The internal structure of the matter composing each terrace consists in an " oblique lamination " or slant bedding of the layers of gravel, sand, and other sediment which constitute it, such as geologists fami liarly recognise as the result of a strong current pushing forward the fragmentary material which it is depositing, and which is hold by them to indicate, iu the direction towards which the laminfe dip, the direc tion towards which the current has moved. The " dip," or downward slant, is almost invariably up the glen, or towards its head, and net down the glen, or towards the Atlantic.
In all previous hyssitheses, the agency of standing water is assumed, either the ocean in its ordinary state of repose, or lakes pent within the glens, as explained in the former article. But to these Professor Rogers opposes the facts, that these level shelves are not true marine beaches, exhibiting not a vestige of any marine organic remains, no rippled sands, no shingle, and no sea-cliffs; that they display, in like manner, a total abscise° of the distinctive marks of lake-sides, not one laeustrine organism, neither fresh-water plant nor animal having ever been discovered imbedded in them. Nur has any feasible cause of blockage of the glens at different stations above their months, to pond the waters to the respective heights; of the terraces, been assigned ; there are uo traces of former barriers in any of those localities where alone we can assume them to have existed, to produce the required embaying of the waters. The hypothesis of successive " sea-margins," or sea-levels, is stated to be overthrown by the now well-established deduction from the recent measurements of Professor Rogers himself, that none of the several shelves, or "roads" of Glen Roy correspond in level with any of those seen in the adjacent valley, Glen Glnoi, a marked discrepancy separating the two groups of terraces into two independently produced systems.
After adducing further objections, both of fact and reasoning, to former hypotheses on the origin of the parallel roads, Professor Rogers concludes by sketching, in the following terms, the action to which he ascribes their formation :—" He supposes the several terraces to have been cut or grooved in the sides of the hills by a great inundation from the Atlantic, engendered by some wide earthquake disturbance of the ocean's bed, and forced against the western slope of Scotland. The
features of the country indicate that, while a portion of such a vast sea-tide entering the Frith of Liunhe rushed straight across the island through the deep natural trench, Glen Mor or the great Caledonian valley, a branch current was deflected from this, and turned by the Spew' valley and its tributary glens, Glen Roy and Glen Gluoi, into the valley of the Spey, and so across to the German Ocean. In this transit, the deflected waters first embayed in these glens, and then filling and pouring through them, would, upon rising to the levels of the successive water-sheds, or low passes, which open away to the eastern slope of the island, take on a swift current through each notch, and as long as the outpour nearly balanced the influx, this current, temporarily stationary in height, would carve or groove the soft "drift" of the hillside. But the influx increasing, the stationary level and grooving power of the surface-stream would cease, and would only recommence when the flood rising to the brim of another natural dam, a new equilibrium would be established, a new horizontal superficial current set in motion, snd a second shelf or terrace begin to be eroded at the higher level. So each of the parallel roads is conceived to have been produced in the successive stages of the rising of one vast steady incursion of the sea. The lapsing back of the waters, unaccompanied by any sharp localised surface-currents through the passes, could imprint no such defined marks on the surface, nor accomplish more than a faint and partial obliteration of the terraces just previously excavated during their incursion." A succinct view of the controversy respecting the formation of the parallel roads, showing the position of the subject prior to Professor Rogers's investigation, will be found in Sir C. Lyell's Manual of Elementary Geology,' 5th ed.,1855, pp. 86-89.