A NEW FLOKA OF 



by the intrusion of the porphyry of the Cheviots ; but, southward 

 of that mountain range, they spread out and extend to the west- 

 ern 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 pre- 

 dominant 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 dis- 

 integrated, a tough clay sub-soil, several of which are loaded with 

 carbonaceous matter ; but there is no hard line between a sand- 

 stone 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 lime- 

 stones alone, however, do not give us the full measure of car- 

 bonate of lime distributed throughout this formation, for many 

 of the shales are highly calcareous, and abound in marine organ- 

 isms : 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 gi-eat 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 ap- 

 pears,- 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 



