342 Professor Rogers on the Origin [March 22, 



The " Roads," or Shelves, themselves, are of various heights above 

 the sea, the lowest of the three conspicuous ones in Glen Roy having 

 an elevation of about 850 feet, the middle one a height of about 1060 

 feet, and the highest a level of nearly 1140 feet. Other much fainter, 

 still more elevated shelves, are discernible in Glen Gluoi, but all 

 hitherto seen lie below a horizon of 1500 feet above the ocean. These 

 Parallel Roads, as they are called, are apparently level, and therefore 

 parallel, but further instrumental measurements are necessary before 

 the question of their absolute horizontality can be regarded as satis- 

 factorily settled. 



They constitute a most impressive feature in the scenery of the 

 lonely, treeless glens containing them. Winding into all the recesses 

 and round the shoulders of the mountains which they imprint, they 

 present at first view a striking likeness to a succession of raised 

 beaches deserted by their waters. 



Seen in profile, as when looked at horizontally, they resemble so 

 many artificial hill-side cuttings, the back of each terrace lying within 

 the general profile of the mountain slope, while the front or outer edge 

 is protuberant beyond it. Each is indeed a nearly level, wide, deep 

 groove, in the easily eroded boulder drift, or diluvium, which to 

 a greater or less thickness everywhere clothes the sides of these moun- 

 tains. They vary greatly in their relative distinctness, being in some 

 places vaguely discernible, while in other spots they indent the surface 

 very plainly, just as they happen to be narrow and to coincide in slope 

 with the hill, or to be broad and apparently level from front to back. 

 Where most indistinct they are frequently not discernible at all when 

 we stand upon them ; though we may in a favourable light have detected 

 their position and course from the opposite side of the glen, or, better 

 still, from the bed of the valley. The conditions which influence this 

 fluctuation in distinctness, promise, if carefully observed, to dispel 

 much of the obscurity which has hitherto invested the origin of the 

 terraces. The modifying circumstances seem to be all referable to one 

 general condition, that of exposure to a current or inundation, supposed 

 by the speaker to have rushed through these glens from their mouths 

 to their heads, or upper ends. Thus it would appear : 1st, With 

 scarcely an exception, that each terrace or shelf is most deeply im- 

 printed in the hill-side, and is broadest where the surface thus grooved 

 has its aspect down the glen on towards the Atlantic, and is faintest 

 where the ground fronts towards the head of the valley on the German 

 Ocean. 2nd, While conspicuous on the open sides and the westward 

 sloping shoulders of the hills, the terraces disappear altogether in the 

 recesses or deeper corries which scollop the flanks of the mountains. 

 3rd, Each shelf, or " road," grows usually more and more distinct as it 

 approaches the head of its own special glen, until those of the two op- 

 posite sides meet in a round spoon-like point. 



A fact obviously material to a true theory of the origin of the ter- 

 races, is that each of them coincides accurately in level with some water- 

 shed or notch in the hills leading out from its glen into some other glen 



