In trying to realize the general geographical conditions of Cambrian time, the geologist finds himself entirely without any evidence as to the character of the terrestrial vegetation. We can hardly doubt that the land was clothed with plants, probably including lycopods and ferns, possibly even cycads and conifers. But no remains of this flora have yet been recovered. Nor have any traces of land-animals been detected. All that we yet know of the life of the period has been gleaned from marine sediments, which show that the invertebrate population by which the sea was then tenanted embraced some of the leading types of structure that have survived through all the long vista of geological time down to our own day.
Some of the shore-lines of the Cambrian waters may still be traced, and it is possible to say where the land of the time stood and where lay the sea. In the British area the largest relic of Cambrian land is found in the far north-west of Scotland. Formed partly of the Lewisian Gneiss and partly of the Torridon Sandstone, it takes in the whole chain of the Outer Hebrides and likewise part of the present western seaboard of Sutherland and Ross. Along the margin of that northern land the white sand was laid down which now gleams in sheets of snow-like quartzite on most of the higher mountains from Cape Wrath to Skye. The sea lay to the east and, so far as we know, may have stretched across the rest of Scotland, and the north and centre of England. Another vestige of the land of this ancient era occurs in Anglesey. There, and likewise over scattered tracts in the Midlands, and in the south-west of England, the geologist seems to descry the last relics of islets that rose out of the Cambrian sea, and are now surrounded with its hardened sediments.
While such was the general aspect of the region of the British Isles during Cambrian time, volcanic action manifested itself at various localities over the area, breaking out on the sea-bottom, and pouring forth sheets of lava and showers of ashes, which mingled with the sand and silt that were settling there at the time. In the northern or Scottish tract no trace of this subterranean activity has been found; but in the English Midlands and over much of Wales abundant evidence has been obtained to show that in those districts the Cambrian period was marked by frequent and prolonged eruptions.
As its name denotes, the Cambrian system is typically developed in Wales. It was there that Sedgwick first worked out the stratigraphical relations of its ancient sediments, and that Murchison demonstrated the succession of organic remains contained in them, applying to them the principles of classification laid down by William Smith in regard to the Secondary formations. It was there too that some of the earliest and most memorable achievements were made in the investigation of ancient volcanic rocks. Sedgwick and Murchison, besides the admirable work which they accomplished in establishing the stratigraphy of the older Palæozoic formations, clearly recognized that among these formations there were preserved the records of contemporaneous submarine eruptions. Sedgwick showed that the mountainous masses of eruptive rock in North Wales were really lavas and ashes, which had been discharged over the sea-floor at the time when the ancient sediments of that region were deposited, while Murchison established the same fact by numerous observations in the east and south of Wales, and in the bordering English counties. De la Beche had found similar evidence among the "grauwacke" rocks of Devonshire.[82]
[82] For early researches on the older Palæozoic volcanic rocks of Britain, see Sedgwick, Proc. Geol. Soc. vols. ii. (1838) pp. 678, 679, iii. (1841) p. 548, iv. (1843) p. 215; Quart. Journ. Geol. Soc. vols. i. (1845) pp. 8-17, iii. (1847) p. 134. Murchison, Proc. Geol. Soc. vol. ii. (1833-34) p. 85; Silurian System (1839) pp. 225, 258, 268, 287, 317, 324, 401; Siluria, 4th edit. (1867) p. 76 et seq. De la Beche, Mem. Geol. Survey, vol. i. (1846) pp. 29-36. A. C. Ramsay in the Maps and Horizontal Sections of Wales published by the Geological Survey; also Descriptive Catalogue of the Rock-Specimens in the Museum of Practical Geology, 1st edit. (1858), 2nd edit. (1859), 3rd edit. (1862); "The Geology of North Wales," forming vol. iii. of Memoirs of the Geological Survey, 1st edit. (1866), 2nd edit. (1881).
Following in the track thus opened up by these great masters, the officers of the Geological Survey were enabled to unravel, as had never before been attempted, the complicated structure of the old volcanic regions of Wales. At the outset of the following discussion I wish to express my admiration of the labours of the early pioneers who thus laid for us the foundation of our knowledge of volcanic action in the Palæozoic periods. To De la Beche and his associates in the Survey a special measure of gratitude is due from all who have followed in their steps and profited by their work. When we consider the condition of geological science, and especially of the department of petrography, in this country at the time when these early and detailed investigations were carried on, when we remember the imperfection of much of the topography on the old one-inch Ordnance maps (which were the only maps then available), when we call to mind the rugged and lofty nature of the ground where some of the most complicated geological structures are displayed, we must admit that at the period when these maps and sections were produced they could not have been better done; nay, that as in some important respects they were distinctly in advance of their time, their publication marked an era in the progress of structural, and especially of volcanic, geology. The separation of lavas and tuffs over hundreds of square miles in a mountainous region, the discrimination of intrusive sheets and eruptive bosses, the determination of successive stratigraphical zones of volcanic activity among some of the oldest fossiliferous formations, were achievements which will ever place the names of Ramsay, Selwyn, Jukes and their associates high in the bede-roll of geological science. No one ever thinks now of making a geological excursion into Wales without carrying with him the sheets of the Geological Survey map. These form his guide and handbook, and furnish him with the basis of information from which he starts in his own researches.
But science does not stand still. The most perfect geological map that can be made to-day will be capable of improvement thirty or forty years hence. The maps of the Geological Survey are no exception to this rule. In criticizing and correcting them, however, let us judge them not by the standard of knowledge which we have now reached, but by that of the time when they were prepared. It is easy to criticize; it is not so easy to recognize how much we owe to the very work which we pronounce to be imperfect.
The ancient volcanoes of Wales, thanks mainly to the admirable labours of my former friend and chief, Sir Andrew C. Ramsay, have taken a familiar place in geological literature. But a good deal has been learnt regarding them since he mapped and wrote. The volcanic history, as he viewed it, began in the Arenig period. The progress of subsequent inquiry, however, has shown that there are volcanic rocks in Wales of much older date. I shall show that the Cambrian period, both in South and North Wales, was eminently volcanic.
Much controversy having arisen as to the respective limits and nomenclature of the older Palæozoic rocks, let me state, at the outset of the inquiry into the volcanic eruptions of Cambrian time, that under the term "Cambrian" I class all the known Palæozoic rocks which lie below the bottom of what is termed the Arenig group. It was maintained by Sir Andrew Ramsay and his colleagues on the Geological Survey that on the mainland of Wales no base is ever found to the Cambrian system. More recently certain conglomerates have been fixed upon as the true Cambrian base, both in South and North Wales, and endeavours have been made to trace an unconformability at that line, all rocks below it being treated as pre-Cambrian. But conglomerates do not necessarily mark a stratigraphical discordance, and in South Wales there is no trace of any unconformability between the strata above and below the supposed line of break.[83] Professor Bonney has shown that in North Wales several zones of conglomerate have been erroneously identified as the supposed basal platform of the Cambrian series, and more recently Mr. Blake has pointed out that some of these conglomerates are unquestionably Lower Silurian.
[83] See a discussion of this subject in Quart. Journ. Geol. Soc. vol. xxxix. (1883), p. 305.