Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/329

 Glen Roy, or the division between the upper and lower vallies, is seen projecting at right angles to the right hand side of the glen, and then turning westward so as to form a promontory parallel to that side; having a cul de sac on one hand and giving passage to the river on the other. No line is visible on the rock itself, but from its junction with the side of the valley (as the plan will show) the two lines commence, and are seen running on far along the face of the hill, the uppermost one being precisely even with the flat parts of the surface of the rock just described. It is proper here to remark that the surface of this rock rises higher in some places than that line, yet it is not marked by any corresponding one. The drawings accompanying this paper will render intelligible that which words alone cannot describe; and I must here premise once for all, that this minuteness of description, however superfluous it may at first sight appear, is absolutely required, as the circumstances thus dwelt on will be of essential use in investigating the cause of the appearances under discussion. It is by an attention to circumstances which at the first glance appear trivial, that abstruse truths are often discovered; and it is precisely where leading and obvious phenomena offer no clue to guide us, that a ray of light will often be thrown on the subject from appearances at first neglected. Had the greater features of Glen Roy been capable of explaining the singular phenomena which it exhibits, this paper would have perhaps been altogether superfluous, since all observers would have been agreed respecting their causes.

These level and parallel lines are scarcely to be seen in this place, except by looking from below upwards, a position by which they are foreshortened to the spectator's eye. They may sometimes