Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/49

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Dr. Berger on the Isle of Man.

it is ferruginous on the joints, but the colour of the basis is greenish grey, of a very fine grain, and full of cubic iron pyrites. It is a fact worth noticing, that the parts of the rock contiguous to the pyrites are whitish and earthy.

The last sort of clay slate mentioned as occurring between Slieau-y-Carnane and little Snei-feldt, has a violet reddish tinge. The structure is fine slaty, the texture more clayey; talcose linings impart to the spontaneous joints a glossy character. Its readier decomposition makes it a better support for vegetation. The mean specific gravity of the different sorts of clay slate enumerated here, is 2,757.

(b) Transition Rocks.

We pass from the clay slate formation to transition rocks through the most insensible gradations, and in this instance it is to be remarked, that it is the grey-wacke and not the limestone which forms the oldest member of the series.

Grey-Wacke.

In the Isle of Man this rock, as far as my observation has gone, never contains organic remains. It is farther to be observed that the tract of land occupied by the grey-wacke is less elevated generally speaking, than that appertaining to the clay slate. We can trace the grey-wacke all along the contour of the island, and, except those places where it slopes down gradually, and is outskirted by the flœtz-limestone receding into the sea, it forms a range of bold cliffs; at Maughold-head, Banks-how, Douglas'-head, Walberry-how, Spanish-head, the Calf of Man, Brada-head,