6 



Prof. J. Prestwich. On the Origin 



[May 1, 



where Qi is the biaxal harmonic of the ith. degree, whose pole is in OP, 

 and which may therefore be expressed in the nsual way, according to 

 Laplace's addition formula. The differentiations are then easily 

 performed. 



III. " On the Origin of the Parallel Roads of Lochaber, and their 

 bearing on other Phenomena of the Glacial Period." By 

 Joseph Prestwich, M.A., F.R.S., F.G.S., &c, Professor of 

 Geology in the University of Oxford. Received March 27, 

 1879. 



(Abstract.) 



Of the various hypotheses that have been brought forward since the 

 time of Macculloch and Dick-Lauder to account for the origin of the 

 Parallel Roads of Glen Roy, the one so ably propounded by 

 Mr. Jamieson, in 1863, has been most generally received and adopted. 

 It is a modification of the views originally expressed by Agassiz, to 

 the effect that the barriers of the lakes, — to the shore action of which 

 both the above-named geologists attributed the "roads," but were at 

 a loss to account both for the formation and removal of barriers, — 

 had been formed during the glacial period by glaciers issuing from 

 Glen Treig and Glen Arkaig-, supplemented by others from Ben Nevis. 

 The subsequent determination, by the Scotch geologists, of an inter- 

 mediate milder period succeeded by a second cold period, led 

 Mr. Jamieson, with whom the preglacial and glacial deposits of 

 Scotland had been a subject of especial investigation, to conclude that 

 the extension of these two glaciers took place during the second cold 

 period, which he thinks Was of little less intensity than the first, and 

 that, while the glacier from Glen Arkaig blocked up Glen Gluoy, the 

 glacier from Glen Treig formed a barrier to Glen Roy. He observes, 

 " Grant, then, these two ice-streams, one in the Great Caledonian 

 Valley and the other at Glen Treig, and the problem of the Parallel 

 Roads can be solved, provided we allow that glaciers have the power 

 to dam such deep bodies of water as must have occupied Glen Gluoy 

 and Glen Roy." 



Mr. Jamieson, in support of this view, adduces the extensive glacia- 

 tion apparent at the entrance both of Glen Arkaig and of Glen Treig, 

 and shows that near the entrance of Glen Gluoy there are ice striae, 

 pointing W. 5° 1ST., or in the direction that a glacier coming from Glen 

 Arkaig would take, and that, in the Spean Valley, opposite Glen 

 Treig, the ice striae are transverse to the valley, or in the direction of 

 the axis of Loch Treig, while on either side they point respectively up 

 and down Glen Spean. He infers, consequently, that the central 



