CHAPTER XIV.. The Cambrian System.

THE CAMBRIAN SYSTEM.

Classification. The rocks of the Cambrian system when found reposing on Precambrian rocks in Britain are always separated from the latter by an unconformity. The typical development of the rocks of the system, as the name implies, is in the hilly region of Caernarvonshire and Merionethshire in North Wales, and they are also well represented in South Wales, the border counties between England and Wales, and the North-West Highlands of Scotland. Two distinct classifications of the Cambrian rocks of Britain are in use, the original one founded on variations of lithological character, whilst the second depends upon faunistic differences, but the original lithological classification has been to some extent modified to make it locally correspond with the classification based upon palæontological grounds. The following table will shew the differences:--

Lithological Classification. Palæontological Classification.

Tremadoc Slate Series Beds with Intermediate Fauna

Lingula Flags Series Beds with Olenus Fauna

Menevian beds (formerly included } in Lingula Flags) } Beds with Paradoxides Fauna } Formerly grouped } Solva beds } together as Harlech Caerfai beds } or Llanberis beds Beds with Olenellus Fauna

The original lithological classification was essentially the result of Prof. Sedgwick's work in North Wales, while the classification according to faunas is the outcome of the researches of Dr Hicks in South Wales.

Description of the Strata. The Cambrian rocks of North Wales occur in two complex anticlines, separated by an intermediate syncline of Ordovician strata occupying the Snowdonian hills. The southerly or Harlech anticline forms a part of Merionethshire to the east of Harlech, whilst the northern one is developed around Bangor and Llanberis. The South Welsh Cambrian rocks are chiefly found on either side of the Pembrokeshire axis of Precambrian rocks which runs through St David's. As the corresponding rocks of the two regions were deposited in bathymetrical zones of much the same depth, it will be convenient to give a general account of the rocks of the two regions at the same time, leaving the student to acquire information of the detailed variations in the larger text-books and in special memoirs.

The strata of the Caerfai and Solva groups show the prevalence of the shallow-water phase almost uninterruptedly through the whole of the time occupied by their accumulation in the Welsh areas. They consist chiefly of basal conglomerates, succeeded by alternations of grits and shales, though the latter are often converted into slates, owing to the subsequent production of cleavage. The basal conglomerates of the Caerfai beds are frequently marked by the existence of enormous pebbles, composed of fragments of the rocks of the underlying Precambrian groups, and the possibility of the occurrence of glacial action during their accumulation as advocated by Dr Hicks must be taken into account. Above these beds are various coloured grits, with alternations of muddy sediments often coloured red. The Solva group consists of massive grits, of various colours, also with alternations of mud, which have prevalent purple and green hues. The great thickness of the strata of the Caerfai and Solva Series, which sometimes exceeds 10,000 feet, must also be noted.

The Menevian beds consist essentially of very fine, well laminated black and grey muds, which are of a texture favourable for the production of a somewhat regular jointing, causing the rock to break into small rectangular blocks. They are thin, not exceeding 600 feet in thickness, and indicate the incoming of the general deep-water phase of the Lower Palæozoic epoch. The Lingula Flags mark a local return to shallower water conditions, especially in the central portion. The total thickness is over 3,000 feet, of which the lower stage (locally the Maentwrog series) is over 500 feet, and consists of blackish muds, the middle (Festiniog stage) is about 2,000 feet thick, and is composed chiefly of shallower water gritty flags, whilst the upper (Dolgelly) stage is of about the same thickness as the lower stage and has similar lithological characters.

The Tremadoc Slates are about 1,000 feet thick. They are divided into a lower and upper stage, of about equal thickness, and are essentially composed of iron-stained slates, with a considerable admixture of calcareous matter in some parts of South Wales, when they furnish the nearest approach to a limestone which has been found amongst the Welsh Cambrian strata. They were probably formed in a fairly deep sea.

Much pyroclastic rock and some lava flows are intercalated amongst the Welsh Cambrian sediments. Tuffs are formed in the lower beds of St David's, and lavas and ashes have been found amongst the Lingula Flags and Tremadoc Slates of North Wales, while the Lingula Flags of South Wales have furnished several bands of ash to the north of Haverfordwest. Much of the material of the grits and muds may be derived from volcanic rocks, though how far this is so cannot be stated in the absence of information obtained by detailed petrological examination of the rocks.

The various isolated outcrops of Cambrian strata amongst the counties of the Welsh borders and adjoining Midland counties indicate a great thinning of the Cambrian rocks in this direction.

The probable equivalents of the Caerfai rocks occur at Nuneaton, Comley, and on the flanks of the Wrekin and Malvern hills. The thin basal conglomerates are succeeded by quartzites, and sometimes red calcareous sandstones (Comley sandstone). These rocks are succeeded by thin arenaceous and calcareous beds which represent either the Solva or Menevian beds of Wales. The Lingula Flags are represented by the Malvern Shales of the Malvern area and the Stockingford Shales of Nuneaton, whilst the Tremadoc Slates have as their equivalents the Shineton Shales. The exact thicknesses of these deposits do not seem to have been recorded, but Prof. Lapworth observes that in central Shropshire "the Comley and Shineton groups which ... have a collective thickness of perhaps less than 3,000 feet, we have apparently a condensed epitome of the entire Cambrian system as at present generally defined."

The Cambrian rocks of the North-west Highlands consist of a thin conglomerate succeeded by grits and flags with shaley beds, and above these a mass of limestone, which may represent some of the Ordovician deposits as well as those of Cambrian age. Pending a complete description of the faunas of these rocks, it is sufficient to state that the only fauna which has hitherto been described in detail indicates the existence of Lowest Cambrian rocks. Further remarks will be made on this head when describing the character of the Cambrian faunas. The Cambrian rocks of the North-west Highlands are also very thin as compared with those of Wales, so that the Highland and Welsh borderland regions appear to have existed as a deeper sea area than that which is indicated by the Cambrian rocks of Wales, an inference which is to some extent borne out by study of the Cambrian rocks of extra-British areas, to which we may now turn.

The principal European developments of Cambrian rock are found in Scandinavia, Russia, Bohemia and Spain, and of these the Scandinavian one is by far the most fully developed, as there is a complete sequence in the rocks of that peninsula. They occur both in Norway and Sweden, but the Swedish exposures are the most interesting in most respects, especially those of Westrogothia and Scania. The rocks are of no great thickness, and consist essentially of black carbonaceous shales, with inconstant bands of impure black limestone composed almost entirely of the remains of trilobites or more rarely of brachiopods. These Alum Shales, as they are termed, rest unconformably upon Precambrian rocks, and have arenaceous and conglomeratic deposits at the base. In Russia the rocks are still further attenuated, and have not yielded the relics of so many faunas as have been found in the Scandinavian Cambrian rocks.

The Bohemian development is incomplete, owing apparently to an unconformity at the base of the overlying Ordovician rocks, while the Spanish deposits which seem fairly thick and composed largely of mechanical sediments have not been worked out in very great detail.

The American development of Cambrian rocks resembles the European one in many striking particulars, and as in the case of Europe, there are lateral variations in the lithological characters of the rocks, though in the opposite direction, the shallow-water deposits occurring on the east coast, and the deep-water deposits further west.

The general distribution of the different types of Cambrian strata in Europe and North America has been accounted for on the supposition that in Cambrian times a tract of land lay over much of the present site of the North Atlantic Ocean, and that the detritus of that land formed the shallow-water accumulations of Wales and the east of Canada, whilst further away from it were deposited the open-sea accumulations of Scandinavia and Russia on one side and of the more westerly regions of North America on the other, as indicated in Fig. 16.

P. Precambrian Rocks. A. Land. X, X´. Sea level. BB´. Shore deposits. CC´. Deep-water deposits. DD´. Abyssal deposits. ]

The Cambrian Faunas. The Cambrian Period has been termed the age of trilobites, for they are the dominant forms of the time, but they are associated with many other forms of invertebrata; indeed all the great groups of this division are represented in the earliest Cambrian fauna. Dr C. D. Walcott records representatives of Spongiae, Hydrozoa, Echinodermata, Annelida, Brachiopoda, Lamellibranchiata, Gastropoda, Pteropoda, Crustacea and Trilobita as occurring in the Olenellus beds of North America and other groups are represented in the rocks of this age in the Old World. The Cambrian trilobites as a whole are of more generalised types than those of the later systems which furnish their remains, as indicated especially by the looseness of the body, and the large number of body rings in many of the genera, while the tail or pygidium was small and formed of only a few coalesced segments, as pointed out by Barrande. In the later trilobites the test is more compact, there are on the whole fewer body rings, as more of these have become fused into a tail which is therefore larger than that of the average tail of the Cambrian trilobite.

Taking the faunas in order, the oldest or Olenellus fauna has furnished a great variety of forms in the North-west Highlands of Scotland, Shropshire, Scandinavia, Esthonia, Sardinia, Canada, and Newfoundland, whilst representative species of the fauna have been recorded also from Worcestershire, Warwickshire, Pembrokeshire, India, China, and Australia.

The dominant form is the trilobite of the genus or group Olenellus, which contains a great variety of species referable to three or four divisions which have been ranked as separate genera by some writers. Associated with Olenellus are trilobites belonging to other genera, which are found in higher deposits, though there represented by different species.

Brachiopods are fairly abundant, especially those provided with a horny shell; of these, the genus Kutorgina is widely distributed.

The zoological relationships of several of the fossils of this horizon are as yet doubtful. The Archæocyathinæ show affinities with certain corals; a number of tests, included in the genus Hyolithes and its allies are doubtfully referred to the Pteropods, and the position of the genus Volborthella is uncertain. Special attention is directed to these doubtful relationships, as it is possible that a number of 'generalised forms' of organisms occur in these strata.

It should be noticed here that faunas have been discovered which are possibly of earlier date than the Olenellus fauna, as they do not correspond with it, or with those of newer strata. One, the Neobolus fauna of the Salt Range of India, occurs in beds below those with Olenellus, though it is not yet clear that Olenellus will not be eventually discovered associated with it, whilst the other, the Protolenus fauna of Canada, is of unknown age.

The Olenellus beds are succeeded by beds containing the Paradoxides fauna, which have been found in North and South Wales, Shropshire, Scandinavia, Bohemia, Spain, and North and South America. Olenellus and its allies became extinct (or else so scarce that no relics of them have been discovered in the Paradoxides beds) before the commencement of the deposition of the strata containing the Paradoxides fauna, and few genera pass from the beds with the one fauna to that containing the other. The Paradoxides fauna existed for a considerable period, and the beds have been divided into a series of zones characterised by different species of Paradoxides, thus

Dr Hicks records the following zones in Pembrokeshire:--

Zone of Paradoxides Davidis } Menevian. " " Hicksii }

" " Aurora } " " Solvensis } Solva. " " Harknessi }

Dr Tullberg divides the Paradoxides beds of Scania into thirteen zones, though only a few of these are characterised by definite species of Paradoxides. The Olenellus beds have not yet been divided into zones, though this will probably be the outcome of further study.

The strata with Paradoxides are succeeded by those with the Olenus fauna, characterised by the genus Olenus and a large number of allied genera or sub-genera as some prefer to term them. The genus Olenus (sensu stricto) is very abundant in the lower part of the series, whilst the allied forms are more abundant in the upper beds. The genus Paradoxides and its associates disappeared before the deposition of these strata containing Olenus and its allies, and indeed the complete change in the character of the faunas in Europe is very remarkable. The Olenus fauna has been found in North Wales, Pembrokeshire, Warwickshire, Worcestershire, and abroad in Scandinavia and Canada. It is interesting to note among the fossils of the Olenus beds the occurrence of a graptolite which is associated with Olenus in Scandinavia; this is the earliest recorded appearance of a group which is destined to play so important a role amongst the fossils of the succeeding system. The following zones have been detected by Dr S. A. Tullberg amongst the Olenus beds of Scania:--

Zone of Acerocare ecorne. " Dictyograptus flabelliformis. " Cyclognathus micropygus. " Peltura scarabæoides. " Eurycare camuricorne. " Parabolina spinulosa. " Ceratopyge sp. " Olenus (proper). " Leperditia. " Agnostus pisiformis.

The beds with Dictyograptus flabelliformis form a wonderfully constant horizon at or near the top of the Olenus beds. They are found in North Wales, the Border Counties between Wales and England, France, Scandinavia, Russia and Canada.

The passage fauna of the beds which are the equivalents of the Tremadoc Slates may be spoken of as the Ceratopyge fauna, for Ceratopyge forficula, a remarkable species of trilobite, characterises it in Scandinavia, and will probably be found elsewhere. Ceratopyge beds have been found in North and South Wales, Shropshire, Scandinavia, Bavaria and North America, and in each case the fauna is intermediate in character between that of the Cambrian and that of the Ordovician system, containing the loosely-formed trilobites of the former with the more compact ones of the latter. The genus Bryograptus, a many-branched graptolite, also appears to characterise this fauna.

The faunas of the Cambrian rocks have not been studied in sufficient detail, with reference to the physical surroundings of the organisms, to throw much light upon the conditions under which the strata were deposited, though the evidence obtained from an examination of the lithological characters of the deposits is generally corroborated by study of the organic contents.