Page:The Habitat of the Eurypterida.djvu/188

180 Upper Old Red sandstone was deposited over the whole of Scotland. In the words of the two authors mentioned above, "The great mass of this mountain chain, then, must have lain to the northwest of the present Old Red Sandstone area, and we now proceed to show how after this long period of upheaval the mountain mass once more began to sink below the level of the sea, and that gradually the waters of the Old Red Sandstone sea levelled it down to the very core" (159, 109). They consider that all of the deposits were made along shore, but they are then confronted by the problem of the lack of molluscs and other typical marine forms. This absence they thus account for: "The solution of the problem rather lies in the fact that the presence of peroxide of iron in these rocks is inimical to the preservation of fossils with a calcareous test, and that more especially in the case of sandstones, which even when composed of pure sand are well known to be a bad medium for the preservation of molluscan and other similar organic remains" (159, 116).

. Each of the two theories given can explain some facts which the other cannot; but, on the other hand, each has very serious faults due in some cases to incorrect observations, in others to the acceptance of prevalent ideas and in others to unjustifiable deductions. Both theories contain elements of truth, but both are open to many objections. These fall into two groups: (1) Physical, (2) Faunal.

(1) Physical, (a) Red color. Within the last twenty years students of sedimentation have clearly shown that it is impossible for a widespread and thick series of red clastic deposits to be laid down in the sea. The red color, as is well known, is due to the dehydration of sediments which were thoroughly oxidized at the time of deposition. Such oxidation cannot take place under water, but only during exposure to the air. It is not to be supposed that the beds were red when deposited, but that only after dehydration had taken place by the lapse of a long period of time, or through the effect of heat from the interior of the earth, or by pressure was the red color taken on. Of course, certain red beds may receive their final working over under water, but such deposits will be of limited thickness and areal extent. For instance, the Bays sandstone (Upper Ordovicic) of Tennessee, Virginia and adjoining regions is a red calcareous sandstone with a maximum thickness of 1500 feet. Throughout most of the formation organic remains are absent, but in the lower beds marine fossils occur abundantly in a few layers of the red sandstones. These