DIRECTIONS or ICE-FLOW IN THE NORTH OF IRELAND. 827 



48. Directions of Ice-flow in the North of Ireland, as deter- 

 mined hy the Observations of the Geological Survey. Ey J. 

 R. KiLROE, Esq. (Read June 20, 1888.) 



(Communicated by Prof. E. Hull, RE.S., F.G.S.) 



The field observations of the Geological Survey, Ireland, being 

 completed, it was considered desirable by the Director to represent 

 those bearing on glacial phenomena in the northern half of the 

 country on a general map, scale ten miles to one inch. Data for 

 this were amply furnished by the one-inch sheets already published, 

 and by those in course of preparation, upon which the usual map- 

 indications of glacial striae are numerous and distinctly shown. 

 These indications consist of circles denoting the places of observation, 

 lines crossing those circles giving the trends of stride, and arrow- 

 heads attached ^thereto indicating the directions of ice-flow when 

 these are determinable. 



In carrying out this transfer a remarkable circumstance became 

 apparent, namely, that the striae were capable of being resolved into 

 two distinct sets, nearly at right angles to each other ; which 

 rendered it convenient to appropriate two copies of the general map 

 to the two sets of striae. The striae thus transferred have been further 

 connected by continuous lines about three eighths of an inch apart, or 

 have served as guides when the lines do not happen to pass through 

 the exact points of observation. So constant in direction are the 

 striae of each set over an area of some 16,000 square miles, that they 

 maintain an almost unswerving parallelism to those continuous, 

 though slightly waved lines. Comparatively few deviate more than 

 a few degrees to one side or the other of the general direction, 

 such deviation being adequately accounted for by local inequalities 

 of the surface. Of more than 600 recorded observations, the trend 

 of striae at some nine or ten points cannot be thus accounted for, and 

 they have been relegated to a third copy of the general map. They 

 are doubtless attributable to local ice-flows : and as an instance 

 may be cited evidence for a glacial moNQmeni southeastwardirom. the 

 Sperrin mountains of Londonderry, which at some period thus seem 

 to have been the centre of an independent glacial system. Probably 

 several other minor systems existed, evidences for which are now 

 wanting, or merge with those going to establish more extended 

 and general systems of glacial ion. 



Now, while the striae to the east of a line drawn from Strangford 

 Lough to Galway Bay all trend in one direction, two sets of striae 

 occur north-west of that line, and these generally at right angles 

 to each other, as above mentioned. This, it will be conceded, is an 

 interesting fact, and cannot be accounted for, as has been attempted, 

 by conceiving that the same general ice-flow could have produced 

 both. The remarkable absence of two sets south-east of the line 

 mentioned is perhaps the strongest argument against this supposition. 

 The sequel, too, makes manifest how unnecessary such a supposition 



