[81] Since this page was in type, Professor Lapworth has found a worm-burrow low down in the Brand Series, and one or two additional examples have since been obtained by Mr. J. Rhodes of the Geological Survey. These are the first undoubted organisms from the Charnwood Forest rocks. Mr. Watts, Geol. Mag. 1896, p. 487.
BOOK III
THE CAMBRIAN VOLCANOES
CHAPTER IX
CHARACTERISTICS OF THE CAMBRIAN SYSTEM IN BRITAIN
The Physical Geography of the Cambrian Period—The Pioneers of Palæozoic Geology in Britain—Work of the Geological Survey in Wales—Subdivisions of the Cambrian System in Britain.
In leaving the investigation of the pre-Cambrian rocks and entering upon that of the Palæozoic systems, that is, the great series of sedimentary formations which include the earliest records of organized life upon the surface of the globe, the geologist feels much as the historian when, quitting the domain of legend and tradition, he can tread firmly in the region of documentary evidence. From the bottom of the Cambrian system upward through the long series of geological formations, the chronicle, though often sadly incomplete, is usually clear and legible. As we follow the lowest fossiliferous strata across a territory, we recognize that they bear witness to the same processes of denudation and deposition which have been going on uninterruptedly on the face of the globe ever since. The beds of conglomerate represent the gravels and shingles of old coast-lines and river-beds. The sandstones recall the familiar features of sandy sea-bottoms not far from land. The shales bear witness to the fall of fine sediment in stiller water, such as now takes place in the deeper parts of seas and lakes. Notwithstanding their vast antiquity, the strata themselves exhibit no exceptional peculiarities of origin. They seem to be just such familiar deposits as are gathering under fitting conditions at the present time.
Some writers have speculated on the far greater intensity of all geological activities in the early times of the planet's history. But if we may interpret the record of the stratified formations by the phenomena of to-day, there is for these speculations no confirmation in the sedimentation of the oldest stratified deposits. It is of course quite intelligible, if not probable, that many geological forces may have been more vigorous in primeval times than they afterwards became. But of the gigantic tides, prodigious denudation and violent huddling together of the waste of the earth's surface, which have been postulated for the early Palæozoic ages, there is assuredly nowhere any indication among the stratified formations. In those vast orderly repositories, layer succeeds layer among thinly-laminated shales, as gently and equably as the fine silt of each tide sinks to-day over the floor of a sheltered estuary. At the primeval period of which these sediments are the memorial, the waters receded from flat shores and left tracts of mud bare to the sky, precisely as they do still. Then as now, the sun shone and dried such mud-flats, covering their surfaces with a network of cracks; the rain fell in heavy drops, that left their imprints on the drying mud; and the next tide rose so gently as to overflow these records of sunshine and shower without effacing them, but spreading over them a fresh film of sediment, to be succeeded by other slowly-accumulating layers, under which they have lain preserved during the long cycles of geological history.
That organized creatures had already appeared upon the earth's surface before the beginning of the Cambrian period cannot be doubted. The animal remains in the lowest Cambrian strata are far from being the simple forms which might be expected to indicate the first start of animal life upon the surface of the earth. On the contrary, though they are comparatively scanty in types, and often rare or absent throughout a thick mass of sedimentary deposits, they show beyond dispute that, when they flourished, invertebrate life had already reached such a stage of advancement and differentiation that various leading types had appeared which have descended, in some cases with generic identity, down to our own day. There must have been a long pedigree to these organisms of the oldest known fossiliferous rocks. And somewhere on the earth's surface we may yet hope to find the remains of their progenitors in pre-Cambrian deposits.
The researches of many explorers in Europe and North America have brought to light an interesting series of organic remains from the Cambrian system. Of the plants of the time hardly any traces have survived, save some markings which have been referred to sea-weeds. The earliest known sponges and corals occur in this system, likewise the ancestors of the graptolites, which played so prominent a part in the life of the next or Silurian period. There were already representatives of crinoids and star-fishes, besides examples of the extinct group of cystideans. Sea-worms crawled over the muddy and sandy sea-bottom, for they have left their trails and burrows in the hardened sediments. Molluscs had by this time appeared in their four great divisions of Brachiopods, Lamellibranchs, Gasteropods and Cephalopods, though the forms yet discovered among Cambrian rocks are comparatively few. The most abundant and characteristic inhabitants of the Cambrian seas were the trilobites, of which many genera have been disinterred from the strata. In the lowest fossiliferous Cambrian group the trilobitic genus Olenellus, already referred to, is the characteristic form. Higher up Paradoxides is predominant, while towards the top of the system the most characteristic genus is Olenus.
From the organic remains which have been preserved, we may legitimately infer the existence of others which have entirely disappeared. There seems no reason to doubt that the leading grades of invertebrate life which are wanting in the known Cambrian fauna were really represented in the Cambrian seas. The chance discovery of a band of limestone may any day entirely alter our knowledge as to the relative proportions of the several divisions of the animal kingdom in the earliest Palæozoic rocks. Sand is rather adverse to the preservation of a varied representation of the organisms of the overlying sea-water. Mud is generally favourable, but calcareous accumulations are greatly more so, and they usually consist almost entirely of organic remains. Thus in the Cambrian series of the north-west of Scotland the quartzites that form the lower group, though sometimes crowded with worm-burrows, contain hardly any other sign of organisms. The overlying shales, besides their abundant worm-castings, have yielded perfect specimens of Olenellus and other fossils. But in the uppermost group, consisting of limestones, every particle of the sediment appears to have passed through the intestines of worms, and as it gathered on the sea-bottom it enclosed and has preserved a varied and abundant assemblage of organisms, including trilobites, gasteropods and a number of cephalopods. While in the Cambrian rocks of Europe calcareous bands are comparatively rare, in those of North America they are not infrequent. Hence it is largely from American deposits that our knowledge of the Cambrian fauna has been derived.
Not a vestige of any vertebrate organism has yet been detected among the earlier Palæozoic sediments. So far as we know, there were no fishes in the Cambrian seas. The highest organisms then existing were chambered shells, a once abundant and singularly varied class, of which the living Nautilus is now the sole representative.