Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/188



The above table will convey an accurate idea of the general configuration of the circumference of the basaltic area.

The aggregate mass of all the formations constituting the third system of mountains, appears to form a species of basin of which the lowest point is situated near the center of the valley of the Ban; hence they rise, towards the south-east, where they lean against the exterior chains of the Down mountains; towards the north-east, where the primitive rocks of Cushleak emerge; and towards the south-west where they abut on the great central chain of Londonderry. In this last direction, all the strata attain their greatest height, the chalk standing in Slieve Gallion at 1460 feet and upwards, and the basalt in Cragnashoack at 1864 feet. It is rather remarkable that the cap of basalt grows gradually thinner in proceeding towards the same point. On the north-east edge of this basin, the inferior limit of the basalt, or its junction with the chalk, is 500 feet lower, but the thickness of the cap of trap, increasing in the inverse ratio of this diminution of level, raises the summit of Knocklead till it nearly rivals Cragnashoack. In the south-east border, at Cave hill, the line of junction is about 200 feet lower than in Knocklead; the cap of trap is there very thin, but in the adjacent summit of Divis nearly equals in thickness that which covers Knocklead. At Divis the line of junction is depressed 200 feet below the level which it occupies at Cave hill.

The high chains which form the borders of this basin on the east and west, present a far less rapid slope towards its interior than towards its circumference.

Thus between the towns of Belfast and Antrim, the watershed or highest point of the road which there crosses the eastern chain, is situated three miles and a half from Belfast, and eight miles and a half from Antrim., at the elevation of 997$$\scriptstyle \frac 12$$ feet above the level of