referred some discrepancies and errors in the measurements of the remarkable rocks which are seen in its bed. This cause is the variable state of the water, which at times exposing more or less of these projections in its bottom, alters their apparent magnitude as well as their number. I have been rendered sensible of these differences by examining its course at, different periods, but since some confusion would have resulted from any attempt to correct the first examination which I made, I have preferred giving it as it was first recorded, with the caution against implicit reliance in the measurements which I have now laid down. It is only necessary to say that the observations were made in a kind of medium height of the water, and that as the geological consequences are in no wise affected by this sort of inaccuracy, I held it unworthy of correction. The statement of the circumstance was however necessary, lest the discovery of inaccuracies of this nature might lead so doubts with regard to more important observations. The measurement given in yards is merely a rude computation from pacing, but it is fully adequate to the purposes in view; since neither the facts nor reasonings can in any way be altered by a greater or less nicety of admeasurement.
Descending therefore the course of the river from Poll Tarff (See Map plate 13) the junction of that stream with the Tilt, a large body of granite will be seen crossing it above the Shepherd's hut, and a small granite vein may also be observed above it at a little distance. The bed of the river from Poll Tarff to this point, wherever it is visible consists of dark blue limestone alternating with quartz rock. At the distance of four or five hundred yards from the burn of Aldianachie, a large rock consisting of a complicated mixture of granite and schist crosses the Tilt again. I shall not here dwell upon the circumstances which attend this mixture, as better opportunities of describing them will occur hereafter.
