[410] Small 8vo, Edin. 1838, first partly published as articles in the Scotsman newspaper. A second edition, which was little more than a reprint of the first, appeared in 1866.
Various papers of more local interest, to some of which allusion will be made in the sequel, appeared during the next quarter of a century. But no systematic study of the volcanic phenomena of any part of Scotland was resumed until the extension in 1854 of the Geological Survey to the north of the Tweed by A. C. Ramsay. The volcanic rocks of the Lothians and Fife were mapped by Mr. H. H. Howell and myself. The maps of that district began to be published in the year 1859, and the Memoirs two years later. In 1861, in a chronological grouping of the whole of the volcanic phenomena of Scotland, I gave an outline of the Carboniferous eruptions.[411] By degrees the detailed mapping of the Geological Survey was pushed across the whole of the rest of the south of Scotland, and the Carboniferous volcanic rocks of each area were then for the first time carefully traced and assigned to their various stratigraphical horizons. In the following pages reference will be given to the more important features of the Survey maps and Memoirs. In the year 1879, availing myself of the large amount of information which my own traverses and the work of the Survey had enabled me to acquire, I published a Memoir on the geology and petrography of the volcanic rocks of the basin of the Firth of Forth;[412] and lastly, in my Presidential Address to the Geological Society in 1892, I gave a summary of all that had then been ascertained on the subject of the volcanic rocks of Carboniferous time in the British Isles.[413]
[411] Trans. Roy. Soc. Edin. vol. xxii.
[412] Ibid. vol. xxix. (1879), p. 437.
[413] Quart. Journ. Geol. Soc. xlviii. (1892), p. 104. This summary, with additional details and illustrations, is embodied in the text.
Two well-marked types of volcanic accumulations are recognizable in the British Isles, which may be conveniently termed Plateaux and Puys.
1. Plateaux.—In this type, the volcanic materials were discharged over wide tracts of country, so that they now form broad tablelands or ranges of hills, reaching sometimes an extent of many hundreds of square miles and a thickness of more than 1000 feet. Plateaux of this character occur within the British area only in Scotland, where they are the predominant phase of volcanic intercalations in the Carboniferous system.
It is noteworthy that the Carboniferous plateaux appeared during a well-marked interval of geological time. The earliest examples of them date from the close of the Upper Old Red Sandstone. They were all in vigorous eruption during the time of the Calciferous Sandstones, but in no case did they survive into that of the Hurlet and later limestones. They are thus eminently characteristic of the earliest portion of the Carboniferous period.
2. Puys.—In this type, the ejections were often confined to the discharge of a small amount of fragmentary materials from a single solitary vent, and even where the vents were more numerous and the outpourings of lava and showers of ash more copious, the ejected material usually covered only a small area round the centres of eruption. Occasionally streams of basic lava and accumulations of tuff were piled up into long ridges. Volcanoes of this character were specially abundant in the basin of the Firth of Forth, and more sparingly in Ayrshire and Roxburghshire. They form the persistent type throughout the rest of the British Isles.
The Puys also occupy a well-defined stratigraphical position. They did not begin until some of the volcanic plateaux had become extinct. From the top of the Cement-stone group up into the Carboniferous Limestone series, their lavas and tuffs are met with on many platforms, but none occur above that series save in Ayrshire, where some of the eruptions appear to have been as late as about the beginning of the Coal-measures.