Page:The Habitat of the Eurypterida.djvu/190

182 Old Red Sandstone are equally marine" (160, 221). The chief objections to this theory of marine denudation continuing from the beginning of Upper Siluric to the end of Lower Devonic time, fall into three groups: (1) Tectonic. The tectonic relations between the Old Red sandstone and the underlying rocks show that there was profound folding at the end of the Siluric, followed by a long period of erosion before the earliest Old Red sediments were deposited; therefore the two series are not conformable as claimed by Macnair and Reid. (See further p. 173 above). (2) Lithologic. (See below, sections (d), p. 182, and (1), (2), (3) on p. 189). (3) Faunal. (See below, section (b), p. 191).

(c) Salt indicative of marine deposition. The argument that the presence of a salt-bearing stratum in the Old Red at one locality is undoubted evidence of the marine origin of that bed, is of no value unless supported by critical data on the chemical composition of the salt and associated salts if any are present, and on the organic content. Too much is now known concerning the continental origin of many and perhaps the larger number of past and present salt deposits for anyone to claim that the sea was always or even commonly the immediate source of the material. Macnair and Reid would make the presence of the salt band an a priori reason for its marine origin, for they say: "We find in the Moray Firth area a large stratum of yellow saliferous sandstone, interbedded with shales containing remains of Old Red sandstone fishes . . . . and we think that but one conclusion alone can be drawn therefrom—that the formation and its contained fish remains were marine" (160, 221). This type of reasoning is delightfully ingenuous and one that is met with frequently; while the authors do not explicitly state any reason why the salt is marine, the reader yet receives the impression that the presence of fish remains carries a strong presumption, and thus we have the pleasing circle: "The salt is marine because associated with fish, and the fish are marine because found in bands interbedded with salt-bearing sandstones." This whole argument would fall to the ground were anyone to show that the fish were fiuviatile, or that the salt could have some other origin.

(d) Thickness of deposits. The recurrence in the same place of thick boulder and pebble conglomerates interbedded with sandstone and shales, all being dominantly red and showing a complete absence of unequivocal marine fossils such as brachiopods, molluscs, crinoids, and trilobites, and amounting in thickness to many thousands of feet