Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/18

2 The two counties have an area of 2905 square miles. The surface of the district rises from the sea coast to the westward, as far as the elevated ranges, forming the backbone of England, and the dividing ridge between the eastern and western coasts; and hence all the rivers and streams flow eastward into the German ocean. The western boundary, on the northern part of Northumberland, is formed by the porphyritic range of the Cheviots, the highest elevation being 2676 feet; and further southward by sandstone hills and ridges from 500 feet to 1809 feet in height. Southward of the Tyne, the whole slopes away from the Penine range, whose culminating point, Cross Fell, 2901 feet above the sea level, is but a short distance beyond the district. The physical aspect is therefore very varied : along the coast are some broad plains ; further inland are low rolling hills, and then long broad high ridges, expanding into dreary moorlands; but on the western borders there are, in some parts, fine conical smooth hills, and in others high rugged cliffs. A few wide valleys there are through which larger rivers flow ; but the burns and waters wend their way from the hills through contracted vales and narrow picturesque denes. This inequality of surface is of itself favourable to the production of a varied Flora; and these conditions have been made more favourable still by the variety of the rock formations ; for not only are there series of strata, differing in mineral character and disintegrating capability, from the old Greywacke up to the New Eed Sandstone, but also a number of igneous rocks in mountain ranges, in overlying masses, and in lateral and vertical dikes. In describing the geology of this district I shall begin with the oldest sedimentary strata, and follow the succession of formations up to recent deposits, and then notice the various igneous rocks. The whole are arranged in the following table: —