AND THE FORMATION OF VALLEYS. 99 



tion to the passes through the ridge south of the 

 valley of the Bearhaven mines, and Ballydonegan 

 Bay, the ridge having an average height of about 

 750 feet, while the nearly level floors of the passes 

 are about 30 feet above the sea, and a parallel pass, 

 a little farther west, is occupied by the strait that 

 separates Dursey Island from the mainland. 



If valleys are not connected with breaks in the 

 underlying rocks, how is it that they occur, in 

 regular systems, over large tracts of country ? 

 Examine any, but especially a contoured map of 

 Ireland, and it will be seen that the outlines 

 river- valleys, lake-basins, and bays occur in systems, 

 the general bearing of which may be indicated by 

 lines. If such systems are not caused by breaks in 

 the subjacent rocks, they must be due to chance, an 

 alternative that even the most sceptical among the 

 subserialists could scarcely insist on. From the map 

 of Ireland, it will be learned that the most con- 

 spicuous system is an east and west one, that forms 

 nearly parallel features ; two lines stretching across 

 the island, respectively from Gal way to Dublin Bay, 

 and from Clew to Dundalk Bays ; while to the north 

 are other nearly parallel breaks that only extend 

 eastward as far as the nearly north-and-south valley 

 in which Lough Neagh is situated ; while to the south- 

 ward a break runs along the valley of the Lower Shan- 

 non, and from that to the nearly north-and-south valley 



