Page:The Habitat of the Eurypterida.djvu/191

Rh proves conclusively that the beds could not have been deposited by an advancing sea, as contended by Macnair and Reid, nor yet in a lake, as Geikie holds. It is not even necessary to point to the red color or to the absence of marine fossils; the thickness and coarseness of the deposits absolutely precludes the possibility of their having been formed in the sea. Macnair and Reid hold that the sea transgressed from the south to the north, but in that case, while there might well have been a basal conglomerate a few feet thick, this would inevitably have been succeeded vertically by finer deposits, sands at first and then muds or limestones as the water became deeper, and the zone of coarse near-shore deposits would have advanced pari passu with the transgression of the sea. Thus it would have been impossible for coarse material to have been deposited in southern Scotland in the Upper Devonic when the sea shore stood two hundred miles to the northwest. Greater obstacles arise if we attempt to have these deposits formed in lakes or epicontinental seas. In Forfarshire, the

position of "Lake Caledonia," the estimated thickness given by Hickling is 12,500 feet, including the volcanics, or considerably over 10,000 feet of clastic deposits; in Caithness Geikie estimates the series which he supposed to have been contemporaneously deposited in "Lake Orcadie" at 16,200 feet. These two lakes were separated by the Crystalline Highlands, a strip of land about 90 miles broad, which apparently supplied the sediments for Lake Orcadie. The waves of this great lake, which is estimated to have had at its maximum a surface of about 48,000 square miles, cut back into this old mountain chain which was at the same time being denuded by the rivers which brought their loads into the lake. In its maximum developed Lake Orcadie extended from Nairn to the Shetland Islands, the Orkneys representing a sublacustrine rise. The cross section made by Geikie is here reproduced in order to show his interpretation (fig. 18). It is at once apparent that there was not enough dry land to supply the thousands of feet of flagstones making up the Caithness series. It is even more difficult to surmise whence came