Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/45

Rh in the river, and is there overlaid by a granular white limestone. At High Force and Cauldron Snout the river cuts through it, and tumbles over cliffs, exposing a limestone below, which is metamorphosed, white and crystalline. This basalt extends nearly as far as Middleton, but in the lower part, at a little distance from the river.

Throughout its long range the rock is essentially the same, being composed of felspar and augite; the iron entering into its composition is in the state of a protoxide, and indeed occasionally, as at the Farne, it possesses polarity. At Budle it is amygdaloidal; and at Ratcheugh some portions are porphyritic, having large felspar crystals scattered through it. Where in great mass the rock has a pillared structure, the columns being rude prisms, irregularly jointed; and some even approach the hexagonal forms seen in Fingal's Cave. They are grand and impressive objects, massive, though rude; and, towering majestically to a great height, we could imagine they had been piled up by the fabulous giants of the olden time.

The metamorphic action of the basalt on the strata above, as well as on those below it, is favourable to the view, that the Basaltic Sill is a lateral dyke intruded among the strata after their deposition; and the displacements of strata effected by the eruption lead to the same conclusion. At the Farne Islands, 90 feet of limestones, shales, and sandstones have been torn from the mass with which they were originally connected, and are lifted up and altered in structure. At Howick, too, there are evidences of violent mechanical action; and near to Little Mill there is a marked instance of the same character—for on the western or basset side of the basalt, limestone and shales are highly inclined against it: their dip is to the south-west from 60° to 45°, and the upper shale-beds are bent and thrown over; blocks of limestone, too, are enveloped in the basalt, and metamorphosed and penetrated with veins of the igneous rock, the whole being firmly welded, as it were, into one mass. Though in Northumberland this sill never appears anywhere, excepting in the calcareous division of the Mountain Limestone, and generally among its upper beds, yet its position varies considerably