Page:The Habitat of the Eurypterida.djvu/129

 were continuous at the time of deposition now appear in far separated localities. Over the whole of this much folded and faulted series the Old Red sandstones were deposited by the rivers flowing south from the northern Highlands. Subsequent erosion has carried away large portions of these Devonic beds, and has cut down into even the lower rocks, so that the Ordovicic and Siluric are exposed in broad belts as shown above, while in certain places only inliers in the Old Red have as yet been exposed. To this class belong the isolated outcrops in the

Pentland Hills and in Lanarkshire. It is thought that in all probability the Wenlock and Ludlow in those regions were continuous and extended southwest into Ayrshire and northeast into the Lothians.

. In the lowest Ordovicic, volcanic activities were pronounced in the Girvan area, but throughout the central and northern belts of the tableland open marine conditions prevailed, marked either by submarine volcanoes or by the accumulation of radiolarian ooze, but the presence of fossiliferous mudstones