THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE II. VOL. VI. 



No. XII— DECEMBER, 1879. 



OEIG-IUAL ARTICLES. 



I. — The Parallel Eoads of Glen Eoy. 



By J. E. Dakyns, M.A. ; 



of H.M. Geological Survey of England. 



LAST summer I paid a visit to the Parallel Eoads of Glen Eoy, 

 and saw certain features which must, I think, have hitherto 

 escaped notice. It has been asserted that the roads consist of, or 

 are cut out of, mere superficial detritus ; and that they never appear 

 where the solid rock appears ; and much discussion has arisen as to 

 the mode of formation of the roads on this supposition, viz. that they 

 consist of detritus merely. 



In order to get a section of the roads, I examined several water- 

 courses descending the side of Leana Mhdr, west of the Eiver Eoy. 

 These water-courses were cut to the depth of about ten feet in the 

 ordinary superficial detritus of the mountain side ; and where they 

 crossed the roads, the material exposed on their sides was precisely 

 the same as that along the rest of their course. This did not surprise- 

 me, as I had recently read in the Geological Magazine for July 

 (p. 321), in a notice of a paper by Professor Prestwich, "On the Origin 

 of the Parallel Eoads of Lochaber," that " the Parallel Eoads are 

 terraces composed of perfectly angular fragments of the local rocks." 

 However, as these water-courses nowhere reached the solid rock, 

 I was not satisfied. I accordingly went next day to the east side 

 of the Eiver Eoy, where the roads are very well marked indeed, and 

 are crossed by two or three comparatively large gills. These gills 

 descend along the west face of east Leana Mhor; for there are two 

 hills of this name given on the One-inch Ordnance Map, one on 

 the west, and the other on the east side of the Eiver Eoy. 



I walked along the topmost road northward : and the first gill 

 I came to showed me at once that the road was cut out of the solid 

 rock. Both above and below the road the solid rock came practically 

 to the surface, being covered with a mere film, about a foot thick, of 

 •its own detritus : the surface along the road itself was, however, 

 hidden by a small fan of detritus shot on to the road from the gill 

 above. The section of the second or middle road was obscure along 

 this gill. I walked on along the highest road till I came to a big 

 gill cut deeply into the native rock, and bifurcating a little below 

 the 1250 contour line. The highest road crosses this pair of gills 

 above the point of bifurcation, and was so utterly destroyed as to be 

 quite obscure ; but the second road ran right up to the edge of the 



DECADE II. VOL. VI. — NO. XII. 34 



