THE ORDOVICIAN SYSTEM.
_Classification._ The Ordovician strata were originally divided into
series by Sedgwick as follows:--
Upper Bala,
Middle Bala,
Lower Bala,
Arenig.
The Arenig series was at one time included by some writers with the
Lower Bala under the name Llandeilo, but the word Llandeilo is now
used in the sense of Sedgwick's Lower Bala. The Middle Bala is often
spoken of as Caradoc, but the terms Bala and Caradoc are sometimes
used interchangeably. As much confusion attaches to the use of the
name Bala without explanation, the alternative titles have been
largely adopted, and as the series are well defined there is no
objection to their use, save that some expression is wanted equivalent
to Upper Bala. The local term Ashgill shales was originally applied by
Mr W. Talbot Aveline to beds of this age in Lakeland, and I have
elsewhere suggested the use of this name for the whole series in that
region; its use may well be extended to the series which is developed
in many parts of Britain and the continent. The terms which will be
used here, therefore, for the different series of the Ordovician
system are the following:--
Ashgill Series (= Upper Bala)
Caradoc " (= Middle " )
Llandeilo " (= Lower " )
Arenig "
Adopting a palæontological classification, we may speak of the Arenig
and Llandeilo beds as those containing the _Asaphus_ fauna, whilst the
Caradoc and Ashgill beds possess the _Trinucleus_ fauna; this is the
terminology employed by Angelin for the equivalent strata of Sweden.
It must be noted that here the names applied are not those of
absolutely characteristic genera, as was the case with those adopted
for naming the Cambrian faunas, for both _Asaphus_ and _Trinucleus_
range through the beds of the system; but whereas _Asaphus_ is most
abundant in the beds of the two lower series, _Trinucleus_ occurs most
frequently in those of the two upper series.
_Description of the strata._ The Ordovician rocks are found over large
tracts in North and South Wales, in the counties on the Welsh border,
in Lakeland and the outlying districts in the Southern Uplands of
Scotland, and in detached areas in Ireland. There are three main types
of deposit:--(i) the volcanic type, in which the ordinary sediments
are associated with a large amount of contemporaneous volcanic matter,
(ii) the black shale type, with a fauna consisting largely of
graptolites, and (iii) the ordinary sedimentary type, in which we find
alternations of grits, shales, and more or less impure limestones. We
also find developments which are intermediate between any two or even
all three of these types. The first type is characteristically
developed in Caernarvonshire and Merionethshire, the second in the
Dumfriesshire Uplands, and the third in the Girvan district of
Ayrshire. The variation in the thickness of these three types of
deposit is shown in the accompanying sections of the Caernarvon,
Merioneth, Moffat and Girvan regions (see Fig. 17).
[Illustration: Fig. 17.
Showing the variations in the characters of the Ordovician deposits of
the three principal types.
Scale 1 in. = 1000 feet.
A = Arenig. L = Llandeilo. C = Caradoc.
The thickness of the Arenig rocks of the Scotch areas is unknown.]
The North Welsh area gives two different developments of the
Ordovician strata, one of which is much less volcanic than the other.
In the Merioneth-Caernarvon area, two great masses of volcanic rock
form the Aran and Arenig hills of Merioneth and the Snowdonian group
of Caernarvon. The former are of Arenig, the latter of Caradoc age.
The Merionethshire volcanic rocks consist of a great thickness of
lavas and ashes of intermediate composition (anderites), associated
with sandy and muddy sediments of no great vertical depth. The
Llandeilo beds of this area are chiefly of the nature of black shales,
while the Caradoc series is represented by volcanic lavas and ashes of
acid composition (felsites) with a few thin interbedded sediments. A
calcareous ash forming the summit of Snowdon is of importance as being
on the same horizon as a limestone (the Bala limestone) found in the
other North Welsh area. The Ashgill series is not represented in
Snowdonia.
In the other North Welsh tract, around Bala Lake, the volcanic matter
is much less conspicuous. The Arenig rocks are not seen nearer than
the Arenig mountains which form the western boundary of this second
tract. The Llandeilo beds consist of shaley deposits with a
well-marked limestone, the Llandeilo limestone, in the centre, whilst
the Caradoc beds consist chiefly of muddy sediments with some thin
ashes and a limestone, the Bala limestone, at the top. The Ashgill
series contains a basal limestone, the Rhiwlas limestone, succeeded by
shales, and another thin limestone called the Hirnant limestone at the
summit.
In South Wales the Arenig beds[74] are chiefly composed of slates, and
are divisible into an upper and lower group. The total thickness is
about 2000 feet. The Llandeilo beds contain three series:--
Upper Llandeilo Slates 1000
Llandeilo Limestone 200
Lower Llandeilo Slates 800.
[Footnote 74: A remarkable fauna, fairly well represented in Britain
and exceedingly well developed on the continent, exists in the
Uppermost Arenig and Lower Llandeilo beds, and it is well separated
from the dominant Arenig fauna below and Llandeilo fauna above. To the
beds which contain it Dr Hicks has given the name Llanvirn series.]
The Caradoc beds consist of black graptolitic shales of no great
thickness, succeeded by an impure limestone on the horizon of the Bala
limestone, while the Ashgill series like that of North Wales is
separated into upper and lower limestone stages with an intervening
stage composed of shales.
The deposits of the Welsh borderland are well developed in Shropshire,
where there is practically a repetition of the Caernarvon-Merioneth
development, with variations in detail. The Arenig and Caradoc
volcanic rocks are not so thick as those of the Welsh district, but
are nevertheless of considerable importance[75].
[Footnote 75: For information concerning these beds see Lapworth, C.
and Watts, W. W., "The Geology of South Shropshire," _Proc. Geol.
Assoc._, vol. XIII. p. 297.]
In the hilly region of Cumberland, Westmorland, and the adjoining
parts of Yorkshire the succession differs from that of any of the
Welsh regions, for the great period of volcanicity was during the
formation of the Llandeilo rocks, and there were merely sporadic
outbursts in Arenig and Caradoc times. The Arenig rocks consist of
black shales with interstratified beds of coarser sediment, and some
thin lavas and ashes of intermediate type. The Llandeilo series is
represented by a very great thickness of volcanic rocks, varying in
composition from basic to acid lavas, with associated pyroclastic
rocks. The rocks of the Caradoc period largely consist of impure
limestone with associated argillaceous rocks, and contemporaneous
volcanic rocks of acid character. A marked unconformity is found
locally in the centre of these. The Ashgill series consists of a basal
limestone with shales above, and there is evidence that volcanic
activity had not become extinct during the deposition of the rocks of
this series.
Passing on to Scotland, the graptolitic type is admirably shown in the
southern Uplands of the neighbourhood of Moffat, Dumfriesshire. The
base of the Ordovician system has not been found, but the lowest
series seems to be represented by shales with a graptolite possibly of
Arenig age. Above this are volcanic beds succeeded by a group of black
shales known as the Moffat shales. They are only about six hundred
feet in thickness, and yet represent much of the Ordovician and part
of the Silurian strata as developed elsewhere. The beds belonging to
the Ordovician system are divided into two series, the Glenkiln shales
below and the Hartfell shales above. The former consist of intensely
black muds with few fossils save graptolites, and a deposit of chert
at the base which is composed of radiolaria. The graptolites of the
black shales are Upper Llandeilo forms, but the thin deposit of
radiolarian chert may represent the rest of the Llandeilo period and
part of the Arenig period also. The Hartfell shales are also usually
black graptolite shales with lighter deposits nearly barren of organic
remains; they represent the Caradoc and Ashgill series and pass
conformably into the deposits of Silurian age[76]. The ordinary
sedimentary type of Ordovician rocks is found in Ayrshire, though a
few thin graptolitic seams are intercalated with the conglomerates
and shelly sands, clays and limestones of the region, which is
therefore peculiarly valuable as affording a means of comparison of
the shelly type with the graptolitic type of Ordovician deposits. The
Arenig series consists of black shales with graptolites, and these
rocks are succeeded by a volcanic group which is probably of Llandeilo
age. Above these volcanic beds, as in Dumfriesshire, we find three
great divisions, two of which are of Ordovician, the third of Silurian
age. The Ordovician divisions are respectively termed the Barr series,
which is the equivalent of the Glenkiln shales, and the Ardmillan
series above, equivalent to the Hartfell shales[77].
[Footnote 76: The Moffat beds are described in a paper by Prof.
Lapworth entitled "The Moffat Series" in the _Quarterly Journal of the
Geological Society_, vol. XXXIV. p. 239. This paper, which is a
masterpiece of detailed work, has furnished a clue to many problems.
Few students will be able to follow the numerous details, and for
general information concerning the beds they are recommended to read
another paper by the same author "On the Ballantrae Rocks of South
Scotland," _Geol. Mag._ Dec. III. vol. VI. p. 20. An account of the
radiolarian cherts by Dr G. J. Hinde will be found in the _Annals and
Magazine of Natural History_ for July, 1890, p. 40.]
[Footnote 77: See Lapworth, C., "The Girvan Succession," _Quart.
Journ. Geol. Soc._, vol. XXXVIII. p. 537, and also the paper on the
Ballantrae Rocks referred to in the preceding footnote. The latter
paper should be carefully read by all students of the stratigraphy of
the Lower Palæozoic Rocks.]
It is interesting to find that in the north of Ireland the rocks
generally coincide in characters with those which are found along the
same line of strike in Great Britain; thus, the Girvan type appears in
Londonderry, Tyrone and Fermanagh, the Moffat type in County Down, and
the Lake District type in the counties of Dublin and Kildare.
On the continent the volcanic material which plays so important a part
in the constitution of the Ordovician accumulations of Britain is
practically absent, and the strata are largely composed of
accumulations of shale and limestone with occasional coarser deposits.
In Scandinavia, the Arenig beds consist of limestones with a few
shales, the Llandeilo deposits are largely calcareous, those of
Caradoc age are partly calcareous and towards the top usually
argillaceous, while the equivalents of the British Ashgill series are
calcareous at the base and argillaceous at the summit. In Russia the
calcareous matter preponderates over the argillaceous material.
Ordovician strata are also found in Belgium, France, Bohemia, and
other places, and are largely composed of mechanical sediments of
varying degrees of fineness mixed occasionally with some calcareous
matter.
The variation in the characters of the Ordovician strata of Britain
points to accumulation in a fairly deep sea, usually at some distance
from the land, but dotted over with volcanoes which often rose above
the water, causing the addition of much volcanic material to the
ordinary sediments, and the existence of minor unconformities at
different horizons along their flanks. As these unconformities are not
always associated with volcanic material it is obvious that uplifts
must have occurred occasionally during the deposition of the rocks;
one important uplift is indicated by the occurrence of an unconformity
in the Arenig rocks of Wales, while another is seen amongst the
Caradoc rocks of the Welsh borders. On the whole, however, the period
was one of slow subsidence, the deposition of material generally
keeping pace with this subsidence, and accordingly there is a great
uniformity of characters amongst the strata over wide areas. The
probable continuation through the Ordovician period of the tract of
land over the present site of the N. Atlantic ocean which as we have
reason to suppose existed during Cambrian times, is indicated by
similar changes of lithological character amongst the strata when
traced from Britain eastward to Russia in both Cambrian and Ordovician
times, and the continuance of these conditions over the American area
is also indicated by study of the variations amongst the American
Ordovician deposits.
_The Ordovician Faunas._ The Ordovician period has justly been termed
the Period of Graptolites, which are the dominant forms of the time,
and continue in abundance throughout the period. The abundance of
graptolites in black shales associated with few other organisms has
often been noted. It appears to be due to a large extent to the slow
accumulation of the graptolitic deposits, allowing an abundance of
these creatures to be showered upon the ocean floor, after death, for
the evidence derived from detailed examination of their structure
points to their existence as floating organisms. The tests of other
creatures largely calcareous may well have been dissolved before
reaching the sea-floor. In support of the view that these black shales
are abysmal deposits may be noted the singular persistence of their
lithological characters over wide areas, their replacement by much
greater thicknesses of normal sediments along the ancient coast-lines,
the frequent occurrence together of blind trilobites with those having
abnormally large eyes when these creatures are associated with
graptolites in the black shales, and lastly the interstratification of
the black shales with radiolarian cherts similar to the modern abysmal
radiolarian oozes. If this be so, we ought to find graptolites in
marine deposits of all kinds, and indeed they are found there, though
largely masked by the mass of sediment and the hosts of other included
fossils, so that their discovery is rendered much more difficult than
when they occur in the black shales,--a state of things which is
familiar in the case of other pelagic organisms as _Globigerinæ_,
radiolaria, and pteropods, whose tests abound in the abysmal deposits
and are comparatively rare in those of terrigenous origin[78].
[Footnote 78: The importance of the graptolites as indices of the
geological age will be seen by perusal of Prof. Lapworth's paper "On
the Geological Distribution of the Rhabdophora," _Ann. and Mag. Nat.
Hist._, Ser. 5, vol. III. (1897).]
The characters of the Ordovician trilobites have already been noticed.
These organisms are abundant, and occur in sediments of all kinds. Of
other groups, the significance of the radiolaria has been referred to
above. Corals occasionally form reef-like masses of limestone as in
the limestones of the Caradoc epoch; the echinoderms are well
represented, cystids being locally abundant; of the crustacea, many
remains of tests of phyllocarida have been recorded; the brachiopods
are very abundant, and of the mollusca, lamellibranchs, gastropods and
cephalopods all occur with frequency though none of these groups is
very prevalent. Certain forms have been referred to pteropods though
with doubt, and other shells seem to be referable to the heteropods.
The existence of vertebrates during Ordovician times is not, in the
opinion of many geologists, proved, though remains of fishes have been
recorded from the Ordovician strata of North America; but it is
desirable that more evidence of this occurrence should be given[79].
[Footnote 79: Walcott, C. D., "Preliminary Notes on the Discovery of a
Vertebrate Fauna in Silurian (Ordovician) Strata," _Bulletin Geol.
Soc. America_, vol. III. p. 153.]
The distribution of the Ordovician faunas like that of the sediments
points to the prevalence of open ocean conditions over wide areas
during the period, with occasional approaches to land, which was often
of a volcanic nature. Around this land clustered the ordinary
invertebrates, building up coral-reefs and shell-banks, whilst away in
the open oceans the graptolites floated, almost alone, and sank to the
ocean floor after death.