the North East of Ireland » 215 



The facts stated in the introduction to this paper (see pages 123, 

 125.) are of such a nature as leave no doubt in the rnind 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 con- 

 struction 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 

 mountain chains ; th€ heights of the principal summits are in- 

 serted 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 be- 

 came 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 distin- 

 guished 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. 



