Page:The Habitat of the Eurypterida.djvu/116

 forming the Jurassic Plattenkalke of Solnhofen, is shown by the utter absence in this horizon or vicinity of reefs which could furnish such deposits, and again by the presence in the composition of the silica and alumina. In the Bertie the silica and the alumina is intimately mixed with the lime, as is shown by the relative constancy in composition and character of specimens from different parts of the formation. In the Plattenkalke of the Solnhofen, on the other hand, where the siliceous material represents the impure dust blown from the land, it is found in clayey layers (Fäulen) between the thin, bedded (Quicksteine) and thick bedded (Flinze) limestones, and not in intimate mixture with the other constituents, as is the case in the Bertie (293, 144, 209).

(2') The only remaining source of the deposit is the land, from which clastic material might be brought by the wind or by the rivers. If brought by the wind and deposited far enough from shore to be free from coarse material, the deposit would not have a circumscribed areal distribution. Such a restricted distribution is, however, possible if the material has been supplied by the rivers. If carried into the sea, it may be deposited in quiet water, and this may produce such a fine-grained rock as the waterlime, which is free from coarse clastics. Such regions of deposition would be found either far out at sea where all of the nearer-shore, coarser clastics were absent, or else near the shore, but in sheltered bays. If these river-borne muds were not carried into the sea, then they must have been deposited on land in the river flood-plains.

We may consider for a moment the possibility of this formation having been deposited at a sufficient distance from land to allow of the quiet accumulation of fine sediments, or else in sheltered areas along shore. Such deposits at the present time are represented by the blue or slate-colored muds, and these are the ones which are spread over the floors of shallow seas and out to the edge of the continental shelf. Murray and Renard (194) have estimated that these muds cover 14,500,000 square miles of the ocean floor. An average analysis shows the following composition: