Page:The Habitat of the Eurypterida.djvu/87

 At a depth of about 400 feet below the surface, an abrupt change was observed in the character of the strata, which were composed in great part of sand, shingle, and boulders, the only fossils observed being the vertebrae of a crocodile, shell of a Trionyx, and fragments of wood very little altered, and similar to that buried in beds far above" (154, 281). This boring was very evidently through the subaërial portion of the delta, which was deposited at a time when the land stood higher and when, probably, hilly areas now removed by erosion or covered by deposits supplied coarser material near the seashore. The variability in the types of deposits is shown and it is seen that neither nodular limestones or conglomerates imply the presence of the sea for their formation. The sediments of the present delta are all fine-grained, the coarse deposits being found only at the foot of the mountains. Moreover, the fine sediments are carried far out to sea. "The sea, where the Ganges and Brahmapootra discharge their main stream at the flood season, only recovers its transparency at the distance of from 60 to 100 miles from the delta" (154, 279). In speaking of the Mississippi river Lyell says: "The prodigious quantity of wood annually drifted down by the Mississippi and its tributaries, is a subject of geological interest . . . . as illustrating the manner in which abundance of vegetable matter becomes, in the ordinary course of nature, imbedded in submarine and estuary deposits" (154, 268).

When the enormous transporting power of rivers is considered, when we think of the amount and variety of sediments together with terrestrial and fluviatile organic remains annually brought down to the sea by rivers there to be mixed with the marine sediments and the organisms living in the sea, we find it not so difficult to realize that the same phenomena happened in the past. The wonder would be if such intermingling had not taken place, and one must indeed be surprised to note how seldom it seems to have come to pass in the Palæozoic. Even admitting that land vegetation was mostly of a primitive, easily destructible, non-vascular nature in the Palæozoic, we still must marvel that so few fluviatile and terrestrial forms were carried out into marine deposits.

If the above characteristics are kept in mind it will not be difficult to formulate a certain number of criteria which may be used in recognizing a fossil delta or flood-plain deposit. That portion of the delta adjacent to the mouth of the river will be characterized by an alternation of marine and continental deposits, and these will be