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CAMBRIAN SYSTEM
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See also:CAMBRIAN See also:SYSTEM , in See also:geology, the name now universally employed to designate the earliest See also:group of Palaeozoic rocks which possesses a connected See also:suite of fossils. The strata of this system See also:rest upon the Pre-Cambrian, and are succeeded by the Ordovician system. Until the See also:fourth See also:decade of the 19th See also:century all stratified rocks older than the Carboniferous had been grouped by geologists into a huge and indefinite "Transition See also:Series." In 1831 See also:Adam See also:Sedgwick and See also:Sir See also:Roderick I. See also:Murchison began the herculean task of studying and sub-dividing this series of rocks as it occurs in See also:Wales and the bordering counties of See also:England. Sedgwick attacked the problem in the See also:Snowdon See also:district, where the rocks are highly altered and displaced and where fossils are comparatively difficult to obtain; Murchison, on the other See also:hand, began to See also:work at the upper end of the series where the stratigraphy is See also:simple and the fossils are abundant. Murchison naturally4tnade 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 See also:peculiar to themselves; as he zealously worked on he followed the fossiliferous rocks further afield and continually See also:lower in the series. This fossil-bearingset of strata he first styled the "fossiliferous See also:greywacke series," changing it in 1835 to " See also:Silurian system." In the same See also: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 See also:Harlech, See also:Llanberis and the See also:Long Mynd. So far, the two workers had been in agreement; but in his presidential address to the See also:Geological Society of See also:London in 1842 Murchison stated his See also:opinion that the Cambrian contained no fossils that differed from those of the Lower Silurian. Where-upon Sedgwick undertook a re-examination of the Welsh rocks with the assistance of J. W. See also:Salter, the palaeontologist; and in 1852 he included the Llandeilo and See also:Bala beds (Silurian) in the Upper Cambrian. Two years later Murchison brought out his Siluria, in which he treated the Cambrian system as a See also:mere See also:local facies of the Silurian system, and he included in the latter, under J. See also:Barrande's See also:term " Primordial See also:zone," all the lower rocks, although they had a distinctive See also:fauna.
Meanwhile in See also:Europe and See also: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 See also:British See also:official geological maps.
Nevertheless, from 1851 to 1855, Sedgwick, in his writings on the British palaeozoic deposits, insisted on the See also:independence of the Cambrian system, and though Murchison had pushed his Silurian system downward in the series of rocks, Sedgwick adhered to the See also:original grouping of his Cambrian system, and even proposed to limit the Silurian to the See also:Ludlow and See also:Wenlock beds with the May See also: See also:Renevier proposed to use Silurique to See also:cover the same See also:period with the Cambrian as the lowest series, but these See also:differences of treatment are merely nominal. Jules See also:Marcou and others have used Taconic (Taconian) as the See also:equivalent of Cambrian, and C.See also:Lapworth proposed to apply the same term to the lowest sub-See also:division only; he had also used " Annelidian " in the same sense. These names are of See also:historical See also: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 See also:Scotland, See also:North America and See also:Canada important deposits of See also:limestone occur and subordinate limestones are found in the Cambrian of central Europe.
In some, regions, notably in the Baltic See also:province and in parts of the See also: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 See also:world they See also:bear abundant evidence of the many movements and stresses to which they have been exposed through so enormous a period of See also:time. Thus, we find them more frequently, folded, tilted and cleaved; the muds. have become shales, slates, phyllites or See also:schists, the See also:grey and red sands and conglomerates have become quartzites and greywackes, while the limestones are very generally dolomitized. In the Cambrian limestones, as in their more
See also:recent analogues, layers and nodules of See also:chest and phosphatized material are not wanting.
Igneous rocks are not extensively See also:developed; in Wales they See also:form an important feature and occur in considerable thickness;
Mt., De Lopp.ent See also:Emery See also: In Bohemia, North America and Canada igneous rocks have been observed. In See also:China, on the Yang-tse See also:river, a thick See also:deposit has been found full of boulders of diverse kinds of See also:rock, striated in the manner that is typical of glacial See also:action. A similar deposit occurs in the Gaisa beds near the Varanger See also:Fjord in See also:Norway. These formations See also: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 See also:floor of crystalline rocks. Cambrian See also:Life.—In a See also:general survey of the life of this period, as it is revealed by the fossils, three outstanding facts are apparent: (r) the great divergence between the Cambrian fauna and that of the See also: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 See also: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 See also:groups of organisms. Although in the Cambrian strata we have the See also:oldest known fossiliferous rocks—if we leave out of See also: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 See also:protozoa appear in the form of See also:Radiolaria; Lithistid See also:sponges are represented by such forms as Archaeoscyphia, Hexactinellid sponges by Protospongia; See also:Graptolites (Dictyograptus (Diclyonema)) come on in the higher parts of the system. See also:Medusa-like casts have been found in the lower Cambrian of Scandinavia (Medusina) and in the See also:mid-Cambrian of See also:Alabama (Brooksella). See also:Corals, Archaeocyathus, Spirocyathus, &c., lived in the Cambrian seas along with starfishes (Palaeasterina), Cystideans, Protocystiles, Trochocystites and possibly Crinoids, Dendrocrinus. Annelids left their traces in burrows and casts on the See also:sea-floor (Arenicolites, Cruziana, Scolithus, &c.). See also:Crustacea occupied an extremely prominent See also:place; there were Phyllocarids such as Hymenocaris, and' Ostracods like Entomidella; but by far the most important in See also:numbers and development were the Trilo-87 bites, now See also:extinct, but in palaeozoic times so abundant. In the Cambrian period See also:trilobites had already attained their maximum See also:size; some See also:species of Paradoxides were nearly 2 ft. long, but in See also:company with these monsters were tiny forms like Agnostus and Microdiscus. Many of the Cambrian trilobites appear to have been See also:blind, and they had not at this period developed that flexibility in the See also: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, Orlhis, Rhynchonella. It is a striking fact that certain of these non-articulate "See also:lamp-shells" are See also:familiar inhabitants of our present seas. Each of the See also:principal groups of true See also:mollusca was represented: Pelecypods (Modioloides); Gasteropods (Scenella, Pleurotomaria, Trochonema); Pteropods (Hyolithellus, Hyolitltes, Salterella) ; Cephalopods (Orthoceras, Cystoceras). Of See also:land See also: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 See also: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 See also:worms. Oldhamia, a delicate branching See also:body, abundant in the Cambrian of the See also:south-See also:east of See also: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 See also:Britain or by the Diclyonema beds or Euloma-See also:Niobe fauna elsewhere. Three regions deserve See also:special See also:attention: (r) Great Britain, the See also: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 See also:mass of limestone and See also:dolomite, the Durness limestone (rgoo ft.). In the lower series are, in descending See also:order, the " Serpulite grits " or " Salterella beds," the " Fucoid beds " and the " Eriboll See also:quartzite," which is divided into an upper " See also:Pipe rock " and lower " Basal quartzite." The Cambrian rocks of Ireland, a great series of See also:purple and See also: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 See also:American See also:continent, as in Europe, the Cambrian system is divisible into three series: (r) the lower or " Georgian," with Olenellus fauna; (2) the middle or " See also:Acadian," with Paradoxides or Dikelocephalus fauna; (3) the upper or " Pots-See also:dam," with Olenus fauna (with Saratogan or St Croix as synonyms for See also:Potsdam). The lower division appears on the See also:Newfoundland and Labrador coasts, and is traceable thence, in a great See also:belt south-See also:west of those points, through See also:Maine and the See also:Hudson-See also:Champlain valley into Alabama, a distance of some 2000 M.; and the rocks are brought up again on the western uplift, in See also:Nevada, See also:Idaho, See also:Utah, western See also:Montana and British See also:Columbia. The middle division covers approximately the same region as the lower one, and in addition it is found in the states of See also:Texas, See also:Oklahoma, and See also:Arizona, in western Montana, and possibly in western See also:Wisconsin. The lower division, in addition to covering the areas already indicated, spreads over the interior of the United States. Bohemia.—The Cambrian rocks ofthis See also: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 Areas in which marine deposits are known. Areas gained by the Sea between the beginning and See also:close of the Period. Unknown. The broken lines indicate the possible See also:distribution of Land and Sea. Additional information and CommentsThere are no comments yet for this article.
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