26 



MR. T. F. JAMIESOlf SUPPLEMENTARY 



Gulban delta. It slopes very gradually back to a level of about 

 50 feet above Loch Laggan, which would be 14 feet above the ave- 

 rage height of the lowest Glen Eoy line and not 40 feet as he makes 

 it. Milne-Home states that the steep front of this bank shows a 

 mass of sand and fine clay in horizontal beds. How a wide platform 

 of such materials should be a moraine, as Prof. Prestwich and Mr. 

 Melvin suggest, is not very easy to see. 



2. Prestwich" s Theory of the Formation of the * Roads J 



This is stated as follows : — " In the Lochaber district, while the 

 exceptional accumulation of ice in the Spean valley heavily barred 

 the entrance of the glens on the north side of that valley, the passes 

 at the head of the glens were also blocked by smaller remnants of 

 the great ice-sheet, and the formation of detrital shelves or terraces 

 is due to the sudden bursting of these minor barriers, when the 

 waters of the lake were discharged with great rapidity until they fell 

 to the level of the col. Under these circumstances the mass of loose 

 debris covering the hillsides gave way and slid after the retreating 

 waters, until stayed with greater or lesser abruptness, according to 

 the angle of slope and the volume of the mass, on the discharge 

 ceasing and the waters coming to rest. The shelves so formed, 

 modified slightly by subsequent subaerial action, constitute the 

 ' roads '." ' 



I have no wish to enter into any lengthened criticism of this new 

 theory, because it has been propounded to meet what I believe to be 

 an imaginary difficulty which has really no existence, that is to say 

 the supposed want of regular horizon tality in the lines. Prof. 

 Prestwich, however, has elaborated it in much detail, and it is due 

 to a former President of the Geological Society, and the author of 

 one of our most esteemed text-books on the science, that I t^hould 

 state some of the reasons why I cannot accept it. 



Even were we to concede the possibility of all these long level 

 lines having been formed by slides of debris in the way Prof. 

 Prestwich explains, it can be shown that there are many places 

 where the conditions he himself lays down as requisite for the pur- 

 pose did not exist. Two conditions are assumed to be necessary, 

 namely, 1st, a higher slope covered with debris above each line, and, 

 2nd, a mass of subsiding water sinking down this slope. Now 

 these conditions were wanting in many places. For example, a 

 • road ' is occasionally met with round the top of a knoll, as at 

 Tom-na-fersit and Meal Derry, where there was no slope above to 

 furnish material for the slide. We have likewise no reason to sup- 

 pose that there was a lake all along Glen Koy above the level of the 

 highest line as the theory requires, neither can it be at all admitted 

 that along Glen Spean, or on the south slope of Eohuntine Hill, 

 there was any mass of water subsiding from a higher level which 

 could have formed the lowest line in the way described. In order 



1 Phil. Trans. Eoy. Soc. for 1879, vol. clxx. p. 725. 



