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numerous and complicated problems. “To the field on every fitting occasion,” should be the guiding maxim of the young geologist; and though new discoveries may not always reward his toil, though he should even fail to obtain what others have obtained before him, he is at least in the way of doing both, and in the excellent practice of training his powers of observation. And when his day's work is done, however little it may have added to his scientific stores, he has secured one luxury at least by his twenty miles' ramble in the pure country air—a blessing which comes to the poor unasked, and which the rich but too seldom enjoy—the luxury of feeling tired.
ling tired the rich buta which SCOTTISH GEOLOGY-ITS PROOFS
By the proofs of Scottish geology we mean what is known, or at least generally admitted, by competent observers; and by its problems, what is still doubtful, and requires further investigation. Our object in the present paper is not to sketch the history of Scottish geology, which about the beginning of this century was an inextricable labyrinth of theories and disputations between “Neptunists and Plutonists," and which at a later period was little else than a bald nomenclature of rocks and minerals ; * but simply to try to distinguish between the established and the doubtful, that the student may better know what to accept, and where to direct his investigations. And yet, while alluding to the history of Scottish geology, one cannot avoid mentioning the names of such men as Hutton, Playfair, Ure, Hall, M‘Culloch, and Jameson, as contributing largely, even amidst these unproductive discussions, to the substantial progress of the science. Geology, such as we now understand it, can date back little more than forty years; and during this period its advancement in Scotland has been indebted in no small degree to the labours of M‘Culloch, Jameson, Fleming, M‘Laren, Cunningham, and Hugh Miller, and especially to those of Lyell, Murchison, Nicol, and others, who are still steadily endeavouring to unravel its many complicated physical as well as palæontological problems. Our present purpose, however, is not the history but the facts of the science; a brief indication, for the guidance of the young geologist, of what is already established, and of what remains doubtful, and to the elucidation of which he could with advantage direct his observation. In a field so vast, and where the labourers are so few, it is ever of importance that the new-comers, instead of treading in the old beaten tracks, should have their attention directed to the determination of the doubtful and the discovery of the unknown.
* This was the grand epoch of rock and mineral collections, the age of magnificent cabinets filled with rocks, minerals, and cut gems—the arranging of these into groups, and genera, and species (after the systems of botany and zoology), being the minute but futile labours of the collectors. A knowledge of Lithology and Mineralogy is no doubt indispensable to the study of Geology ; but the idea of dealing with mixed rocks as with mineral, vegetable, and animal species, was a fancy that retarded rather than accelerated the progress of the science.
If the student will turn to any of the geological maps of Scotland—M'Culloch's, Nicol's, or Knipe's—he will find that the various rock-formations have all a south-west and north-east strike, with their dip to the south-east-thus placing the older rocks to the north-west, and the younger, in regular succession, to the south-east. The same holds good in England, so that a line drawn from London to North Wales would pass in succession through all the formations from the Tertiary to the Cambrian, just as a line drawn from the Firth of Forth to Cape Wrath would pass through the upper coals on through all the underlying formations to the Lower Laurentian. This fact is of use as affording a sort of index to the stratified systems, the traveller passing from younger to older if he begins at the south-east, and the reverse, or from older to younger, should
he begin at the north-west. In this extreme north-west (the Island of Lewis and Cape Wrath), the schists and quartzites are supposed to be of Laurentian age, and the fossiliferous limestones and sandstones of Durness and Loch Maree of Cambrian. We say “supposed,” for there are still doubts as to the true relations of the latter rocks—whether indeed they pass under the gneiss and mica-schists of Sutherland, or are not merely faulted and disrupted portions of the same series. If they be faulted portions of the same series, then we have as yet no ground for separating the great metamorphic masses of the Highlands into systems and formations; but if, on the other hand, these limestones and quartzites do pass continuously under the gneiss and mica-schist, then (as their fossils seem to indicate) they must be regarded as of Cambrian age, and all the gneiss rocks that lie beneath them (the “fundamental gneiss” of Murchison) be held as the equivalents of the Norwegian and Canadian Laurentians. In the mean time, the majority of those who have surveyed the district (Murchison, Harkness, Geikie, &c.) lean to the latter opinion; though Professor James Nicol, a cautious observer, and well acquainted with our Highland rocks, still entertains the belief that the rocks of Lewis, of Durness, and of Sutherland, generally belong to one and the same series of metamorphic schists, whatever be their age-Laurentian, Cambrian, or Silurian. The matter, it must be confessed, requires still further elaboration-more extended examination of the rocks themselves, and more minute discrimination of the fossils which may yet be found in more convincing states of preservation. During the last eight or ten years nothing has been done towards the solution of this problem, and we are tempted, in the interests of geology, to inquire_Is the question to remain in abeyance till the officers of the Geological Survey in the next generation reach these distant lochs and head
lands, or is there no geologist of sufficient enthusiasm and leisure to put it at rest to the satisfaction of existing inquirers ? Between 1856 and 1859 a strong current of interest was directed to these “fundamental gneiss rocks” and Cambrian fossils, but since then nothing has been done either to corroborate the opinions of Sir Roderick Murchison, or to refute those of Professor Nicol—the geological world remaining somewhat divided in opinion, and the nongeological, so far as it takes any interest in the matter, inclined to accept the conclusions of Sir Roderick.
Turning next to the Silurian system, we find, in that broad belt of upland which stretches from St Abb's Head to Port Patrick, that all the members of the formationLower, Middle, and Upper — have been detected; but, strange to say, no one has yet attempted the delineation of these subdivisions on our maps, or done more than merely admit the fact of their existence. In fact, beyond the admission that the whole belt is Silurian, and the finding of certain fossils at Grieston, the Pentlands, Moffat, Nitberry Hills, and Girvan, nothing has been done in a comprehensive way to co-ordinate the Silurian of southern Scotland with those of England, or to say how far the most southerly and most highly metamorphosed belt of them (the old Greywacké proper) must be assigned to the Cambrian system. In the north of Scotland, on the other hand, the gneisses, mica-schists, and clay-slates—the “ metamorphic rocks,” in fine, of the Highland mountains — have recently been coloured on some of our maps (that of Sir R. Murchison and Mr A. Geikie) as Silurian, though not a vestige of fossil organism has yet been detected in any portion of that vast mass and area. This colouring (mainly, we believe, at the suggestion of Sir Roderick) we consider premature and apt to mislead, though we know, from personal observation, that there is an orderly succession among