Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/372

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through Glen Spean at the same time as they flowed through Glen Roy. This case is even more complicated than the one stated at the beginning of the argument, it being impossible that a deluge of this sudden nature and great magnitude could have taken place so as to include Loch Laggan, without changes in the surface of the earth at this place and beyond it, even greater than those which I first pointed out.

I shall not pursue the consequences of this hypothesis further. Every person's imagination can supply additional difficulties, which it would be tedious to follow to their ultimate ramifications.

The next hypothesis which has been offered in explanation of the lines of Glen Roy is, that they are the remains of water terraces similar to those which are of common occurrence in the alluvial straths of Scotland.

Almost all the rivers, whether of greater or smaller dimensions, that flow through these straths with small velocities, or over planes of moderate inclination, are accompanied by lateral banks lying at a distance from the actual bed of the stream. They may in fact be considered as the deserted banks of the river itself, which in forming a deeper course for its waters has at the same time changed its ground laterally, leaving its ancient bed deserted and bounded by one of the banks which formerly confined it, while the other has been undermined and carried away by its opposite lateral deviation.

The ancient bank thus assumes the form of a terrace, with a slope resembling that of a military work, and standing at that angle which the peculiar circumstances of the soil enable it to maintain. Where the action of the river has been such as to cut its way, constantly deepening, through an alluvial plain, the surfaces of these terraces