Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/42

26 ', and '. The basaltic rocks yield a more peculiar and varied Flora.

The Cheviot porphyry is undoubtedly the oldest igneous rock in our district. It was protruded subsequently to the Cambro-Siluria era, for the Greywacke beds are highly inclined against it; but prior to the deposition of the Upper Old Red Sandstones, as the conglomerates are partly composed of rolled Cheviot porphyries. The Cheviots, however, had been further elevated subsequently to the Tuedian period, for beds of that age are highly inclined against the porphyry on the Coquet and in Akeld Burn.

2. On the boundary of Northumberland, near to Carham, there is another outbreak of igneous rock unconnected with the Cheviots, and of a later age, but of a porphyritic character, having a greenish claystone base resembling wacke, with crystals of red felspar. It crosses the Tweed at Carham, and the railway near to Reddon Burn, and then runs along Heddon ridge a distance of about 3 miles. It protrudes through beds of the Tuedian group; and near to it is the peculiar Magnesian Limestone with chert nodules.

The Basaltic Whin Sill is the most remarkable rock in the North of England on account of its long and tortuous range, and of its relation to, and effect upon, the strata it traverses, and among which it has been intruded. A sill it has been called, because sometimes seen, like a stratum, intercalated among stratified rocks. It is not, however, a true stratum, for its thickness varies very much, from 2 or 3 feet to more than 200 feet, and the parallelism of its upper and under surface is preserved only for a short distance; so that, though its extension in the line of direction is great, yet its extension in the line of dip is inconsiderable.

It is traceable through Northumberland, with breaks here and there, from Kyloe to Glenwhelt; and the same kind of rock extends southward into Durham, Cumberland, and Yorkshire.

Let us trace its range, and, as we go along, note any peculiarities. It first appears in the North at Kyloe Crags, in a fine