We need not suppose that these movements of subsidence and upheaval were uninterrupted and uniform. Indeed, the abundant coarse conglomerates, which play so prominent a part in the materials thrown into the basins, suggest that at various intervals during the prolonged sedimentation subterranean disturbances were specially vigorous. But the occurrence of strong unconformabilities among the deposits of the basins sets this question at rest, by proving that the terrestrial movements were so great as sometimes to break up the floor of a lake, and to place its older sediments on end, in which position they were covered up and deeply buried by the succeeding deposits.[312]
[312] An unconformability of this kind occurs between the south end of the Pentland Hills and Tinto in Lanarkshire, and another in Ayrshire.
It is not surprising to discover, among these evidences of great terrestrial disturbance, that eventually groups of volcanoes rose in long lines from the waters of most of the lakes, and threw out enormous quantities of lava and ashes over tracts hundreds of square miles in extent. So vast, indeed, were these discharges, across what is now the Midland Valley of Scotland, that the portions of sheets of lava and tuff visible at the surface form some of the most conspicuous ranges of hills in that district, stretching continuously for 40 or 50 miles and reaching heights of more than 2000 feet above the sea. Exposed in hundreds of ravines and escarpments, and dissected by the waves along both the eastern and western coasts of the country, these volcanic records may be studied with a fulness of detail which cannot be found among earlier Palæozoic formations.
It might have been supposed that a series of rocks so well displayed and so full of interest, would long ere this have been fully examined and described. But they can hardly be said to have yet received, as a whole, the attention they deserve. Without enumerating all the writers who, each in his own measure, have added to the sum of our knowledge of the subject, I may refer to the labours of Jameson,[313] Macknight[314] and Fleming,[315] among the observers who began the investigation. But of the early pioneers, by far the most important in regard to the igneous rocks of the Old Red Sandstone was Ami Boué. While attending the University of Edinburgh, where he took the degree of M.D. in the year 1816, he imbibed from Jameson a love of mineralogy and geognosy, and for several years spent his leisure time in personally visiting many parts of Scotland, in order to study the geological structure of the country. Probably in 1820 he published in French his now classic Essai.[316] The value of this work as an original contribution to the geology of the British Isles has probably never been adequately acknowledged. For this want of due recognition the author himself was no doubt in some measure to blame. He refers distinctly enough to various previous writers, notably to Jameson and Macculloch, but he mingles the results of his own personal examinations with theirs in such a way that it is hardly possible to ascertain what portions are the outcome of his own original observations. Less credit has accordingly been given to him than he could fairly have claimed for solid additions to the subjects of which he treated. In the later years of his life I had opportunities of learning personally from him how extensive had been his early peregrinations in Scotland, and how vivid were the recollections which, after the lapse of half a century, he still retained of them. Judged simply as a well-ordered summary of all the known facts regarding the geology of Scotland, his Essai must be regarded as a work of very great value. Especially important is his arrangement of the volcanic phenomena of the country, which stands far in advance of anything of the kind previously attempted. Under the head of the "Terrain Volcanique," he treats of the basaltic formations, distinguishing them as sheets (nappes, coulées) and dykes; and of the felspathic or trachytic formations, which he subdivides into phonolites, trachytes, porphyries (forming mountains and also sheets) and felspathic or trachytic dykes. In the details supplied under each of these sections he gives facts and deductions which were obviously the result of his own independent examination of the ground, and he likewise marshals the data accumulated by Jameson, Macculloch and others, in such a way as to present a more comprehensive and definite picture of the volcanic phenomena of Scotland than any previous writer had ventured to give.
[313] Memoirs of the Wernerian Society, vol. ii. (1811), pp. 178, 217, 618; vol. iii. (1820), p. 220, 225.
[314] Op. cit. vol. ii. pp. 123, 461.
[315] Op. cit. vol. i. (1808), p. 162; vol. ii. (1811), pp. 138, 339.
[316] Essai géologique sur l'Écosse (Paris; no date, but probably about 1820). He acknowledges his indebtedness to Jameson, whose demonstrations of the geology of the Edinburgh district he partly reproduced in his book. Jameson's early writings in the Wernerian Memoirs and in separate works were mere mineralogical or "geognostical" descriptions. His later lectures became more valuable but were never published, save indirectly in so far as they influenced the opinions of his pupils who published writings on the same subjects. See, for instance, Hay Cunningham's Geology of the Lothians, p. 59, footnote. Compare an article on Boué, Edinburgh Review for May 1823 (vol. xxxviii. p. 413).
The account which Boué wrote of the Old Red Sandstone and its associated igneous rocks marked the first great forward step in the investigation of this section of the geological record. He was the earliest observer to divide what he calls the "roches feldspathiques et trappéennes" into groups according to their geological position and mineralogical character, and to regard them as of igneous origin and of the age, or nearly of the age, of the red sandstone of Central Scotland.
Of later writers who have treated of the volcanic rocks of the Old Red Sandstone, my old friend Charles Maclaren deserves special recognition. His survey and description of the Pentland Hills embodied the first detailed and accurate investigation of any portion of these rocks, and his Geology of Fife and the Lothians may still be read with pleasure and instruction.[317] Boué had indicated roughly on the little sketch-map accompanying his Essai the chief bands of his felspathic and trappean rocks of the Old Red Sandstone, but their position and limits were more precisely defined in Macculloch's "Geological Map of Scotland," which was published in 1840, five years after the sudden and tragic death of its author. The observers who have more recently studied these rocks have been chiefly members of the Geological Survey, and to some of the more important results obtained by them I shall refer in the sequel.