PROCEEDINGS OF GEOLOGICAL SOCIETIES. 



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

 ficial 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 mountains. They varv 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 wc 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 in- 

 vested the origin of the terraces. The modifying circumstances seem to be all 

 referrable 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 imprinted in the hill-side, 

 and is broadest where the surface thus grooved has its aspect doicti the glen or 

 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 oppo- 

 site sides meet in a round spoon-like point. 



A fact obviously material to a true theory of the origin of the terraces 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 or valley adjoining, 

 a coincidence suggestive of the notion that they were formed by the grooving 

 agency of a flood pouring through the glens while it was embayed at the re- 

 spective levels of these natural waste weirs. In confirmation of this view that 

 they were transiently caused by erosive currents held successively at the 

 heights of the barriers on whose levels the terraces terminate, we have as 

 another interesting general feature, a remarkable ruggedness of the bed of each 

 external glen just outside the water-shed or barrier closing the glen which con- 

 tains the terrace. These rough and deep ravines, contrasting strikingly with 

 the smooth spoon-like terminations of the terrace-lined glens which head 

 against them, strengthen the suggestion already awakened by the marks of 

 horizontal erosion in the terraces themselves, that the notches or passes which 

 determined the grooving of the hill-sides on their one hand were externally the 

 sites of so many stupendous cataracts. 



The internal structure or disposition of the matter composing each terrace, 

 affords a further and striking corroboration of this hypothesis of the passage of 

 an erosive flood. It consists in an " oblique lamination," or slant bedding of 

 the constituents of the shelves — viz., the layers of gravel, sand, and other 

 sediment, such as geologists familiarly recognise as the result of a strong 

 ciirrent pushing forward the fragmentary material which it is depositing, and 

 which is held by them to indicate in the direction towards which the laminae 

 dip, the direction towards which the current has moved. Now, it is a most 

 suggestive peculiarity in the oblique bedding of these terraces, that the " dip," 

 or downward slant, is almost invariably up the glen, or towards its head, and 

 not down the glen, or towards the Atlantic, as we must suppose it would have 

 been, had the glen been a bay of the sea, and these materials but portions of 



