Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/326

 justify. The necessity of investigating their probable or possible origin from natural causes will, in consequence of the extent of their geographical connections, lead to rather a wide range of enquiry, not however wider than the importance of the subject will be found to justify. I have attempted to keep clear of all speculations purely hypothetical, and wherever physical evidence and analogy have entirely failed, have rather chosen to leave the question in its natural obscurity than to involve it in more profound darkness by assigning imaginary causes. To avoid any bias which the original and not sufficiently descriptive term, Parallel roads, might preserve in the reader's mind, I have substituted that of Lines, a term less exceptionable and sufficiently expressive of their appearance. I think it right to remark that every precaution was taken in ascertaining both the levels and the elevations which will be referred to in the arguments hereafter to be used. Those which could not be accurately determined, on account of the nature and extent of the ground requiring a survey, have been estimated by such approximations as were attainable, and these are distinguished wherever they occur. However imperfect, they are no where so lax as to affect the arguments, even if the errors were much greater than any which could have occurred, as no undue stress as any where laid on hypothetical assumptions. The levels of the lines were observed by a spirit level, and the vertical distances between them were measured by the same instrument. One barometer of Ramsden's construction was applied for the measurement of the elevations; and such differences between any two altitudes as were required, were ascertained by observations repeated at very short intervals. For determining the absolute altitudes above the level of the sea, the barometric observations were compared with a register kept purposely for these experiments by Lord Gray at Kinfauns Castle, those observations, with the required corrections, being made at the same hours with instruments of similar construction. To remove still more any chance of error, the principal altitudes were deduced from a medium of nine observations taken on four different days, the greatest variations between the extremes not exceeding one twelfth of the whole. The altitude of the great Caledonian valley is known from the measurements belonging to the canal.

Notwithstanding all this care however, I can only consider these altitudes as approximations, since I am convinced from a careful comparison of barometric registers that this instrument cannot be relied on when used in this manner, as I have attempted to show in late communication to this Society.

The principal map does not pretend to be an accurate survey. The defective nature of all the maps of Scotland hitherto constructed, as well as the smallness of their scales, prevented me from making any use of them for this purpose; but as the description would have been unintelligible without some sketch of the ground, I have given one which must however be considered merely in the light of a military reconnaissance. The sections do not pretend to be real. The transverse ones are, like the map, delineated without attention to their true proportions, and are merely intended to mark the important variations of the form of the bottom of the valley, and more particularly the points in which the lines and the terraces coincide. The curved longitudinal sections are equally artificial, but they assist the imagination in pursuing the wide connections of Glen Roy with the sea. The profiles of the lines are deduced from actual measurement by the spirit level.

Of the accompanying views I need only say that they are intended to elucidate several parts of the description, and to convey a slight notion of the nature of the appearances in question. The purposes of the two other maps in showing the connections of Glen Roy with the neighbouring and with the more distant country will be obvious.