The facts stated in the introduction to this paper (see pages 123, 125.) are of such a nature as leave no doubt in the mind of the writer, of the former continuity of the Irish and Scotch coasts.
The greatest depth of the channel between Rathlin and Antrim is 53 fathoms, and between the north-east of Ireland and south-west of Scotland 90 fathoms.
It now only remains to explain the principles followed in the construction of the geological maps accompanying this paper.
The larger of these comprehends the whole of the district which has been described, as it is limited, in the Introduction, page 121.
Much attention has been bestowed on the delineation of the mountains chains; the heights of the principal summits are inserted from Dr. Berger's measurements.
The more important soundings on the coast are added.
In a map on so small a scale it was not found practicable to distinguish every minute formation by different colours; it became therefore necessary to assume as the basis of the colouring, the prevailing rocks which, in association with other subordinate formations, constitute districts of considerable extent.
Thus the districts to which separate colours have been assigned, are
1. The granitic district of the Mourne mountains, &c. The hornblende or primitive trap rocks, on the border of this district, are distinguished by an appropriate mark.
2. The mica slate districts of Londonderry and Antrim, including primitive limestone, felspar, porphyry, and sienite, each distinguished by peculiar signs.
3. The transition district, encircling the Mourne mountains. The lead mines in this district are marked.
4. The shell limestone underlying the coal.
