Having at this part of' Glen Roy arrived at the point -where all the three lines on each side arc visible, we must now remark, that there is a perfect correspondence of level between the opposite and corresponding pairs, wher ever they are found. The few exceptions from anoma lous or supernumerary lines were already noticed ; and we may here observe that no other instances of these ir regularities occur, from this point down to the termina tion of the valley of the Roy in that of the Spean. They are often absent altogether for a certain space, but wher ever th y are present they obey the general law.
The correspondence of level in question was ascertained by so many observations with the spirit level, as to leave no doubt of the truth of this fact ; which is also a ne cessary one, according to the view which we have taken of the causes of these appearances. It is by this regu larity of horizontal disposition in the whole, and the con tinuity of the respective lines, that has produced that as pect of parallelism and regularity by which they are cha racterized, and to which their very peculiar effect is ow ing. This parallelism, it must, however, be obvious, is in the vertical plane only. On the horizontal one they cannot be parallel, because of the varying slopes of the hills or breadths of the valley, which place the corres ponding ones at very different distances from each other in different places. This circumstance, it will hereafter be seen, is of importance as it regards one of the theories of their origin which has been given. It is unnecessary to give any nice account of these variations ; but we may remark in a general way, that the nearest horizontal dis tance between the uppermost and nearest lines may be ta ken at 150 or 160 feet, and the greatest between the two lowermost and most distant ones at 1000 ; an approxima tion which is sufficiently accurate for any useful purpose.
Their vertical distances from each other form an im portant point in their history ; and as they were measured by levelling, they cannot*fail to be correct, as far as it is possible that they should be so, where the surfaces to be measured from are so irregular, so wide, and of a slope so much varying from the horizontal plane. The point for the level in these experiments was fixed in the mid dles of the lines, as a medium ; and thus, omitting frac tions, the distance was found to be 82 feet between the two uppermost, and that between the middle one and the lowermost, 212.
In conducting these experiments, observations were also made by the barometer, with as much care as could be bestowed on this object, for the purpose of ascertaining various other elevations connected with the due under standing of these phenomena. We need not detail the
methods and precautions that were used; and as these measurements will often be referred to in the subsequent remarks on this spot, we cannot do better than give a ta bular view of them in this place, as they are intimately connected with the relative vertical distances of the lines from each other. Even those which are not very imme diately connected with this part of the subject, could not conveniently be omitted, as it is highly necessary towards a due appreciation of all the circumstances connected with them, to form a correct notion of the elevations of the adjoining country.
We shall merely remark, that the only two very doubt ful points in this table relate to the difference of level be tween the upper line of Glen by and Loch Spey, and the height of this last point above the German ocean. There are occasionally circumstances in the state of the weather which render it impossible to depend on barometrical measurements, when the stations of the instrument are far asunder; but whatever errors there may be here, they do not affect either the essential parts of the history of these phenomena, or the reasonings which will hereafter be deduced from them.
To return to the description of the valley. Having passed Glen Turit, the three lines now become distinct and well marked on the right side, where the hill is co vered with a thick alluvium. On the opposite side, they are also distinct, although here and there slight marks of irregularity and of supernumerary lines occur. The bot tom of the glen continues to exhibit an alluvial flat, for about three miles from the entrance of Glen Turit down wards; and the terraces which border the glen gradually disappear as its bottom contracts. A few interruptions occasionally occur, apparently connected with the rocki ness and irregularity of the ground, and these are most remarkable on the right side; but shortly before the glen turns to the south, and till we arrive at Glen Fintec, all the three lines are strongly marked on both sides. They are particularly conspicuous on the slope of a brown hill about this place, on account of their continuity, their pre servation, and the almost absolute equality of their di mensions, not only through the course of each individual line, but relatively to each other.