Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/332

 angle, or at the inner and outer edge of the line, that no precise limit for the measure of breadth can be assigned; different modes of measurement may therefore produce differences of many feet. It is sufficient if they agree generally, and in general sixty feet may be assumed as an average breadth: by far the largest portion of all the lines will be found to conform to this measurement.

Great terraces are now visible on the right. These are not precisely on the same level with that which I before mentioned as corresponding to the course of a third and lower line, but they do not differ materially from it.

I shall not describe the various rivers which enter the glen, the principal ones being marked in the plan, but may mention that in this upper part of the valley, both before and at the junction of Glen Turit with Glen Roy, they are generally accompanied by their own lateral terraces.

On the left hand going down the glen many marks or fragments of lines are seen between the principal ones; but these are short, and are remarkable for many obscurities and deficiencies. In a few places there are errors of level to be seen in the lines. Examining these however there appears no doubt of their having been produced by partial subsidences of the whole alluvial face; and this is confirmed by the appearance of one great slide on the left, which has descended many feet, and which the imagination can readily replace. It is necessary to be cautious in examining these instances of errors of level, as the laws of perspective are apt to lead to mistake when the lines pass curved surfaces elevated high above the horizon.

Where the faces of the hills have been furrowed by the long continued action of descending torrents, the lines enter these hollows