THE DEVONIAN SYSTEM.
_Classification._ As a result of the movements which were briefly
described in the last chapter, two types of Devonian deposit are found
in the British Isles, and are called respectively the Devon type and
the Old Red Sandstone type. The latter rocks, formerly divided into
three divisions, are now separated into two only, the upper and lower
Old Red Sandstone, and the exact relation of these to the different
subdivisions of the rocks of Devon type remains to be settled. The
Devon type itself has given rise to much difference of opinion, two
local classifications have been applied, one for the rocks of North
Devon and another for those of South Devon. The classification which
has been most generally adopted is as follows:--
N. Devon. S. Devon[83].
{ Pilton Beds { Entomis Slates
Upper Devonian { Cucullæa (Marwood) { Goniatite Limestones
(Clymenian) { Beds { and Slates
{ Pickwell Down Sandstone { Massive Limestones
Middle Devonian { Morte Slates { Middle Devonian
(Eifelian) { Ilfracombe Beds { Limestones
{ Ashprington Volcanic
{ Series
{ Eifelian Slates and
{ Shaly Limestones
{ Lower Devonian
Lower Devonian { Hangman Grits { Slates
(Coblenzian) { Lynton Slates { Lincombe and Warberry
{ Foreland Grits { Grits and
{ Meadfoot Sands
[Footnote 83: An account of the South Devon rocks by Mr Ussher will be
found in the _Quart. Journ. Geol. Soc._, vol. XLVI. p. 487; from it
the above classification of the rocks of S. Devon is taken.]
The division into Lower Middle and Upper Devonian is generally
adopted, though the alternative titles given to these divisions are
not always used with the same signification, and the distribution of
the different local stages given in the above classifications is
usually adopted in the main, though a detailed comparison of the
Devonian beds of North and South Devon is still attended with
difficulty.
More than once an attempt has been made to prove that the apparent
succession of the North Devon rocks, which is that given in the above
table, is not the true one, and of recent years Dr Hicks has obtained
a number of fossils from the Morte Slates which had hitherto yielded
none, and he believes that these fossils indicate that the Morte
Slates are on a lower horizon than the beds on which they rest.
Whatever be the ultimate verdict, we can, at any rate, say that the
"Devonian Question," as it is termed, is not settled[84].
[Footnote 84: See Hicks, H., "On the Morte Slates and Associated Beds
in North Devon and West Somerset," _Quart. Journ. Geol. Soc._, vols.
LII. p. 254, LIII. p. 438.]
_Description of the Strata._ The general variations in the
lithological characters of the deposits of Devonian age will be seen
from the accompanying figure which represents the deposits of Britain
as they occurred from north to south before they had been affected by
subsequent earth-movements (Fig. 19). The conventional signs which are
used are similar to those which have been used in other parts of this
work, and will save description of the section.
[Illustration: Fig. 19.
A. Lower Palæozoic and Precambrian Rocks.
N.S. North of Scotland }
C.V. Central valley of ditto } Old Red Sandstone Type.
W. Wales }
N.D. North Devon } Devon Type.
S.D. South Devon }
]
The ridges separate different deposits of Devonian rocks, which were
possibly deposited in isolated areas, though there was probably
connexion between them at any rate at times.
The Old Red Sandstone type consists to a large extent, as the name
implies, of sandstones which are coloured red by a deposit of peroxide
of iron around the sand grains. They are separable into a lower and
upper division with an unconformity often occurring between them. The
lower Old Red passes down in places into the Silurian rocks with
perfect conformity, and the upper Old Red similarly passes up into the
Carboniferous strata. The existence of pebble beds at different
horizons is a noteworthy feature. They are frequently found at or near
the base of the two divisions. The sandstones of the lower division
are often accompanied by flagstones, while the red sandstones of the
upper division usually have deposits of yellow and brown sandstone
intercalated between them. Inconstant beds of limestone, known as
cornstones, are found in both divisions, and Prof. Sollas has shown
that some of these, at any rate, are true mechanical deposits, formed
by the destruction of pre-existing strata of limestone and the
deposition of the resulting fragments from a state of suspension. In
Scotland a great thickness of volcanic material of various kinds is
associated with the two divisions. For the sake of simplicity this is
omitted from Fig. 19[85]. It is not known how far normal sediments are
associated with the Old Red Sandstone type of deposit. The existence
of some in South Wales is suggested by evidence supplied by the late
Mr J. W. Salter.
[Footnote 85: For an account of these and all other British volcanic
rocks the reader is referred to Sir A. Geikie's work on _The Ancient
Volcanoes of Great Britain_. Macmillan and Co., 1897.]
The Devon type, as will be seen in the figure, consists of rocks which
are to a great extent of normal character. We find in Devonshire
alternations of sandstones, shales and limestones, but even here, red
sandstones, which are comparable with those of the Old Red type occur
in diminished amount: the Foreland Grits and Pickwell Down Sandstones
are both coloured red, and are like the sandstones formed further
north. The recognition of this fact induces one to believe that the
contrast between the two types of rock which are found at a short
distance from one another on opposite sides of the Bristol Channel is
not so marked as one is sometimes led to suppose.
The rocks of North Devon differ from those of South Devon chiefly
owing to the amount of calcareous sediment found in the two areas, for
limestones occur in South Devon to a great extent, and in North Devon
there is a comparative poverty of this kind of sediment. Here, again,
the apparent difference is possibly greater than the real one. The
North Devon limestones have in places been stretched out after their
formation and thus rendered thinner, and the highly-cleaved limestones
are occasionally mistaken for shales, while in South Devon there is
evidence of thickening of the limestones by folding subsequently to
their deposition. Allowing for these changes, however, there is still
a marked diminution in the amount of coarse mechanical sediments and
increase in the quantity of calcareous matter as one passes from North
to South Devon, and this prepares one for the condition of things met
with on parts of the continent, where the mechanical sediments become
finer and thinner on the whole as one travels southward, until, when
we reach the Bohemian area, the Devonian rocks are found to be largely
composed of calcareous sediments.
It is interesting to find that in North America the two types of
Devonian strata recur, and present characters generally similar to
those which they possess upon this side of the Atlantic.
Passing now to a consideration of the conditions under which the
Devonian rocks were deposited, we may examine the bearing of the
character of the strata as a whole, and then proceed to more detailed
consideration of the nature and conditions of deposits of the two
types.
The gradual increase in calcareous matter and dying out of mechanical
sediments as one travels southward points to recession from land in
that direction, and we have already seen that the epeirogenic and
orogenic movements of this continental period elevated the Silurian
sea-floor in the north, and gave rise to a Northern Continent, while
oceanic conditions continued further South, and allowed the
accumulation of sediments lying conformably upon those of Silurian
age, and giving indications of the prevalence of physical conditions
during Devonian times which were in the main similar to those of the
preceding Silurian period.
In the shallow waters adjoining the land of the Northern Continent the
Old Red Sandstones were laid down, and the exact conditions under
which they were accumulated is a matter of some interest. The late Sir
Andrew Ramsay gave reasons for supposing that many red deposits were
accumulated in the waters of inland lakes, which underwent rapid
evaporation, and his views have been applied, with much corroborative
evidence by Sir A. Geikie, to account for the red sandstones of
Devonian age, which he believes to have been accumulated in a series
of inland lakes, though others hold a different opinion, and consider
that the Old Red Sandstone waters had a direct connexion with those of
the open ocean; the question is too intricate to be discussed at
length here. Besides the difference of physical characters of the two
types of strata, the difference in the nature of their included
organisms is significant. The ordinary invertebrates, as corals,
crinoids, brachiopods and molluscs are extremely rare in the Old Red
Sandstone, which contains remarkable remains of Agnatha fishes and
eurypterids, and although these are also found associated with a true
marine fauna in Russia, Germany and Bohemia, the rarity or apparent
absence of the ordinary marine invertebrates, though only negative
evidence, which is proverbially dangerous, must be regarded.
The North Devon rocks are sediments which might well be accumulated on
the shores of a continent, while those of South Devon, with their
abundant coral reefs, and other organic limestones were no doubt
deposited in a clearer sea, at a greater distance from the land, and
the clear water deposits of Germany and still more of Bohemia, were
accumulated in the open ocean. It is interesting to note in these
Bohemian deposits abundance of shells of a Pteropod _Styliola_ which
has been proved by Prof. H. A. Nicholson to form masses of limestone
in the Devonian system of Canada. The modern distribution of the
Pteropoda suggests the open ocean character of the deposits which
contain them even so far back as Devonian times, though one cannot
conclude that these deposits are really analogous to the so-called
Pteropod ooze of modern seas which, as a matter of fact, is largely
composed of foraminiferal tests with a considerable percentage of
pteropod shells.
_The Devonian flora and faunas._ The plant remains in the Lower
Palæozoic rocks are few in number. Some undoubted terrestrial plants
have been discovered, but the prevalent flora of lower Palæozoic
times, so far as yet known, was one consisting of Algæ. In Devonian
times we begin to meet with a number of Cryptogams of higher type,
allied to those which form the dominant flora of the succeeding
period. The fauna is in many ways remarkable. The Devonian period has
been termed the period of ganoid fishes, and the remarkable remains,
so graphically described by the late Hugh Miller, are indeed
peculiarly characteristic of Devonian times, but they are largely
though by no means exclusively entombed in rocks of the Old Red
Sandstone type[86]. The Devon type of rock contains a great abundance
and variety of the problematical group, the Stromatoporoids, which
contribute extensively to the formation of many of the limestones,
and although these organisms are not by any means confined to Devonian
strata, their abundance and variety therein might lead one to speak of
the period as that of Stromatoporoids. The remains of corals are very
abundant in the limestones, and, as already stated, frequently give
rise to true reef-masses. The graptolites, as remarked in the previous
chapter, disappear in the rocks of the Devonian period, and as only
one or two fragments have been found, we may assert that the group was
practically extinct at the end of Silurian times, though species of
one genus, _Monograptus_, lingered for a short time in greatly
diminished quantity. The trilobites which played so important a part
amongst the faunas of Lower Palæozoic times still occur fairly
abundantly amongst the rocks of the Devonian system, and there is a
very interesting point to be noticed in connexion with them. They seem
to have become practically extinct in the succeeding Carboniferous
period, where few genera are found, and the decadence of the group
began in Devonian times. In these circumstances it is interesting to
note the tendency displayed by the creatures to possess spiny
coverings. It is true that _Acidaspis_, the most spinose of all
trilobites, is abundant in Ordovician and Silurian strata, and that
other spinose trilobites are found there, but the peculiarity of the
Devonian trilobites is, that genera which were previously smooth, or
rarely possessing one or few spines, are found represented by
extremely spinose species in these beds,--the spines being developed
from all parts of the test, sometimes as a fringe to head or tail,
sometimes as prominent projections from glabella and neck segment, and
frequently in rows down the body segments. Besides _Acidaspis_, we
find spinose species of _Phacops_, _Homalonotus_, _Cyphaspis_,
_Bronteus_ and _Encrinurus_ in Devonian strata, and the occurrence of
these forms is so frequent and world-wide, that one might perhaps
infer with confidence that an unknown fauna containing many spiny
trilobites was of Devonian age.
[Footnote 86: For an account of these see A. S. Woodward's _Vertebrate
Palæontology_.]
The abundance of Eurypterids has been previously noted. Occurring as
they do in Silurian rocks, they are far more abundant in those of
Devonian age, and are found indifferently in sediments of Old Red and
Devon types. Of air breathers, several insects have been found in the
strata of different parts of the world.
The ordinary marine faunas are otherwise intermediate in character
between those of the Silurian and Carboniferous periods, but there are
several characteristic Devonian genera, and no one who is acquainted
with the peculiarity of the Devonian fauna would deny to the Devonian
strata the right to rank as a separate system, containing a fauna as
well marked in its way as that of the Silurian system below or that of
the Carboniferous above. Special stress is laid upon this point
because it has been suggested that the Devonian system should be
abolished, and its strata either divided between the Silurian and
Carboniferous systems or referred exclusively to the latter
system[87].
[Footnote 87: The literature of the fauna of the Devonian rocks is a
rich one. For an account of the Devonian rocks of Britain, the reader
may consult the Monograph of the Devonian Fossils of the South of
England by Rev. G. F. Whidbourne, which is now appearing in the series
of Monographs of the Palæontographical Society, and in the
publications of the same Society he will find a Monograph of the
Eurypterids from the pen of Dr Henry Woodward. The richest Devonian
fauna is undoubtedly that of the Bohemian area, for the work of Dr E.
Kayser has conclusively proved that the stages _F_, _G_ and _H_ of
that basin, formerly referred to the Silurian, are of Devonian age,
and an excellent idea of the richness of the Devonian fauna may be
obtained by studying the descriptions of the fossils from those stages
which have appeared and are appearing in Barrande's classic work.]