Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/141



It should be added, that in the county of Londonderry waterworn fragments of basalt really occur in considerable quantity, particularly near the valley of the Roe.

It is impossible to dismiss the subject of denudation without remarking, that appearances analogous to those so forcibly described by Saussure as occurring in the Jura mountains, (where scattered fragments of the alpine rocks attest the subsequent excavation of the great valley now separating the two chains) are also presented, though on a much smaller scale, among the hills we have just examined, where rolled fragments of primitive rocks are often found on the summit of high basaltic ridges, which are at present cut off from all communication with the primitive districts by numerous intervening valleys. (See Sampson's Statistical Account of the County of Londonderry.)