Page:Proceedings of the Royal Society of London Vol 4.djvu/152

Rh land was subjected to these conditions, it appears that its surface would have been modeled in a manner exactly similar, even in its minute details, to the existing structure of the valleys in Lochaber. Considering that he has thus established his theory, the author pro- ceeds to remove the objections which might be urged against its truth, derived from the non-extension of the shelves, and the ab- sence of organic remains at great altitudes. He then shows how various details respecting the structure of the glens of Lochaber, such as the extent of corrosion of the solid rock, the quantity of shingle, the numerous levels at which water must have remained, the forms of the heads of the valley, where the streams divide, and especially their relation with the shelves, and the succession of ter- races near the mouth of Glen Spean, are all explicable on the suppo- sition that the valleys had become occupied by arms of a sea which had been subject to tides, and which had gradually subsided during the rising of the land ; two conditions which could not be fulfilled in any lake. From the attentive consideration bestowed by the au- thor on these several and independent steps of the argument, he re- gards the truth of the theory of the marine origin of the parallel roads of Lochaber (a theory, of which the foundation stone may be said to have been laid by the important geological researches of Mr. Lyell, establishing the fact of continents having slowly emerged from be- neath the sea) as being sufficiently demonstrated.

The author states, in the concluding part of his paper, the follow- ing as being the chief points which receive illustration from the examination of the district of Lochaber by Sir Thomas Lauder Dick, Dr. Macculloch, and himself. It appears that nearly the whole of the water-worn materials in the valleys of this part of Scotland were left, as they now exist, by the slowly retiring waters of the sea ; and the principal action of the rivers since that period has been to remove such deposits ; and when this had been effected, to excavate a wall-sided gorge in the solid rock. Throughout this entire district, every main, and most of the lesser inequalities of surface are due, primarily to the elevating forces, and, secondarily, to the modeling power of successive beach-lines. The ordinary alluvial action has been exceedingly insignificant ; and even moderately sized streams have w^orn much less deeply into the solid rock than might have been anticipated, during the vast period which must have elapsed since the sea was on a level with the upper shelves : even the steep slopes of turf over large spaces, and the bare surface of certain rocks, having been perfectly preserved during the same lapse of time. The elevation of this part of Scotland to the amount of at least 1278 feet was extremely gradual, and was interrupted by long intervals of rest. It took place either during the so called " erratic block pe- riod," or afterwards ; and it is probable that the erratic blocks were transported during the quiet formation of the shelves. One of these was found at an altitude of 2200 feet above the present level of the sea. The most extraordinary fact is, that a large tract of country was elevated to a great height, so equably, that the ancient beach- lines retain the same curvature, or nearly so, which they had when forming the margin of the convex surface of the ancient waters.