some arthropods or insects; among the plants, leaves, algæ, reeds and grasses would be expected. Such a deposit would be difficult to correlate with a marine deposit, because it might contain none of the contemporaneous marine organisms. If, perchance, some of the river organisms or fragments of them had been blown or carried to sea, their remains could be entombed with the typical marine fauna and the age would thus be determinable. It is not unlikely, too, that stray molluscan shells might be blown from the shore