THE CAMBRIAN PERIOD.
The traces of life in the Laurentian period, as we have seen,
are but scanty; but the _Cambrian Rocks_--so called from their
occurrence in North Wales and its borders ("Cambria ")--have
yielded numerous remains of animals and some dubious plants.
The Cambrian deposits have thus a special interest as being the
oldest rocks in which occur any number of well-preserved and
unquestionable organisms. We have here the remains of the first
_fauna_, or assemblage of animals, of which we have at present
knowledge. As regards their geographical distribution, the Cambrian
Rocks have been recognised in many parts of the world, but there
is some question as to the precise limits of the formation, and
we may consider that their most typical area is in South Wales,
where they have been carefully worked out, chiefly by Dr Henry
Hicks. In this region, in the neighbourhood of the promontory
of St David's, the Cambrian Rocks are largely developed, resting
upon an ancient ridge of Pre-Cambrian (Laurentian?) strata, and
overlaid by the lowest beds of the Lower Silurian. The subjoined
sketch-section (fig. 27) exhibits in a general manner the succession
of strata in this locality.
From this section it will be seen that the Cambrian Rocks in
Wales are divided in the first place into a lower and an upper
group. The _Lower Cambrian_ is constituted at the base by a great
series of grits, sandstones, conglomerates, and slates, which
are known as the "Longmynd group," from their vast development
in the Longmynd Hills in Shropshire, and which attain in North
Wales a thickness of 8000 feet or more. The Longmynd beds are
succeeded by the so-called "Menevian group," a series of sandstones,
flags, and grits, about 600 feet in thickness, and containing
a considerable number of fossils. The _Upper Cambrian_ series
consists in its lower portion of nearly 5000 feet of strata,
principally shaly and slaty, which are known as the "Lingula
Flags," from the great abundance in them of a shell referable
to the genus _Lingula_. These are followed by 1000 feet of dark
shales and flaggy sandstones, which are known as the "Tremadoc
slates," from their occurrence near Tremadoc in North Wales;
and these in turn are surmounted, apparently quite conformably,
by the basement beds of the Lower Silurian.
[Illustration: Fig 27. GENERALIZED SECTION OF THE CAMBRIAN ROCKS
IN WALES.]
The above may be regarded as giving a typical series of the Cambrian
Rocks in a typical locality; but strata of Cambrian age are known in
many other regions, of which it is only possible here to allude to
a few of the most important. In Scandinavia occurs a well-developed
series of Cambrian deposits, representing both the lower and
upper parts of the formation. In Bohemia, the Upper Cambrian, in
particular, is largely developed, and constitutes the so-called
"Primordial zone" of Barrande. Lastly, in North America, whilst the
Lower Cambrian is only imperfectly developed, or is represented by
the Huronian, the Upper Cambrian formation has a wide extension,
containing fossils similar in character to the analogous strata
in Europe, and known as the "Potsdam Sandstone." The subjoined
table shows the chief areas where Cambrian Rocks are developed,
and their general equivalency:
TABULAR VIEW OF THE CAMBRIAN FORMATION.
_Britain._ | _Europe._ | _America._
| |
/a. Tremadoc Slates. | a. Primordial zone | a. Potsdam
| | of Bohemia. | Sandstone.
| b. Lingula Flags. | b. Paradoxides | b. Acadian
Upper < | Schists, Olenus | group of New
Cambrian. | | Schists, and | Brunswick.
| | Dictyonema schists |
\ | of Sweden. |
| |
/a. Longmynd Beds. | a. Fucoidal | Huronian
| | Sandstone of Sweden | Formation?
| b. Llanberis Slates.| b. _Eophyton_ |
| | Sandstone of Sweden.|
Lower < c. Harlech Grits. | |
Cambrian. | d. _Oldhamia_ | |
| Slates of Ireland.| |
| e. Conglomerates and| |
| and Sandstones of | |
| Sutherlandshire? | |
\f. Menevian Beds. | |
Like all the older Palæozoic deposits, the Cambrian Rocks, though
by no means necessarily what would be called actually "metamorphic,"
have been highly cleaved, and otherwise altered from their original
condition. Owing partly to their indurated state, and partly to
their great antiquity, they are usually found in the heart of
mountainous districts, which have undergone great disturbance,
and have been subjected to an enormous amount of denudation. In
some cases, as in the Longmynd Hills in Shropshire, they form
low rounded elevations, largely covered by pasture, and with few
or no elements of sublimity. In other cases, however, they rise
into bold and rugged mountains, girded by precipitous cliffs.
Industrially, the Cambrian Rocks are of interest, if only for
the reason that the celebrated Welsh slates of Llanberis are
derived from highly-cleaved beds of this age. Taken as a whole,
the Cambrian formation is essentially composed of arenaceous
and muddy sediments, the latter being sometimes red, but more
commonly nearly black in colour. It has often been supposed that
the Cambrians are a deep-sea deposit, and that we may thus account
for the few fossils contained in them; but the paucity of fossils
is to a large extent imaginary, and some of the Lower Cambrian
beds of the Longmynd Hills would appear to have been laid down
in shallow water; as they exhibit rain-prints, sun-cracks, and
ripple-marks--incontrovertible evidence of their having been a
shore-deposit. The occurrence, of innumerable worm-tracks and
burrows in many Cambrian strata is also a proof of shallow-water
conditions; and the general absence of limestones, coupled with
the coarse mechanical nature of many of the sediments of the
Lower Cambrian, maybe taken as pointing in the same direction.
The _life_ of the Cambrian, though not so rich as in the succeeding
Silurian period, nevertheless consists of representatives of
most of the great classes of invertebrate animals. The coarse
sandy deposits of the formation, which abound more particularly
towards its lower part, naturally are to a large extent barren
of fossils; but the muddy sediments, when not too highly cleaved,
and especially towards the summit of the group, are replete with
organic remains. This is also the case, in many localities at any
rate, with the finer beds of the Potsdam Sandstone in America.
Limestones are known to occur in only a few areas (chiefly in
America), and this may account for the apparent total absence
of corals. It is, however, interesting to note that, with this
exception, almost all the other leading groups of Invertebrates
are known to have come into existence during the Cambrian period.
Fig. 28.--Fragment of _Eophyton Linneanum_, a supposed land-plant.
Lower Cambrian, Sweden, of the natural size.
Of the land-surfaces of the Cambrian period we know nothing;
and there is, therefore, nothing surprising in the fact that
our acquaintance with the Cambrian vegetation is confined to
some marine plants or sea-weeds, often of a very obscure and
problematical nature. The "Fucoidal Sandstone" of Sweden, and the
"Potsdam Sandstone" of North America, have both yielded numerous
remains which have been regarded as markings left by sea-weeds or
"Fucoids;" but these are highly enigmatical in their characters,
and would, in many instances, seem to be rather referable to the
tracks and burrows of marine worms. The first-mentioned of these
formations has also yielded the curious, furrowed and striated
stems which have been described as a kind of land-plant under
the name of _Eopkyton_ (fig. 28). It cannot be said, however,
that the vegetable origin of these singular bodies has been
satisfactorily proved. Lastly, there are found in certain green
and purple beds of Lower Cambrian age at Bray Head, Wicklow,
Ireland, some very remarkable fossils, which are well known under
the name of _Oldhamia_, but the true nature of which is very
doubtful. The commonest form of _Oldhamia_ (fig. 29) consists of
a thread-like stem or axis, from which spring at regular intervals
bundles of short filamentous branches in a fan-like manner. In
the locality where it occurs, the fronds of _Oldhamia_ are very
abundant, and are spread over the surfaces of the strata in tangled
layers. That it is organic is certain, and that it is a calcareous
sea-weed is probable; but it may possibly belong to the sea-mosses
(_Polyzoa_), or to the sea-firs (_Sertularians_).
Amongst the lower forms of animal life (_Protozoa_), we find the
Sponges represented by the curious bodies, composed of netted
fibres, to which the name of _Protospongia_ has been given (fig.
32, a); and the comparatively gigantic, conical, or cylindrical
fossils termed _Archoeocyathus_ by Mr Billings are certainly
referable either to the _Foraminifera_ or to the Sponges. The
almost total absence of limestones in the formation may be regarded
as a sufficient explanation of the fact that the _Foraminifera_
are not more largely and unequivocally represented; though the
existence of greensands in the Cambrian beds of Wisconsin and
Tennessee may be taken as an indication that this class of animals
was by no means wholly wanting. The same fact may explain the
total absence of corals, so far as at present known.
[Illustration: Fig. 29.--A portion of _Oldhamia antiqua_, Lower
Cambrian, Wicklow, Ireland, of the natural size. (After Salter.)]
The group of the _Echinodermata_ (Sea-lilies, Sea-urchins, and
their allies) is represented by a few forms, which are principally
of interest as being the earliest-known examples of the class.
It is also worthy of note that these precursors of a group which
subsequently attains such geological importance, are referable to
no less than three distinct _orders_--the Crinoids or Sea-lilies,
represented by a species of _Dendrocrinus_; the Cystideans by
_Protocystites_; and the Star-fishes by _Palasterina_ and some
other forms. Only the last of these groups, however, appears
to occur in the Lower Cambrian.
[Illustration: Fig. 30.--Annelide-burrows (_Scolithus linearus_)
from the Potsdam Sandstone of Canada, of the natural size. (After
Billings.)]
The Ringed-worms (_Annelida_), if rightly credited with all the
remains usually referred to them, appear to have swarmed in the
Cambrian seas. Being soft-bodied, we do not find the actual worms
themselves in the fossil condition, but we have, nevertheless,
abundant traces of their existence. In some cases we find vertical
burrows of greater or less depth, often expanded towards their
apertures, in which the worm must have actually lived (fig. 30),
as various species do at the present day. In these cases, the
tube must have been rendered more or less permanent by receiving
a coating of mucus, or perhaps a genuine membranous secretion,
from the body of the animal; and it may be found quite empty,
or occupied by a cast of sand or mud. Of this nature are the
burrows which have been described under the names of _Scolithus_
and _Scolecoderma_, and probably the _Histioderma_ of the Lower
Cambrian of Ireland. In other cases, as in _Arenicolites_ (fig.
32, b), the worm seems to have inhabited a double burrow, shaped
like the letter U, and having two openings placed close together
on the surface of the stratum. Thousands of these twin-burrows
occur in some of the strata of the Longmynd, and it is supposed
that the worm used one opening to the burrow as an aperture of
entrance, and the other as one of exit. In other cases, again,
we find simply the meandering trails caused by the worm dragging
its body over the surface of the mud. Markings of this kind are
commoner in the Silurian Rocks, and it is generally more or less
doubtful whether they may not have been caused by other marine
animals, such as shellfish, whilst some of them have certainly
nothing whatever to do with the worms. Lastly, the Cambrian beds
often show twining cylindrical bodies, commonly more or less
matted together, and not confined to the surfaces of the strata,
but passing through them. These have often been regarded as the
remains of sea-weeds, but it is more probable that they represent
casts of the underground burrows of worms of similar habits to
the common lob-worm (_Arenicola_) of the present day.
The _Articulate_ animals are numerously represented in the Cambrian
deposits, but exclusively by the class of _Crustaceans_. Some
of these are little double-shelled creatures, resembling our
living water-fleas (_Ostracoda_). A few are larger forms, and
belong to the same group as the existing brine-shrimps and
fairy-shrimps (_Phyllopoda_). One of the most characteristic of
these is the _Hymenocaris vermicauda_ of the Lingula Flags (fig.
32, d). By far the larger number of the Cambrian _Crustacea_
belong, however, to the remarkable and wholly extinct group of
the _Trilobites_. These extraordinary animals must have literally
swarmed in the seas of the later portion of this and the whole of
the succeeding period; and they survived in greatly diminished
numbers till the earlier portion of the Carboniferous period.
They died out, however, wholly before the close of the Palæozoic
epoch, and we have no Crustaceans at the present day which can be
considered as their direct representatives. They have, however,
relationships of a more or less intimate character with the existing
groups of the Phyllopods, the King-crabs (_Limulus_), and the
Isopods ("Slaters," Wood-lice, &c.) Indeed, one member of the
last-mentioned order, namely, the _Serolis_ of the coasts of
Patagonia, has been regarded as the nearest living ally of the
Trilobites. Be this as it may, the Trilobites possessed a skeleton
which, though capable of undergoing almost endless variations,
was wonderfully constant in its pattern of structure, and we
may briefly describe here the chief features of this.
[Illustration: Fig. 31.--Cambrian Trilobites: a, _Paradoxides
Bohemicus_, reduced in size; b, _Ellipsocephalus Hoffi_; c, _Sao
hirsuta_; d, _Conocorypke Sultzeri_ (all the above, together with
fig. g, are from the Upper Cambrian or "Primordial Zone" of
Bohemia); e, Head-shield of _Dikellocephalus Celticus_, from the
Lingula Flags of Wales; f, Head-shield of _Conocoryphe Matthewi_,
from the Upper Cambrian (Acadian Group) of New Brunswick; g,
_Agnostus rex_, Bohemia; h, Tail-shield of _Dikellocephalus
Minnesotensis_, from the Upper Cambrian (Potsdam Sandstone) of
Minnesota. (After Barrande, Dawson, Salter, and Dale Owen.)]
The upper surface of the body of a Trilobite was defended by a
strong shell or "crust," partly horny and partly calcareous in
its composition. This shell (fig. 31) generally exhibits a very
distinct "trilobation" or division into three longitudinal lobes,
one central and two lateral. It also exhibits a more important and
more fundamental division into three transverse portions, which
are so loosely connected with one another as very commonly to be
found separate. The first and most anterior of these divisions
is a shield or buckler which covers the head; the second or middle
portion is composed of movable rings covering the trunk ("thorax
"); and the third is a shield which covers the tailor "abdomen." The
head-shield (fig. 31, e) is generally more or less semicircular
in shape; and its central portion, covering the stomach of the
animal, is usually strongly elevated, and generally marked by
lateral furrows. A little on each side of the head are placed
the eyes, which are generally crescentic in shape, and resemble
the eyes of insects and many existing Crustaceans in being
"compound," or made up of numerous simple eyes aggregated together.
So excellent is the state of preservation of many specimens of
Trilobites, that the numerous individual lenses of the eyes have
been uninjured, and as many as four hundred have been counted
in each eye of some forms. The eyes may be supported upon
prominences, but they are never carried on movable stalks (as
they are in the existing lobsters and crabs); and in some of the
Cambrian Trilobites, such as the little _Agnosti_ (fig. 31 g),
the animal was blind. The lateral portions of the head-shield
are usually separated from the central portion by a peculiar
line of division (the so-called "facial suture") on each side;
but this is also wanting in some of the Cambrian species. The
backward angles of the head-shield, also, are often prolonged
into spines, which sometimes reach a great length. Following
the head-shield behind, we have a portion of the body which is
composed of movable segments or "body-rings," and which is
technically called the "thorax," Ordinarily, this region is strongly
trilobed, and each ring consists of a central convex portion,
and of two flatter side-lobes. The number of body-rings in the
thorax is very variable (from two to twenty-six), but is fixed
for the adult forms of each group of the Trilobites. The young
forms have much fewer rings than the full-grown ones; and it
is curious to find that the Cambrian Trilobites very commonly
have either a great many rings (as in _Paradoxides_, fig. 31,
a), or else very few (as in _Agnostus_, fig. 31, g). In some
instances, the body-rings do not seem to have been so constructed
as to allow of much movement, but in other cases this region of
the body is so flexible that the animal possessed the power of
rolling itself up completely, like a hedgehog; and many individuals
have been permanently preserved as fossils in this defensive
condition. Finally, the body of the Trilobite was completed by
a tail-shield (technically termed the "pygidium"), which varies
much in size and form, and is composed of a greater or less number
of rings, similar to those which form the thorax, but immovably
amalgamated with one another (fig. 31, h).
The under surface of the body in the Trilobites appears to have
been more or less entirely destitute of hard structures, with the
exception of a well-developed upper lip, in the form of a plate
attached to the inferior side of the head-shield in front. There
is no reason to doubt that the animal possessed legs; but these
structures seem to have resembled those of many living Crustaceans
in being quite soft and membranous. This, at any rate, seems to
have been generally the case; though structures which have been
regarded as legs have been detected on the under surface of one
of the larger species of Trilobites. There is also, at present,
no direct evidence that the Trilobites possessed the two pairs
of jointed feelers ("antennæ") which are so characteristic of
recent Crustaceans.
The Trilobites vary much in size, and the Cambrian formation
presents examples of both the largest and the smallest members
of the order. Some of the young forms may be little bigger than
a millet-seed, and some adult examples of the smaller species
(such as _Agnostus_) may be only a few lines in length; whilst
such giants of the order as _Paradoxides_ and _Asaphus_ may reach
a length of from one to two feet. Judging from what we actually
know as to the structure of the Trilobites, and also from analogous
recent forms, it would seem that these ancient Crustaceans were
mud-haunting creatures, denizens of shallow seas, and affecting
the soft silt of the bottom rather than the clear water above.
Whenever muddy sediments are found in the Cambrian and Silurian
formations, there we are tolerably sure to find Trilobites, though
they are by no means absolutely wanting in limestones. They appear
to have crawled out upon the sea-bottom, or burrowed in the yielding
mud, with the soft under surface directed downwards; and it is
probable that they really derived their nutriment from the organic
matter contained in the ooze amongst which they lived. The vital
organs seem to have occupied the central lobe of the skeleton,
by which they were protected; and a series of delicate leaf-like
paddles, which probably served as respiratory organs, would appear
to have been carried on the under surface of the thorax. That
they had their enemies may be regarded as certain; but we have
no evidence that they were furnished with any offensive weapons,
or, indeed, with any means of defence beyond their hard crust,
and the power, possessed by so many of them, of rolling themselves
into a ball. An additional proof of the fact that they for the
most part crawled along the sea-bottom is found in the occurrence
of tracks and markings of various kinds, which can hardly be
ascribed to any other creatures with any show of probability.
That this is the true nature of some of the markings in question
cannot be doubted at all; and in other cases no explanation so
probable has yet been suggested. If, however, the tracks which have
been described from the Potsdam Sandstone of North America under
the name of _Protichnites_ are really due to the peregrinations
of some Trilobite, they must have been produced by one of the
largest examples of the order.
As already said, the Cambrian Rocks are very rich in the remains
of Trilobites. In the lowest beds of the series (Longmynd Rocks),
representatives of some half-dozen genera have now been detected,
including the dwarf _Agnostus_ and the giant _Paradoxides_. In
the higher beds, the number both of genera and species is largely
increased; and from the great comparative abundance of individuals,
the Trilobites have every right to be considered as the most
characteristic fossils of the Cambrian period,--the more so as
the Cambrian species belong to peculiar types, which, for the
most part, died out before the commencement of the Silurian epoch.
All the remaining Cambrian fossils which demand any notice here
are members of one or other division of the great class of the
_Mollusca_, or "Shell-fish" properly so called. In the Lower
Cambrian Rocks the Lamp-shells (_Brachiopoda_) are the principal
or sole representatives of the class, and appear chiefly in three
interesting and important types--namely, _Lingulella, Discina,_
and _Obolella_. Of these the last (fig. 32, i) is highly
characteristic of these ancient deposits; whilst _Discina_ is
one of those remarkable persistent types which, commencing at
this early period, has continued to be represented by varying
forms through all the intervening geological formations up to the
present day. _Lingulella_ (fig. 32, c), again, is closely allied
to the existing "Goose-bill" Lamp-shell (_Lingula anatina_), and
thus presents us with another example of an extremely long-lived
type. The _Lingulelloe_ and their successors; the _Linguloe_, are
singular in possessing a shell which is of a horny texture, and
contains but a small proportion of calcareous matter. In the Upper
Cambrian Rocks, the _Lingulelloe_ become much more abundant, the
broad satchel-shaped species known as _L. Davisii_ (fig. 32,
e) being so abundant that one of the great divisions of the
Cambrian is termed the "Lingula Flags." Here, also, we meet for
the first time with examples of the genus Orthis (fig. 32, f,
k, l) a characteristic Palæozoic type of the Brachiopods, which
is destined to undergo a vast extension in later ages.
[Illustration: Fig 32.--Cambrian Fossils: a, _Protospongia
fenestrata_, Menevian Group; b, _Arenicolites didymus_, Longmynd
Group; c, _Lingulella ferruginea_, Longmynd and Menevian, enlarged;
d, _Hymenocaris vermicauda_, Lingula Flags; e, _Lingulella Davisii_,
Lingula Flags; f, _Orthis lenticularis_, Lingula Flags; g, _Theca
Davidii_, Tremadoc Slates; h, _Modiolopsis Solvensis_, Tremadoc
Slates; i, _Obolela sagittalis_, interior of valve, Menevian;
j, Exterior of the same; k, _Orthis Hicksii_, Menevian; l,
Cast of the same; m, _Olenus micrurus_, Lingula Flags. (Alter
Salter, Hicks, and Davidson.)]
Of the higher groups of the _Mollusca_ the record is as yet but
scanty. In the Lower Cambrian, we have but the thin, fragile,
dagger-shaped shells of the free-swimming oceanic Molluscs or
"Winged-snails" (_Pteropoda_), of which the most characteristic
is the genus _Theca_ (fig. 32, g). In the Upper Cambrian, in
addition to these, we have a few Univalves (_Gasteropoda_), and,
thanks to the researches of Dr Hicks, quite a small assemblage
of Bivalves (_Lamellibranchiata_), though these are mostly of no
great dimensions (fig. 32, h). Of the chambered _Cephalopoda_
(Cuttle-fishes and their allies), we have but few traces; and these
wholly confined to the higher beds of the formation. We meet,
however, with examples of the wonderful genus _Orthoceras_, with
its straight, partitioned shell, which we shall find in an immense
variety of forms in the Silurian rocks. Lastly, it is worthy of
note that the lowest of all the groups of the _Mollusca_--namely,
that of the Sea-mats, Sea-mosses, and Lace-corals (_Polyzoa_)--is
only doubtfully known to have any representatives in the Cambrian,
though undergoing a large and varied development in the Silurian
deposits.
[Illustration: Fig. 33.--Fragment of _Dictyonema sociale_,
considerably enlarged, showing the horny branches, with their
connecting cross-bars, and with a row of cells on each side.
(Original.)]
An exception, however, may with much probability be made to this
statement in favour of the singular genus _Dictyonema_ (fig.
33), which is highly characteristic of the highest Cambrian beds
(Tremadoc Slates). This curious fossil occurs in the form of
fan-like or funnel-shaped expansions, composed of slightly-diverging
horny branches, which are united in a net-like manner by numerous
delicate cross-bars, and exhibit a row of little cups or cells,
in which the animals were contained, on each side. _Dictyonema_
has generally been referred to the _Graptolites_; but it has a
much greater affinity with the plant-like Sea-firs (_Sertularians_)
or the Sea-mosses (_Polyzoa_), and the balance of evidence is
perhaps in favour of placing it with the latter.
LITERATURE.
The following are the more important and accessible works and
memoirs which may be consulted in studying the stratigraphical
and palæontological relations of the Cambrian Rocks:--
(1) 'Siluria.' Sir Roderick Murchison. 5th ed., pp. 21-46.
(2) 'Synopsis of the Classification of the British Palæozoic Rocks.'
Sedgwick. Introduction to the 3d Fasciculus of the 'Descriptions
of British Palæozoic Fossils in the Woodwardian Museum,'
by F. M'Coy, pp. i-xcviii, 1855.
(3) 'Catalogue of the Cambrian and Silurian Fossils in the Geological
Museum of the University of Cambridge.' Salter. With a Preface
by Prof. Sedgwick. 1873.
(4) 'Thesaurus Siluricus.' Bigsby. 1868.
(5) "History of the Names Cambrian and Silurian." Sterry
Hunt.--'Geological Magazine.' 1873.
(6) 'Système Silurien du Centre de la Bohême.' Barrande. Vol. I.
(7) 'Report of Progress of the Geological Survey of Canada, from its
Commencement to 1863,' pp. 87-109.
(8) 'Acadian Geology.' Dawson. Pp. 641-657.
(9) "Guide to the Geology of New York," Lincklaen; and "Contributions
to the Palæontology of New York," James Hall.--'Fourteenth
Report on the State Cabinet.' 1861.
(10) 'Palæozoic Fossils of Canada.' Billings. 1865.
(11) 'Manual of Geology.' Dana. Pp. 166-182. 2d ed. 1875.
(12) "Geology of North Wales," Ramsay; with Appendix on the
Fossils, Salter.--'Memoirs of the Geological Survey of Great
Britain,' vol. iii. 1866.
(13) "On the Ancient Rocks of the St David's Promontory, South Wales,
and their Fossil Contents." Harkness and Hicks.--' Quart.
Journ. Geol. Soc.,' xxvii. 384-402. 1871.
(14) "On the Tremadoc Rocks in the Neighbourhood of St David's,
South Wales, and their Fossil Contents." Hicks.--'Quart.
Journ. Geol. Soc.,' xxix. 39-52. 1873.
In the above list, allusion has necessarily been omitted to numerous
works and memoirs on the Cambrian deposits of Sweden and Norway,
Central Europe, Russia, Spain, and various parts of North America,
as well as to a number of important papers on the British Cambrian
strata by various well-known observers. Amongst these latter
may be mentioned memoirs by Prof. Phillips, and Messrs Salter,
Hicks, Belt, Plant, Homfray, Ash, Holl, &c.