Page:The Habitat of the Eurypterida.djvu/145

Rh show. Even in the Pentland Hills the base of the Wenlock is nowhere visible, the beds standing on end for the most part and sticking up through the Old Red Sandstone. In Lanarkshire, however, there occurs in the Lesmaghagow inlier only, below the Ludlow, a series of blue greywackes with shale partings which is 1300 feet thick and has proved unfossiliferous throughout except in one locality where a few specimens of Murchisonia (specifically undeterminable) and some doubtful forms called Orthis have been found. It has been thought by the Scottish Survey that part at least of this formation was Wenlock in age. I should like to offer the following purely theoretical suggestion. It has been shown that during Tarannon time rivers flowing from the Eastern Highlands carried down pebbles and boulders which were deposited in the Central Belt. Probably the whole of central and northern Scotland was above water then, and either subaerial erosion or deposition was going on. The 1300 feet of greywackes and shales below the Ludlow in Lanarkshire might represent in their lower part delta or torrential deposits accumulated during Tarannon time and in their later part similar deposits during Wenlock time. Their unfossiliferous character and great thickness would thus be accounted for. Future study in those rocks should be directed towards the search for cross-bedding, if any, and the type represented, for plant remains, tracks, and eurypterids. As the Wenlock sea advanced northwards—there is little reason to doubt that it did, for the same marine fossils are found in the Southern Belt and in the Pentland Hills—it reworked the Tarannon, and a basal sandstone, conglomerate or shale was formed, depending upon the nature of the Tarannon continent where the sea transgressed. Thus along the northern border of the Southern Belt the basal Wenlock consists of "greenish grey, flaggy grits, separated by grey shale bands, some of which are crowded with Crossopodia, Nemertites, and other tracks, resembling those found in the Hawick Rocks." On the Slitrig Water the shales and surfaces of the greywackes are crowded with tracks. These rocks pass conformably downwards into the Tarannon rocks of Hawick, indicating that the actual seashore at the close of Tarannon and beginning of Wenlock time must have been just about in this region. The unfossiliferous grits and greywackes (the first division of the Wenlock) appear along the northern border, while the second division occupies all of the rest of the belt to the south except for small patches or inliers in the extreme south where the third division of the Wenlock is seen; there are also inliers in various