Page:The Habitat of the Eurypterida.djvu/187

Rh of fish obtained from the head waters of the four rivers, five were found by Mr. Cope on both sides of the water-shed. There is likewise considerable disparity in the genera represented in the different rivers. The still more important barrier of the Rocky Mountains separates ichthyological areas yet more sharply marked off from each other. Such isolated basins as Lake Baikal, Lake Titicaca, and the Caspian Sea show by their peculiar assemblages of fishes how much ichthyic types may be modified by prolonged isolation. The differences, therefore, between the fauna of Lake Orcadie and Lake Caledonia during the Old Red Sandstone, as I venture to hold, are not incompatible with the idea that the two lakes were in a general and geological sense contemporaneous, though separated from each other by the barrier of the Grampian Mountains, which formed an effectual boundary between two ichthyic faunas" (71, 364, 365).

Deposition in the Sea. To certain geologists it will appear that I am wasting paper in setting forth a theory which has as its thesis the deposition of the Old Red sandstone in the sea, and that it is a further useless expenditure of ink and of the reader's time for me to voice the objections to such a theory. Indeed, I would agree with anyone who raised such a protest were it not for the deplorable fact that there are still not a few geologists who claim that this much-talked-of red sandstone was deposited in the sea, and further that other sandstones with similar striking lithological and faunal characteristics could have been formed nowhere else but in that region where all sediments have been deposited since the world began, namely, in the littoral zone of the sea.

The chief advocates for the theory of marine deposition are Macnair and Reid who brought out two papers in the Geological Magazine for 1896, one entitled "On the Physical Conditions under which the Old Red Sandstone of Scotland Was Deposited" (159), and the other "Palaeontological Considerations on the Old Red Sandstone of Scotland," (160), in which they sought to prove that physical, stratigraphical and palaeontological evidence all pointed to the marine origin of the Old Red. In a few words their interpretation may be summarized: in pre-Devonic time there was a large land-mass to the northwest of Scotland which supplied the material for much of the marine deposits during Cambric, Ordovicic and Siluric time. At the end of the Siluric the sea began to transgress across Scotland and the land-mass was at the same time depressed until by sinking and by marine erosion the whole area disappeared beneath the sea and the