OLD RED SANDSTONE, OR DEVONIAN GROUP.
Old Red Sandstone of Scotland, and borders of Wales--Fossils usually
rare--"Old Red" in Forfarshire--Ichthyolites of Caithness--Distinct
lithological type of Old Red in Devon and Cornwall--Term
"Devonian"--Organic remains of intermediate character between those of
the Carboniferous and Silurian systems--Corals and shells--Devonian
strata of Westphalia, the Eifel, Russia, and the United States--Coral
reef at Falls of the Ohio--Devonian flora.
It was stated in Chap. XXII. that the Carboniferous formation is
surmounted by one called the "New Red," and underlaid by another called
the "Old Red Sandstone."[342-C] The British strata of the last mentioned
series were first recognized in Herefordshire and Scotland as of great
thickness, and immediately subjacent to the coal; but they were in
general so barren of organic remains, that it was difficult to find
paleontological characters of sufficient importance to distinguish them
as an independent group. In Scotland, and on the borders of Wales, the
"Old Red" consists chiefly of red sandstone, conglomerate, and shale,
with few fossils; but limestones of the same age, peculiarly rich in
organic remains, were at length found in Devonshire.
I shall first advert to the characters of the group as developed in
Herefordshire, Worcestershire, Shropshire, and South Wales. Its thickness
has been estimated at 8000 feet, and it has been subdivided into--
1st. A quartzose conglomerate passing downwards into chocolate-red and
green sandstone and marl.
2d. Cornstone and marl--red and green argillaceous spotted marls, with
irregular courses of impure concretionary limestone, provincially
called Cornstone.
Here, as usual, fossils are extremely rare in the clays and sandstones in
which the red oxide of iron prevails; but remains of fishes of the genera
_Cephalaspis_ and _Onchus_ have been discovered in the Cornstone.
The whole of the northern part of Scotland, from Cape Wrath to the
southern flank of the Grampians, has been well described by Mr. Miller
as consisting of a nucleus of granite, gneiss, and other hypogene rocks,
which seem as if set in a sandstone frame.[343-A] The beds of the Old
Red Sandstone constituting this frame, may once perhaps have extended
continuously over the entire Grampians before the upheaval of that
mountain range; for one band of the sandstone follows the course of the
Moray Frith far into the interior of the great Caledonian valley; and
detached hills and island-like patches occur in several parts of the
interior, capping some of the higher summits in Sutherlandshire, and
appearing in Morayshire like oases among the granite rocks of
Strathspey. On the western coast of Ross-shire, the Old Red forms those
three immense insulated hills before described (p. 67.), where beds of
horizontal sandstone, 3000 feet high, rest unconformably on a base of
gneiss, attesting the vast denudation which has taken place.
But in order to observe the uppermost part of the Old Red, we must travel
south of the Grampians, and examine its junction with the bottom of the
Carboniferous series in Fifeshire. This upper member may be seen in Dura
Den, south of Cupar, to consist of a belt of yellow sandstone, in which Dr.
Fleming first discovered scales of _Holoptychius_, and in which species of
fish of the genera _Pterichthys_, _Pamphractus_, and others, have been met
with. (For genus _Pterichthys_, see fig. 400. p. 345.)
The beds next below the yellow sandstone are well seen in the large zone of
Old Red which skirts the southern flank of the Grampians from Stonehaven to
the Frith of Clyde. It there forms, together with trap, the Sidlaw Hills
and the strata of the valley of Strathmore. A section of this region has
been already given (p. 48.), extending from the foot of the Grampians in
Forfarshire to the sea at Arbroath, a distance of about 20 miles, where the
entire series of strata is several thousand feet thick, and may be divided
into three principal masses: 1st, and uppermost, red and mottled marls,
cornstone, and sandstone (Nos. 1. and 2. of the section); 2d, Conglomerate,
often of vast thickness (No. 3. ibid.); 3d, Roofing and paving stone,
highly micaceous, and containing a slight admixture of carbonate of lime
(No. 4. ibid.). In the first of these divisions, which may be considered as
succeeding the yellow sandstone of Fifeshire before mentioned, a gigantic
species of fish of the genus _Holoptychius_ has been found at Clashbinnie
near Perth. Some scales (see fig. 395.) have been seen which measured 3
inches in length by 2-1/2 in breadth.
At the top of the next division, or immediately under the conglomerate
(No. 3. p. 48.), there have been found in Forfarshire some remarkable
crustaceans, with several fish of the genus named by Agassiz _Cephalaspis_,
or "buckler-headed," from the extraordinary shield which covers the head
(see fig. 396.), and which has often been mistaken for that of a trilobite,
of the division _Asaphus_.
[Illustration: Fig. 395. Scale of _Holoptychius nobilissimus_, Agas.
Clashbinnie. Nat. size.]
Species of the same genus are considered in England as characteristic of
the second or Cornstone division (p. 343.).
[Illustration: Fig. 396. _Cephalaspis Lyellii_, Agass. Length 6-3/4 inches.
From a specimen in my collection found at Glammiss, in Forfarshire. See
other figures, Agassiz, vol. ii. tab. 1. _a_. and 1. _b_.
_a._ One of the peculiar scales with which the head is covered when
perfect. These scales are generally removed, as in the specimen
above figured.
_b, c._ Scales from different parts of the body and tail.]
[Illustration: Fig. 397. _Eggs of gasteropodous mollusk?_ Lower beds of
Old Red, Ley's Mill, Forfarshire.]
[Illustration: Fig. 398. _Fucoids and eggs of gasteropodous mollusk?_
Lower Old Red, Fife.]
In the same grey paving-stones and coarse roofing-slates, in which the
_Cephalaspis_ occurs, in Forfarshire and Kincardineshire, the remains of
marine plants or fucoids abound. They are frequently accompanied by groups
of hexagonal, or nearly hexagonal markings, which consist of small
flattened carbonaceous bodies, placed in a slight depression of the
sandstone or shale. (See figs. 397 and 398.) They much resemble in form the
spawn of the recent Natica (see fig. 399.), in which the eggs are arranged
in a thin layer of sand, and seem to have acquired a polygonal form by
pressing against each other. The substance of the egg, if fossilized, might
give rise to small pellicles of carbonaceous matter.
[Illustration: Fig. 399. Fragment of spawn of British species of _Natica_.]
These fossils I have met with, both to the north of Strathmore, in the
vertical shale beneath the conglomerate, and in the same beds in the Sidlaw
hills, at all the points where fig. 4. is introduced in the section, p. 48.
[Illustration: Fig. 400. _Pterichthys_, Agassiz; upper side, showing mouth;
as restored by H. Miller.[345-A]]
Beds of red shale and red sandstone, sometimes associated with
pudding-stone (older than No. 3., fig. 62. p. 48.), and destitute of
organic remains, separate, in the region of Strathmore, the above-described
fossiliferous strata from the older crystalline rocks of the Grampians.
But, in the north of Scotland, we find, at the base of the Old Red, other
grey slaty sandstones, in the counties of Banff, Nairn, Moray, Cromarty,
Caithness, and in Orkney, rich in ichthyolites of peculiar forms, belonging
to the genera _Pterichthys_ (fig. 400.), _Coccosteus_, _Diplopterus_,
_Dipterus_, _Cheiracanthus_, and others of Agassiz.
Five species of _Pterichthys_ have been found in this lowest division of
the Old Red. The wing-like appendages, whence the genus is named, were
first supposed by Mr. Miller to be paddles, like those of the turtle;
but Agassiz regards them as weapons of defence, like the occipital
spines of the River Bull-head (_Cottus gobio_, Linn.); and considers the
tail to have been the only organ of motion. The genera _Dipterus_ and
_Diplopterus_ are so named, because their two dorsal fins are so placed
as to front the anal and ventral fins, so as to appear like two pairs of
wings. They have bony enamelled scales.
_South Devon and Cornwall._--A great step was made in the classification of
the slaty and calciferous strata of South Devon and Cornwall in 1837, when
a large portion of the beds, previously referred to the "transition" or
most ancient fossiliferous series, were found to belong in reality to the
period of the Old Red Sandstone. For this reform we are indebted to the
labours of Professor Sedgwick and Sir R. Murchison, assisted by a
suggestion of Mr. Lonsdale, who, in 1837, after examining the South
Devonshire fossils, perceived that some of them agreed with those of the
Carboniferous group, others with those of the Silurian, while many could
not be assigned to either system, the whole taken together exhibiting a
peculiar and intermediate character. But these paleontological observations
alone would not have enabled us to assign, with accuracy, the true place in
the geological series of these slate-rocks and limestones of South Devon,
had not Messrs. Sedgwick and Murchison, in 1836 and 1837, discovered that
the culmiferous or anthracitic shales of North Devon belonged to the Coal,
and not, as preceding observers had imagined, to the transition period.
As the strata of South Devon here alluded to are far richer in organic
remains than the red sandstones of contemporaneous date in Herefordshire
and Scotland, the new name of the "Devonian system" was proposed as a
substitute for that of Old Red Sandstone.
The rocks of this group in South Devon consist, in great part, of green
chloritic slates, alternating with hard quartzose slates and sandstones.
Here and there calcareous slates are interstratified with blue crystalline
limestone, and in some divisions conglomerates, passing into red sandstone.
The link supplied by the whole assemblage of imbedded fossils, connecting
as it does the paleontology of the Silurian and Carboniferous groups, is
one of the highest interest, and equally striking, whether we regard the
_genera_ of corals or of shells. The _species_ are almost all distinct.
Among the more abundant corals, we find the genera _Favosites_ and
_Cyathophyllum_, common on the one hand to the Mountain limestone, and on
the other to the Silurian system. Some few even of the _species_ are common
to the Devonian and Silurian groups, as, for example, _Favosites
polymorpha_ (fig. 401.), very abundant in South Devon.
[Illustration: Fig. 401. _Favosites polymorpha_, Goldf., S. Devon.
From a polished specimen.
_a._ portion of the same, magnified to show the pores.]
The _Cyathophyllum caespitosum_ (fig. 402.) and _Porites pyriformis_ (fig.
424. p. 356.) are more peculiarly characteristic of the Devonian rocks.
In regard to the shells, all the brachiopodous genera, such as
_Terebratula_, _Orthis_, _Spirifer_, _Atrypa_, and _Productus_, which are
found in the Mountain limestone, occur, together with those of the Silurian
system, except the _Pentamerus_. Some forms, however, seem exclusively
Devonian, as for example, _Calceola sandalina_ (fig. 403.) and
_Strygocephalus Burtini_ (fig. 404.), which have been met with both in the
Eifel, in Germany, and in Devonshire, in the very lowest Devonian beds.
[Illustration: Fig. 402. Cyathophyllum.
_a._ _Cyathophyllum caespitosum_, Goldf., Plymouth.
_b._ a terminal star.
_c._ vertical section exhibiting transverse plates, and part of
another branch.]
Among the peculiar lamellibranchiate bivalves, also common to Devonshire
and the Eifel, we find _Megalodon cucullatus_ (fig. 405.). Several spiral
univalves are abundant, among which are many species of _Pleurotomaria_ and
_Euomphalus_. Among the Cephalopoda we find _Bellerophon_ and _Orthoceras_,
as in the Silurian and Carboniferous groups, and _Goniatite_ and
_Cyrtoceras_, as in the Carboniferous. In some of the upper Devonian beds,
a shell, resembling a flattened _Goniatite_, occurs, called _Clymenia_, by
Munster (_Endosiphonites_, Ansted.[347-A]).
[Illustration: Fig. 403. _Calceola sandalina_, Lam. Eifel; also South
Devon.
_a._ both valves united.
_b._ inner side of opercular valve.]
[Illustration: Fig. 404. _Strygocephalus Burtini_. (_Terebratula porrecta_,
Sow.) Eifel; also South Devon.
_a._ valves united.
_b_. side view of same.
_c._ interior of larger valve, showing thick partition, and thinner one
continued from it.]
[Illustration: Fig. 405. _Megalodon cucullatus_, Sow. Eifel; also
Bradley, S. Devon.
_a._ the valves united.
_b._ interior of valve, showing the large cardinal tooth.]
[Illustration: Fig. 406. _Clymenia linearis_, Munster. (_Endosiphonites
carinatus_, Ansted.) Cornwall.]
A peculiar species of trilobite, called _Brontes flabellifer_ (fig. 407.),
is found in the Devonian strata of the Eifel and in South Devon. It should
be observed, however, that the head in the specimen here figured by
Goldfuss, the most perfect which could be obtained, is incomplete, and a
restoration has been attempted by Mr. Salter in fig. 408., from data
supplied by other species of the same genus occurring in older rocks.
[Illustration: Fig. 407. _Brontes flabellifer_, Goldf. Eifel;
also S. Devon.]
[Illustration: Fig. 408. Restored outline of head of
_Brontes flabellifer_.]
For determining the true equivalents of the Devonian group in the Rhenish
provinces and adjacent parts of Germany, we are indebted to the labours of
Messrs. Sedgwick and Murchison, in 1839, from which it appears that rocks
of that age emerge from beneath the coal-field of Westphalia, and are also
found in troughs among the Silurian rocks in Nassau. Many of the
limestones, particularly those on the river Lahn, are identical, both in
structure and in coralline remains, with the beautiful marbles of
Babbacombe, Torquay, and Plymouth.
The limestones of the Eifel, long ago celebrated for their fossils, and
which lie in a basin supported by Silurian rocks, are found to be referable
to the lower part of the Devonian system.
In Russia, also, Messrs. Murchison and De Verneuil have shown (1840) that
the "Old Red" group occupies a wide area south from St. Petersburg. It was
formerly supposed to be the New Red Sandstone, on account of its saliferous
and gypseous beds; but it is now proved to be the Old Red by containing
ichthyolites of genera which characterize this group in the British Isles,
as, for example, _Holoptychius_, _Coccosteus_, _Diplopterus_, &c.[349-A],
associated with mollusca found in the Devonian of Western Europe. Among the
fish are also many species of sharks of the Cestraciont division, a fact
worthy of notice, because the squaloid fishes of the present day offer the
highest organization of the brain and of the generative organs, and make,
in these respects, the nearest approach to the higher vertebrate classes.
_Devonian Strata in the United States._
The position of this formation between the carboniferous rocks of
Pennsylvania and Ohio, is pointed out in the section, fig. 379. p. 327.,
and it is a remark of M. de Verneuil that in no European country is
there so complete and uninterrupted a development of the Devonian system
as in North America. At the falls of the Ohio, at Louisville, in
Kentucky, there is a grand display of one of the limestones of this
period, resembling a modern coral reef. A wide extent of surface is
exposed in a series of horizontal ledges, at all seasons, when the water
is not high; and the softer parts of the stone having decomposed and
wasted away, the harder calcareous corals stand out in relief, and many
of them send out branches from their erect stems precisely as if they
were living. Among other species I observed large masses, not less than
5 feet in diameter, of _Favosites gothlandica_, with its beautiful
honeycomb structure well displayed, and, by the side of it, the
_Favistella_, combining a similar honeycombed form with the star of the
_Astrea_. There was also the cup-shaped _Cyathophyllum_, and the
delicate network of the _Fenestella_, and that elegant and well-known
European species of fossil, called "the chain coral," _Catenipora
escharoides_, with a profusion of others (see fig. 423. p. 355.). These
coralline forms were mingled with the joints, stems, and occasionally
the heads, of lily encrinites. Although hundreds of fine specimens have
been detached from these rocks, to enrich the museums of Europe and
America, another crop is constantly working its way out, under the
action of the stream, and of the sun and rain, in the warm season when
the channel is laid dry. The waters of the Ohio, when I visited the
spot in April, 1846, were more than 40 feet below their highest level,
and 20 feet above their lowest, so that large spaces of bare rock were
exposed to view.[349-B]
_Devonian Flora._
With the exception of the fucoids above mentioned (p. 344.), but little
is known with certainty of the plants of the Devonian group. Those found
in the department of La Sarthe in France, and in various parts of
Brittany, formerly referred to the Devonian era, have been shown (in
1850), by M. de Verneuil, to belong to the carboniferous series. The
same may be said of the species of _Lepidodendron_, _Knorria_,
_Calamite_, _Sagenaria_, and other genera recently figured (1850), by
Mr. F. A. Roemer, from the formation called "Greywacke a Posodonomyes" in
the Hartz.[350-A] They are accompanied by _Goniatites reticulatus_
Phillips, _G. intercostatus_ Phil., and other mountain limestone
species, and had been previously assigned to the oldest part of the
carboniferous series by Messrs. Murchison and Sedgwick.
If hereafter we should become well acquainted with the land plants of the
Devonian era, we may confidently expect that nearly all of them will agree
generically with those of the carboniferous period, but the species will be
as different as are the Devonian vertebrate and invertebrate animals from
the fossil species of the Coal.
FOOTNOTES:
[342-C] See section, fig. 318. p. 287.
[343-A] The Old Red Sandstone, by Hugh Miller, 1841.
[345-A] Old Red Sandstone. Plate 1. fig. 1. Mr. M.'s description of the
fish is most graphic and correct.
[347-A] Camb. Phil. Trans., vol. vi. pl. 8. fig. 2.
[349-A] See Proceedings of Geol. Soc., and the anniversary speech of
Dr. Buckland, P. G. S., for 1841.
[349-B] Lyell's Second Visit to the United States, vol. ii. p. 277.
[350-A] Memoir on the Hartz, Palaeontographica of Dunker and Von Meyer,
part iii.