Page:The Habitat of the Eurypterida.djvu/189

Rh fossils are all mollusca and brachiopods, are numerous and wellpreserved, a fact not compatible with the reasoning of Macnair and Reid. As has been fully explained by Grabau, the main mass of the Bays represents an alluvial fan spread out on the land and having its western and southernmost margins extending into the sea. Thus it was possible for some of the highly oxidized sands to be carried out to sea, where they were deposited and where marine fossils were entombed with them. There is, therefore, nothing inherent in potential red deposits to prevent marine shells from being preserved in them; the difficulty lies in the fact that great thicknesses of potential red beds can not be deposited under conditions where it is possible for marine animals to leave their record, because such deposits must be formed on the land. As for the inimical effects of iron peroxide, it need only be stated that the reddest of deposits contain only a small amount of iron and that it is not the amount but the fineness and perfection of dissemination of the iron that are responsible for the color (Grabau, 87, 621). That sandstones made up of grains of pure silica are bad media for the preservation of molluscs is easily disproven, for one need only recall such highly fossiliferous formations as the Oriskany sandstone and the Schoharie grit of the Devonic of New York, or the Miocenic sands and conglomerates of the Vienna Basin. The reason why so many sandstones are unfossiliferous is generally that they were deposited as terrestrial sediments either fluviatile or eolian.

(b) Marine denudation. A second argument advanced by Macnair and Reid is based upon the assumption that the erosion of the Siluric rocks in the Highlands of Scotland was due to marine denudation and upon faulty observations at certain localities. They argue "The marine denudation of the Silurian rocks of the Highlands of Scotland is not in dispute, but Ramsay and Geikie have assumed a subsequent lake or fresh-water denudation." The conformable deposition, however "of the Old Red Sandstone upon the preceding Upper Silurian deposits in the counties of Edinburgh and Lanark, the Welsh area, and in the St. Lawrence basin, precludes any such idea; for from the base of the Upper Silurian to the top of the Lower Old Red sandstone the sequence of these deposits is unbroken. It therefore follows that the denudation of the rocks of the Highland area being marine, the equivalent deposits occurring in the Upper Silurian and Lower