1911 Encyclopædia Britannica/Cambrian System
CAMBRIAN SYSTEM, in geology, the name now universally employed to designate the earliest group of Palaeozoic rocks which possesses a connected suite of fossils. The strata of this system rest upon the Pre-Cambrian, and are succeeded by the Ordovician system. Until the fourth decade of the 19th century all stratified rocks older than the Carboniferous had been grouped by geologists into a huge and indefinite “Transition Series.” In 1831 Adam Sedgwick and Sir Roderick I. Murchison began the herculean task of studying and sub-dividing this series of rocks as it occurs in Wales and the bordering counties of England. Sedgwick attacked the problem in the Snowdon district, where the rocks are highly altered and displaced and where fossils are comparatively difficult to obtain; Murchison, on the other hand, began to work at the upper end of the series where the stratigraphy is simple and the fossils are abundant. Murchison naturally made the most of the fossils collected, and was soon able to show that the transition series could be recognized by them, just as younger formations had fossils peculiar to themselves; as he zealously worked on he followed the fossiliferous rocks further afield and continually lower in the series. This fossil-bearing set of strata he first styled the “ fossiliferous greywacke series, ” changing it in 1835 to “ Silurian system.”
In the same year Sedgwick introduced the name “ Cambrian series ” for the older and lower members. Murchison published his Silurian system in 1839, wherein he recognized the Cambrian to include the barren slates and grits of Harlech, Llanberis and the Long Mynd. So far, the two workers had been in agreement; but in his presidential address to the Geological Society of London in 1842 Murchison stated his opinion that the Cambrian contained no fossils that differed from those of the Lower Silurian. Whereupon Sedgwick undertook a re-examination of the Welsh rocks with the assistance of I. W. Salter, the palaeontologist; and in 1852 he included the Llandeilo and Bala beds (Silurian) in the Upper Cambrian. Two years later Murchison brought out his Siluria, in which he treated the Cambrian system as a mere local facies of the Silurian system, and he included in the latter, under J. Barrande's term “ Primordial zone, ” all the lower rocks, although they had a distinctive fauna.
Meanwhile in Europe and America fossils were being collected from similar rocks which were classed as Silurian, and the use of “ Cambrian ” was almost discarded, because, following Murchison, it was taken to apply only to a group of rocks without a characteristic fauna and therefore impossible to recognize. Most of the Cambrian rocks were coloured as Silurian on the British official geological maps.
Nevertheless, from 1851 to 1855, Sedgwick, in his writings on the British palaeozoic deposits, insisted on the independence of the Cambrian system, and though Murchison had pushed his Silurian system downward in the series of rocks, Sedgwick adhered to the original grouping of his Cambrian system, and even proposed to limit the Silurian to the Ludlow and Wenlock beds with the May Hill Sandstone at the base. This attitude he maintained untilthe year of his death (1873), when there appeared his introduction to Salter's Catalogue of Cambrian and Silurian Fossils.
It is not to be supposed that one of these great geologists was necessarily in the wrong; each had right on his side. It was left for the subsequent labours of Salter and H. Hicks to prove that the rocks below the undoubted lower Silurian of Murchison did indeed possess a characteristic fauna, and their work was confirmed by researches going on in other countries. To-day the recognition of the earliest fossil-bearing rocks, below the Llandeilo formation of Murchison, as belonging to the Cambrian system, and the threefold subdivision of the system according to palaeontological evidence, may be regarded as firmly established. It should be noted that A. de Lapparent classifies the Cambrian as the lowest stage in the Silurian, the middle and upper stages being Ordovician and Gothlandian. E. Renevier proposed to use Silurique to cover the same period with the Cambrian as the lowest series, but these differences of treatment are merely nominal. Jules Marcou and others have used Taconic (Taconian) as the equivalent of Cambrian, and C.Lapworth proposed to apply the same term to the lowest sub-division only; he had also used “ Annelidian ” in the same sense. These names are of historical interest alone.
Cambrian Rocks.—The lithological characters of the Cambrian rocks possess a remarkable uniformity iii all quarters of the globe. Muds, sands, grits and conglomerates are the predominant types. In Scotland, North America and Canada important deposits of limestone occur and subordinate limestones are found. in the Cambrian of central Europe.
In some regions, notably in the Baltic province and in parts of the United States, the rocks still retain their original horizontality of deposition, the muds are scarcely indurated and the sands are still incoherent; but in most parts of the world they bear abundant evidence of the many movements and stresses to which they have been exposed through so enormous a period of time. Thus, we find them more frequently, folded, tilted and cleaved; the muds have become shales, slates, phyllites or schists, the grey and red sands and conglomerates have become quartzite's and grey wackes, while the limestones are very generally dolomitized. In the Cambrian limestones, as in their more recent analogues, layers and nodules of chert and phosphatized material are not wanting.
Igneous rocks are not extensively developed; in Wales they form an important feature and occur in considerable thickness;
they are represented by lavas of olivine-diabase and by contemporaneous tuffs which are traversed by later granite and quartz felsite. In the Cambrian of Brittany there are acid lavas and tuffs. Quartz porphyry, diabase and diorite appear in the Ardennes. In Bohemia, North America and Canada igneous rocks have been observed.
In China, on the Yang-tse river, a thick deposit has been found full of boulders of diverse kinds of rock, striated in the manner that is typical of glacial action. A similar deposit occurs in the Gaisa beds near the Varanger Fjord in Norway. These formations lie at the base of the lowest Cambrian strata and may possibly be included in the pre-Cambrian, though in Norway they are clearly resting upon a striated floor of crystalline rocks.
Cambrian Life.—In a general survey of the life of this period, as it is revealed by the fossils, three outstanding facts are apparent: (1) the great divergence between the Cambrian fauna and that of the present day; (2) the Cambrian life assemblage differs in no marked manner from that of the succeeding Ordovician and Silurian periods; there is a certain family likeness which unites all of them; (3) the extraordinary complexity and diversity not only in the assemblage as a whole but within certain limited groups of organisms. Although in the Cambrian strata we have the oldest known fossiliferous rocks—if we leave out of account the very few and very obscure organic remains hitherto recorded from the pre-Cambrian—yet we appear to enter suddenly into the presence of a world richly peopled with a suite of organisms already far advanced in differentiation; the Cambrian fauna seems to be as far removed from what must have been the first forms of life, as the living forms of this remote period are distant from the creatures of to-day.
With the exception of the vertebrates, every one of the great classes of animals is represented in Cambrian rocks. Simple protozoa appear in the form of Radiolaria; Lithistid sponges are represented by such forms as Archaeoscyphia, Hexactinellid Sponges by Protospongia; Graptolites (Dictyograptus (Dictyonema)) come on in the higher parts of the system. Medusa-like casts have been found in the lower Cambrian of Scandinavia (Medusina) and in the mid-Cambrian of Alabama (Brooksella). Corals, Archaeocyathus, Spirocyathus, &c., lived in the Cambrian seas along with starfishes (Palaeasterina), Cystideans, Protocystites, Trochocystites and possibly Crinoids, Dendrocrinus. Annelids left their traces in burrows and casts on the sea-floor (Arenicolites, Cruziana, Scolithus, &c.). Crustacea occupied an extremely prominent place; there were Phyllocarids such as Hymertocaris, and'Ostracods like Entomidella; but by far the most important in numbers and development were the Trilobites, now extinct, but in palaeozoic times so abundant. In the Cambrian period trilobites had already attained their maximum size; some species of Paradoxides were nearly 2 ft. long, but in company with these monsters were tiny forms like Agnostus and Microdiscus. Many of the Cambrian trilobites appear to have been blind, and they had not at this period developed that flexibility in the carapace that some forms acquired later.
Brachiopods were fairly abundant, particularly the non-articulated forms (Obolus, Lingulella, Acrotreta, Discinopsis, &c.); amongst the articulate genera are Kutorgina, Orthis, Rhynchonella. It is a striking fact that certain of these non-articulate “lamp-shells ” are familiar inhabitants of our present seas. Each of the principal groups of true mollusca was represented: Pelecypods (Modioloides); Gasteropods (Scenetta, Pleurotomaria, Trochonema); Pteropods (Hyolithellus, Hyolithes, Salterella); Cephalopods (Orthoceras, Cystoceras). Of land plants no traces have yet been discovered. Certain markings on slates and sandstones, such as the “ fucoids ” of Scandinavia and Scotland, the Phycoides of the Fichtelgebirge, Eophyton and other seaweed-like impressions, may indeed be the casts of fucoid plants; but it is by no means sure that many of them are not mere inorganic imitative markings or the tracks or casts of worms. Oldhamia, a delicate branching body, abundant in the Cambrian of the south-east of Ireland, is probably a calcareous alga, but its precise nature has not been satisfactorily determined.
Cambrian Stratigraphy.—Wherever the Cambrian strata have been carefully studied it has now been found possible and convenient to arrange them into three series, each of which is characterized by a distinctive genus of trilobite. Thus we have a Lower Cambrian with Olenellus, a middle series with Paradoxides and an Upper Cambrian with Olenus. It is true that these fossils are not invariably present in every occurrence of Cambrian strata, but this fact notwithstanding, the threefold division holds with sufficient constancy. An uppermost series lies above the Olenus fauna in some areas; it is represented by the Tremadoc beds in Britain or by the Dictyonema beds or Euloma-Niobe fauna elsewhere. Three regions deserve special attention: (1) Great Britain, the area in which the Cambrian was first differentiated from the old “ Transition Series ”; (2) North America, on account of the wide-spread occurrence of the rocks and the abundance and perfection of the fossils; and (3) Bohemia, made classic by the great labours of J. Barrande.
Great Britain and Ireland.—The table on p. 88 contains the names that have been applied to the subdivisions of the Cambrian strata in the areas of outcrop in Wales and England; at the same time it indicates approximately their relative position in the system.
In Scotland the upper and middle series are represented by a thick mass of limestone and dolomite, the Durness limestone (1500 ft.). In the lower series are, in descending order, the “ Serpulite grits ” or “ Salterella beds, " the “ Fucoid beds " and the “Eriboll quartzite, ” which is divided into an upper “ Pipe rock" and lower “ Basal quartzite."
The Cambrian rocks of Ireland, a great series of purple and green shales, slates and grits with beds of quartzite, have not yet yielded sufficient fossil evidence to permit of a correlation with the Welsh rocks, and possibly some parts of the series may be transferred in the future to the overlying Ordovician.
North America.—On the North American continent, as in Europe, the Cambrian system is divisible into three series: (1) the lower or “ Georgian, ” with Olenellus fauna; (2) the middle or “ Acadian, " with Paradoxides or Dikelocephalus fauna; (3) the upper or “ Potsdam," with Olenus fauna (with Saratogan or St Croix as synonyms for Potsdam). The lower division appears on the Newfoundland and Labrador coasts, and is traceable thence, in a great belt south-west of those points, through Maine and the Hudson-Champlain valley into Alabama, a distance of some 2000 m.; and the rocks are brought up again on the western uplift, in Nevada, Idaho, Utah, western Montana and British Columbia. The middle division covers approximately the same region as the lower one, and in addition it is found in the states of Texas, Oklahoma, and Arizona, in western Montana, and possibly in western Wisconsin. The lower division, in addition to covering the areas already indicated, spreads over the interior of the United States.
Bohemia.—The Cambrian rocks of this country are now recognized by J. F. Pompeskj to comprise the Paradoxidian and Olenelledian groups. - They were made famous through the researches of Barrande. The Cambrian system is covered by his stages “ B ” and “ C ” ; the former a barren series of conglomerates and quartzite's, the latter a series of grey and green fissile shales 1200 ft. thick with sandstones, greywackes and conglomerates.
Scandinavia.—Here the Cambrian system is only distinguished clearly on the eastern side, where the three subdivisions are found in a thin series of strata (400 ft.), in which black concretion-bearing
|North Wales.||South Wales.||
Midland and West of England.
(1) Dolgelly beds
(2) Ffestiniog beds
(3) Maentwrog beds
and shales with
|Menevian beds||Menevian beds
|Comley or Holly-
|Harlech grits and
|Caerfai group||Lower Comley
ate at the base
Middle and lower
zite and the
quartzite of the
shales play an important part. Limestones and shales with the Euloma-Niobe fauna come at the top. The upper series (Olenus) has been minutely zoned by W. C. Brögger, S. A. Tullberg and J. C. Moberg. In the middle series (Paradoxides) three thin limestone bands have been distinguished, the Fragmenten-Kalk, the Exulans-Kalk and the Andrarums-Kalk. On the Norwegian side the Cambrian is perhaps represented by the Röros schists which lie at the base of a great series of crystalline schists, the probable equivalent of Ordovician and Silurian rocks.
Baltic Province.–The Cambrian rocks in this region are nearly all soft sediments, some 600 ft. thick; they reach from the Gulf of Finland towards Lake Ladoga. At the base is the so-called “ blue clay ” (really greenish) with ferruginous sandstones and with a fucoidal sandstone at its summit. This division is the equivalent of the Lower Cambrian. Above the fucoidal sandstone an important break appears in the system, for the Paradoxides and Olenus divisions are absent. The upper members are the “ Ungulite grit " and about 20 ft. of Dictyonema shale. Cambrian rocks have been traced into Siberia (lat. 71°) and on the island of Vaigatch.
Central Europe.–Besides the Bohemian region previously mentioned, Cambrian rocks are present in Belgium and the north of France, in Spain and the Thüringer Wald. In the Ardennes the system is represented by grits and sandstones, shales, slates and quartz schists, and includes also whet slates and some igneous rocks. A. Dumont has arranged the whole series (Terrain ardennais) into three systems, an upper “ Salmien, " a middle “ Revinien ” and a lower “ Devillien, " but J. Gosselet has subsequently proposed to unite the two lower groups in one.
France.–In northern France Cambrian rocks, mostly purple conglomerates and red shales, rest with apparent unconformability upon pre-Cambrian strata in Brittany, Normandy and northern Poitou. In the Rennes basin limestones—often dolomitic—are associated with quartzite's and conglomerates; siliceous limestones also occur in the Sarthe region. Farther south, around the old lands of Languedoc, equivalents of the two upper divisions of the Cambrian have been recorded; and the uppermost members of the system appear in Herault. Patches of Cambrian rocks are found in the Pyrenees.
In Spain slates and quartzite's, the slates of Rivadeo, more than 9000 ft. thick, are followed by the middle Cambrian beds of La Vega, thick quargzites with limestone, slates and iron ores. Cambrian rocks occur also in the provinces of Seville and Ciudad-Real. Upper Cambrian strata have been found in upper Alemtejo in Portugal.
In Russian Poland is a series of conglomerates, quartzite's and shales; some of the beds yield a Paradoxides fauna. In the Thüringer Wald are certain strata, presumably Cambrian since the uppermost beds contain the Euloma-Niobe fauna.
Sardinia contains both middle and upper Cambrian. The Cambrian system is represented in the Salt Range of India by the Neobolus or Khussack beds, which may possibly belong to the middle subdivision. The same group is probably represented in Corea and the Liao-tung by the thick “Sinisian" formation of F. von Richthofen. In South America upper Cambrian rocks have been recorded from north Argentina. The Lower Cambrian has been found at various places in South Australia; and in Tasmania a thick series of strata appears to be in part at least of Upper Cambrian age.
General Physical Conditions in the Cambrian Period.—The Cambrian rocks previously described are all such as would result from deposition, in comparatively shallow seas, of the products of degradation of land surfaces by the ordinary agents of denudation. Evidences of shallow water conditions are abundant; very frequently on the bedding surfaces of sandstones and other rocks we find cracks made by the sun's heat and pittings caused by the showers that fell from t.he Cambrian sky, and these records of the weather of this remote period are preserved as sharply and clearly as those made only to-day on our tidal reaches. Ripple marks and current bedding further point to the shallowness of the water at the places where the rocks were made.
No Cambrian rocks are such as would be formed in the abysses of the sea—although the absence of well-developed eyes in the trilobites has led some to assume that this condition was an indication that the creatures lived in abyssal depths. At the close of the pre-Cambrian, many of the deposits of that period must have been elevated into regions of fairly high ground; this we may assume from the nature of the Cambrian deposits which are mainly the product of the denudation of such ground. Over the land areas thus formed, the seas in Cambrian time gradually spread, laying down first the series known as Lower Cambrian, then by further encroachment on the land the wider spread Upper Cambrian deposits—in Europe, the middle series is the most extensive. Consequently, Cambrian strata are usually unconformable on older rocks.
During the general advance of the sea, local warpings of the crust may have given rise to shallow lagoon or inland-lake conditions. The common occurrence of red strata has been cited in support of this view.
Compared with some other periods, the Cambrian was free from extensive volcanic disturbances, but in Wales and in Brittany the earlier portions of this period were marked by voluminous outpourings; a condition that was feebly reflected in central and southern Europe.
No definite conclusions can be drawn from the fossils as to the climatic peculiarities of the earth in Cambrian times. The red rocks may in some cases suggest desert conditions; and there is good reason to suppose that in what are now Norway and China a glacial cold prevailed early in the period.
Considerable variations occur in the thickness of Cambrian deposits, which may generally be explained by the greater rapidity of deposition in some areas than in others. Nothing could be more striking than the difference between the thicknesses in western and eastern Europe; in Brittany the deposits are over 24,000 ft. thick, in Wales at least 12,000 ft., in western England they are only 3000 ft., and in northern Scotland 2000 ft., while no farther east than Scandinavia the complete Cambrian succession is only about 400 ft. thick. Again, in North America, the greatest thicknesses are found along the mountainous regions on the west and on the east—reaching 12,000 ft. in the latter and probably nearly 40,000 ft. in the former (in British Columbia)—while over the interior of the continent it is seldom more than 1000 ft. thick.
Any attempt to picture the geographical conditions of the Cambrian period must of necessity be very imperfect. It was pointed out by Barrande that early in Palaeozoic Europe there appeared two marine provinces-a- northern one extending from Russia to the British Isles through Scandinavia and northern Germany, and a southern one comprising France, Bohemia, the Iberian peninsula and Sardinia. It is assumed that some kind of land barrier separated these two provinces. Further, there is a marked likeness between the Cambrian of western Europe and eastern America; many fossils of this period are common to Britain, Sweden and eastern Canada; therefore it is likely that a north Atlantic basin existed. Prof. Kayser suggests that there was also a Pacific basin more extensive than at present; this is borne out by the similarity between the Cambrian faunas of China, Siberia and Argentina. The same author postulates an Arctic continent, bordering upon northern Europe, Greenland and North America; an African-Brazilian continent across the present south Atlantic, and a marine communication between Australia and India, where the faunas have much in common.
References.—The literature devoted to the Cambrian period is very voluminous, important contributions having been made by A. Sedgwick, Sir R. I. Murchison, H. Hicks, C. Lapworth, T. Groom, J. W. Salter, J. E. Marr, C. D. Walcott, G, F. Matthew, E. Emmons, E. Billings, J. Barrande, F. Schmidt, W. C. Brögger, S. A. Tullberg, S. L. Törngrist, G. Linnarsson and many others. A good general account of the period will be found in Sir A. Geikie’s Text-Book of Geology. vol. ii. 4th ed. 1903 (with references), and from an American point of view, in T. C. Chamberlin and R. D. Salisbury’s Geology, vol. ii., 1906 (references to American sources). See also J. E. Marr, The Classification of the Cambrian and Silurian Rocks, 1883 (with bibliography up to the year of publication); A. Geikie, Q. J. Geol. Soc., 1891, xlvii., Ann. address, p. 90; F. Freeh, “ Die geographische Verbreitung und Entwickelung des Cambrium, " Compte Rendu. Congrés Géol. Internat. 1897, St-Pétersbourg (1899); Geological Literature added to the Geological Society’s Library, published annually since 1893. (J. A. H.)