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[64] 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
[Footnote 64: In accordance with the custom usually observed in
Britain, the Tremadoc slates are placed in the Cambrian system; most
continental geologists place them in the succeeding Ordovician system.
The matter is not an important one, as the fauna is an intermediate
one between that of the Lingula Flags and that of the Arenig series of
the Ordovician system, and the beds are true beds of passage. As the
lithological classification is essentially British, it will be as well
to retain the Tremadoc Slates in the Cambrian system.]
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[65].
[Footnote 65: A general account of the Cambrian, Ordovician and
Silurian rocks will be found in the Sedgwick Essay for 1883, _A
Classification of the Cambrian and Silurian Rocks_, though the use of
a cumbrous nomenclature therein will tend to confuse the reader. For a
detailed account of the Cambrian rocks of North Wales the reader is
referred to the Geological Survey Memoir, _The Geology of North
Wales_, by Sir A. Ramsay (2nd edition), he may also consult Belt, T.,
"On the Lingula Flags or Festiniog Group of the Dolgelly district,"
_Geol. Mag._, Dec I. vol. IV. pp. 493, 536, vol. V. p. 5. The geology
of the Cambrian rocks is described in a series of Memoirs in the
_Quarterly Journal of the Geological Society_ by Dr H. Hicks; the
following should be consulted: Harkness, R. and Hicks, H., "On the
Ancient Rocks of the St David's Promontory, South Wales, and their
Fossil Contents," vol. XXVII. p. 384; Hicks, H., "On some Undescribed
Fossils from the Menevian Group," vol. XXVIII. p. 173; and "On the
Tremadoc Rocks in the neighbourhood of St David's, South Wales, and
their Fossil Contents," vol. XXIX. p. 39. See also Hicks, "The
Classification of the Eozoic and Lower Palæozoic Rocks of the British
Isles," _Popular Science Review_, New Series, vol. V., and Hicks,
"Life-zones in the Lower Palæozoic Rocks," _Geol. Mag._ Dec IV. vol.
I. pp. 368, 399 and 441.]
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[66]. 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.
[Footnote 66: In giving this description the red (Glyn) slates of
North Wales are treated as belonging to the Caerfai series, though
this correlation depends on lithological characters only at present.]
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[67]) 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.
[Footnote 67: The term Festiniog has been used for the whole Lingula
Flag series as well as for the middle stage. It will be well to use it
with reference to the stage only.]
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.
[Illustration: 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[68].
[Footnote 68: For an account of the _Olenellus_ fauna see Walcott, C.
D., "The Fauna of the Lower Cambrian or Olenellus Zone," _Tenth Annual
Report of the Director of the United States Geological Survey_,
Washington, 1890. It is possible that some of the fossils mentioned in
that report belong to strata above that containing _Olenellus_.]
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[69].
[Footnote 69: For an account of the _Neobolus_ beds see Noetling, F.,
"On the Cambrian Formation of the Eastern Salt Range," _Records Geol.
Survey, India_, vol. XXVII. p. 71, and for the Protolenus fauna
consult a paper by Matthew, G. F., "The _Protolenus_ Fauna," _Trans.
New York Acad. of Science_, 1895, vol. XIV. p. 101.]
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[70]:--
Zone of _Paradoxides_ _Davidis_ } Menevian.
" " _Hicksii_ }
" " _Aurora_ }
" " _Solvensis_ } Solva.
" " _Harknessi_ }
[Footnote 70: The order here as elsewhere is _ascending_, i.e. the
newest deposit is placed at the top.]
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[71].
[Footnote 71: The _Paradoxides_ fauna is described in the following
works: Britain, Hicks, H. and Salter J. W., _Quart. Journ. Geol.
Soc._, vol. XXIV. p. 510, XXV. p. 51, XXVII. p. 173, and Hicks, H. and
Harkness, R., _ibid._ vol. XXVII. p. 384; Scandinavia, Angelin, N. P.,
_Palæontologia Scandinavica_; Brögger, W. C., _Nyt Magazin for
Naturvidenskaberne_, vol. XXIV., Linnarsson, G., _Sveriges Geologiska
Undersökning_, Ser. C. No. 35; Bohemia, Barrande, J., _Système
Silurien du centre de la Bohême_; Spain, Prado, C. de, "Sur
l'existence de la faune Primordiale dans la chaîne Cantabrique suivie
de la description des Fossiles par MM. de Verneuil et Barrande,"
_Bull. Soc. Geol. France_, 2 Series, vol. XVII. p. 516; America,
Walcott, C. D., _Bull. U. S. Geol. Survey_: "The Cambrian Faunas of
North America," and Matthew, G. F., _Trans. Roy. Soc. Canada_, 1882
and succeeding years.]
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[72]. 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_.
[Footnote 72: For descriptions of the _Olenus_ fauna consult the
following:--Wales, Belt, T., _Geol. Mag._ Dec. I. vol. V. p. 5, and
Salter, J. W., _Decades Geol. Survey_, Decade II. Pl. IX. and Decade
XI. Pl. VIII.; Scandinavia, Angelin, N. P., _Palæontologia
Scandinavica_, and Brögger, W. C., _Die Silurischen Etagen 2 und 3 im
Kristianiagebiet und auf Eker_; Canada, Matthew, G. F., "Illustrations
of the Fauna of the St John Group, No. VI.," _Trans. Roy. Soc.
Canada_, 1891.]
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[73].
[Footnote 73: For accounts of the Tremadoc Slates Fauna in England and
Wales see Ramsay, A. C., _Geology of North Wales_, Appendix; Hicks,
H., _Quart. Journ. Geol. Soc._, vol. XXIX. p. 39; Callaway, C.,
_ibid._ vol. XXXIII. p. 652, whilst many of the foreign fossils are
noticed in Brögger's _Die Silurischen Etagen 2 und 3_ and Barrande's
_Faune silurienne des Environs de Hof en Bavière_.]
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.