Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/24

8 by the intrusion of the porphyry of the Cheviots; but, southward of that mountain range, they spread out and extend to the western border of the county. Sections may be seen in the affluents of North Tyne, in the Irthing, in the sources of the Reed, in burns flowing into the Aln; but more complete series appear along the coast from Alnmouth to Lammerton Sheal; and these, with the pit sections of Lowick, Scremerston, Sunderland, Eglingham, Shilbottle, &c., give pretty full information of the thickness, the succession, and the organic contents of the several beds. From such data I estimate the total thickness at 2600 feet. The predominant rock is sandstone, of which there are about 1400 feet; of shales there are 900 feet; of limestones 230 feet; and of coal about 70 feet. The sandstones are generally free gritstones made up of grains of quartz and felspar, with a little mica; and when in solid beds forming durable building stones. The shales are mud beds, in which alumina predominates, forming, when disintegrated, a tough clay sub-soil, several of which are loaded with carbonaceous matter; but there is no hard line between a sandstone and a shale, for slaty sandstones, by a larger admixture of argillaceous matter, become a kind of shale; and even some shales are so carbonaceous as to be combustible. The limestones are mostly tolerably pure carbonate of lime; but some beds become magnesian, especially when near to basaltic dikes. The limestones alone, however, do not give us the full measure of carbonate of lime distributed throughout this formation, for many of the shales are highly calcareous, and abound in marine organisms: one such bed at Howick is 15 feet in thickness.

The name, Mountain Limestone, is not physically descriptive of the Northumbrian series, for they contain no thick beds of limestone — none exceeding 30 feet; they form no great cliffs, nor rise to high elevations. In the Lowick district, where most of them crop out, they ascend to a height of only 300 or 400 feet above the sea level; some of the lower beds are seen, on the higher moorlands, at an elevation of 600 feet; and one bed appears, above the Plashets Coal, in North Tynedale, at a height of nearly 1000 feet. But even in these loftier positions they have no influence on the features of the country, which are