Page:The Habitat of the Eurypterida.djvu/117

 A comparison of this analysis with that of the Bertie shows that the two types of deposits are as different as could well be imagined, the deep sea mud having combined alumina and silica 77.75 per cent, as opposed to 28.98 per cent, while the combined CaO and MgO is 5.00 per cent as compared to 63.10 per cent in the waterlime. One cannot argue much, however, from this pronounced difference between the two types, because it must be borne in mind that in the late Siluric the greater portion of exposed land areas in northern and western North America was covered with limestones or dolomites and that in consequence the muds which accumulated far out to sea, and which were the finest particles derived by the erosion of those land surfaces, would of necessity have been high in calcium and magnesium, whereas the blue muds accumulating in our present oceans are derived from a great diversity of rocks in which the limestones form a very small part. Thus, while we can find no analogous mud deposit in modern oceans, we are not justified in saying that such a one might not have formed in the past under different conditions; and I can, therefore, see no characteristics in the chemical composition of the rock to preclude the possibility of its deposition at a considerable distance from land. We are not, however, lacking in another criterion when the physical characteristics fail to be restrictive; the type of fauna represented is the safest guide in the interpretation of ancient regions of deposition. There is no region where muds are accumulating in the sea today, whether near shore or farther from land, where an abundance of organic remains is not being included. Along the entire Atlantic coast of North America the muddy facies of the littoral zone swarms with life, and while many of the species are confined to that facies it certainly cannot be claimed that where muds are accumulating there is a paucity of plant and animal life. Detailed studies of restricted areas of the ocean floor have proved that a large and varied fauna flourishes even where muds pour in in great quantities from the land. Thus, Walther (295, 36) has found that the muds in the Bay of Naples contain a fauna of about 1120 species of invertebrates and fishes. The fauna of the Bertie contains not two dozen species and nearly all of these belong to one phylum and to one class in that phylum, namely, the merostomes. Such a fauna cannot be considered as marine in any sense, if we accept the principles for the criteria of fossil faunas, based upon the study of recent faunas (p. 67 above). It is characteristic of no portion of the sea-shore, bays, lagoons, or estuaries, nor of the open sea, whether in