Page:EB1911 - Volume 05.djvu/99

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87
CAMBRIAN SYSTEM

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;

Distribution of Cambrian Rocks

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