1879.] 



of the Parallel Roads of Tjochaber. 



9 



adjacent Ben Aonach. The difference is too slight to allow of so great 

 a variation in the level of the snow line, and the cause suggested by 

 Mr. Jamieson, viz., a great difference in the rainfall such as it now 

 obtains in this district, can scarcely be maintained, for although the 

 annual fall at Fort William is 86 inches, and at Laggan 46 inches, the 

 rainfall at Roy Bridge has now been found to be as much as 62 inches. 

 Further, for the argument to be of any value, it should be shown that 

 in the country further eastward where the rainfall is much less, there 

 was the like absence of glaciers during the second period, whereas 

 Chambers and other geologists, including Mr. Jamieson himself, have 

 shown that during that period local glaciers descended from every 

 mountain range, approaching or exceeding 3,000 feet in height — a 

 height attained as already remarked by the hills to the north as well 

 as by those to the south of the Spean. 



With respect to the so-called " deltas " of the Turret in Glen Boy, 

 and of the Gulban in Glen Spean, which are supposed to have accumu- 

 lated during the long time that the lakes filled the valleys, the author 

 shows that the structure of the former is not in accordance with the 

 bedded structure of deltas, but on the contrary that it is formed of 

 unstratified moraine debris 50 to 80 feet thick, with a thin coatino- of 



o 



gravel water- worn and reconstructed from the underlying mass, and 

 that the angle of terminal slope is not that of original deposition, but 

 is due to wearing back of the terrace by the Roy, and the fall of the 

 debris by weathering. 



The foregoing are the objections raised by the author to the hypo- 

 thesis of Mr. Jamieson, but while objecting to this exposition of the 

 glacial theory, he considers that that theory affords the most satis- 

 factory solution of the problem, only that he would suggest a different 

 interpretation in explanation of the phenomena, the description of 

 which he does not think it necessary to repeat. 



Yieius of the Author. 



Dismissing the hypothesis of local glaciers of the second period of 

 glaciation, the author falls back upon the original idea of Agassiz with 

 the development acquired by more recent research, and assigns the 

 Lochaber lakes to the close of the first period of great glaciation. He 

 considers the phenomena are due to the peculiar physiographical con- 

 ditions of the district, and shows that owing to the configuration of 

 the country, the drainage of the Ben ^Tevis range instead of flowing off 

 from the centre to two or more sides, is all diverted northward into 

 the lower part of the Spean Valley and the Great Glen near Fort 

 William. These conditions which now give this area an excess of 

 water drainage, must in the like manner, during the glacial period, 

 have there led to an exceptional accumulation of ice. 



