Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/379

 cerned in the special arguments for or against the two varieties of the lake -theory.

Whatever view we may take of the origin of these lines, one great fact seems beyond dispute. Long before they originated, the country in which they lie must have had nearly its present form and outline — the same hills and mountains, the same glens and valleys, with nearly their present relative elevations. This fact we may, or rather must, assume in all our reasonings and speculations. This region has also been subjected to a very extensive glacial action. Wherever the rocks are newly exposed they are marked by grooves and striae. The direction of these striae and the form of the rounded rocks show that in a few cases the ice has come down some of the lateral valleys and moved towards the west. But other facts appear to me to indicate that here, as in other parts of the North of Scotland, the great ice-stream has flowed from the west, and probably from a lofty mountain- chain that then existed in that region.

Subsequent to this ice-period, but before the formation of the Glen-Roy lines, the whole region has been submerged in the sea. This is proved by the uniform coat of detritus covering the whole surface, in a thicker or thinner sheet according to the form of the ground. This coat is not the mere surface waste, but matter laid down by water, and is too widespread and general in its distribution, and too much mixed in its composition, to have been formed in any mere lake. Associated with it are numerous boulders of travelled stones, some of them imbedded in its mass, others lying on the surface. As examples of these I may mention some huge blocks of black granite and other smaller masses of red porphyry which occur within a few yards of the summit of Craig Dhu, a conical mountain of mica slate, that rises to more than 2000 feet above the sea, in the angle between the valleys of the Spean and Roy. One block must weigh about forty tons ; and they are evidently ice-borne masses, floated probably far from the west in the ancient ocean.

It is in this detrital cover that the lines are cut ; and the period and mode of its formation are thus of much importance. That it is a marine deposit seems beyond doubt. That it has been formed since the general glacial striation of the land is also proved by the fact that it spreads over the rocks marked in this manner. This is well seen in the Spean valley in very many places. More convincing, or at least more interesting, is the fact that in Glen Roy these striated rocks occur immediately under the lines. The old line or parallel road now passes over the rock-surface that in a former period was worn and striated by the glacier. There is thus the most direct proof that the period of general ice-striation was separated from that of the formation of the lines, by a period in which the land was submerged in the ocean and its general cover of detritus deposited.

This fact, however, does not decide the mode of formation of the lines, or even its relation to ice and glaciers. There is abundant evidence in many parts of Scotland that the great western glaciation

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