TRIAS OR NEW RED SANDSTONE GROUP.
Distinction between New and Old Red Sandstone--Between Upper and Lower
New Red--The Trias and its three divisions--Most largely developed in
Germany--Keuper and its fossils--Muschelkalk--Fossil plants of
Bunter--Triassic group in England--Bone-bed of Axmouth and Aust--Red
Sandstone of Warwickshire and Cheshire--Footsteps of _Chirotherium_ in
England and Germany--Osteology of the _Labyrinthodon_--Identification
of this Batrachian with the Chirotherium--Origin of Red Sandstone and
Rock-salt--Hypothesis of saline volcanic exhalations--Theory of the
precipitation of salt from inland lakes or lagoons--Saltness of the
Red Sea--New Red Sandstone in the United States--Fossil footprints of
birds and reptiles in the Valley of the Connecticut--Antiquity of the
Red Sandstone containing them.
Between the Lias and the Coal, or Carboniferous group, there is interposed,
in the midland and western counties of England, a great series of red
loams, shales, and sandstones, to which the name of the New Red Sandstone
formation was first given, to distinguish it from other shales and
sandstones called the "Old Red" (_c_, fig. 318.), often identical in
mineral character, which lie immediately beneath the coal (_b_).
[Illustration: Fig. 318. Cross section.
_a._ New red sandstone.
_b._ Coal.
_c._ Old red.]
The name of "Red Marl" has been incorrectly applied to the red clays of
this formation, as before explained (p. 13.), for they are remarkably free
from calcareous matter. The absence, indeed, of carbonate of lime, as well
as the scarcity of organic remains, together with the bright red colour of
most of the rocks of this group, causes a strong contrast between it and
the Jurassic formations before described.
Before the distinctness of the fossil remains characterizing the upper and
lower part of the English New Red had been clearly recognized, it was found
convenient to have a common name for all the strata intermediate in
position between the Lias and Coal; and the term "Poikilitic" was proposed
by Messrs. Conybeare and Buckland[286-A], from +poikilos+, poikilos,
_variegated_, some of the most characteristic strata of this group having
been called _variegated_ by Werner, from their exhibiting spots and streaks
of light-blue, green, and buff colour, in a red base.
A single term, thus comprehending both Upper and Lower New Red, or the
Triassic and Permian groups of modern classifications, may still be useful
in describing districts where we have to speak of masses of red sandstone
and shale, referable, in part, to both these eras, but which, in the
absence of fossils, it is impossible to divide.
_Trias or Upper New Red Sandstone Group._
The accompanying table will explain the subdivisions generally adopted for
the uppermost of the two systems above alluded to, and the names given to
them in England and on the Continent.
Synonyms.
German. French.
{ _a._ Saliferous and } }
{ gypseous shales and } Keuper } Marnes irisees.
Trias or Upper { sandstone } }
New Red {
Sandstone { _b._ (wanting in England) } Muschelkalk { Muschelkalk, ou
{ { calcaire
{ { coquillier.
{
{ _c._ Sandstone and } Bunter- } Gres bigarre.
{ quartzose conglomerate } sandstein }
I shall first describe this group as it occurs in South Western and North
Western Germany, for it is far more fully developed there than in England
or France. It has been called the Trias by German writers, or the Triple
Group, because it is separable into three distinct formations, called the
"Keuper," the "Muschelkalk," and the "Bunter-sandstein."
[Illustration: Fig. 319. _Equisetites columnaris._ (Syn. _Equisetum
columnare_.) Fragment of stem, and small portion of same
magnified. Keuper.]
_The Keuper_, the first or newest of these, is 1000 feet thick in
Wuertemberg, and is divided by Alberti into sandstone, gypsum, and
carbonaceous slate-clay.[287-A] Remains of Reptiles, called _Nothosaurus_
and _Phytosaurus_, have been found in it with _Labyrinthodon_; the detached
teeth, also, of placoid fish and of rays, and of the genera _Saurichthys_
and _Gyrolepis_ (figs. 325, 326, p. 289.). The plants of the Keuper are
generically very analogous to those of the lias and oolite, consisting of
ferns, equisetaceous plants, cycads, and conifers, with a few doubtful
monocotyledons. A few species, such as _Equisetites columnaris_, are common
to this group, and the oolite.
_The Muschelkalk_ consists chiefly of a compact, greyish limestone, but
includes beds of dolomite in many places, together with gypsum and
rock-salt. This limestone, a rock wholly unrepresented in England, abounds
in fossil shells, as the name implies. Among the cephalopoda there are no
belemnites, and no ammonites with foliated sutures, as in the incumbent
lias and oolite, but a genus allied to the Ammonite, called _Ceratite_ by
De Haan, in which the descending lobes (see _a_, _b_, _c_, fig. 320.)
terminate in a few small denticulations pointing inwards. Among the
bivalve shells, the _Posidonia minuta_, Goldf. (_Posidonomya minuta_,
Bronn) (see fig. 321.), is abundant, ranging through the Keuper,
Muschelkalk, and Bunter-sandstein; and _Avicula socialis_, fig. 322.,
having a similar range, is very characteristic of the Muschelkalk in
Germany, France, and Poland.
[Illustration: Fig. 320. _Ceratites nodosus._ Muschelkalk.
_a._ Side view.
_b._ Front view.
_c._ Partially denticulated outline of the septa dividing the chambers.]
[Illustration: Fig. 321. _Posidonia minuta_, Goldf. (_Posidonomya
minuta_, Bronn.)]
[Illustration: Fig. 322. Avicula. Characteristic of the Muschelkalk.
_a._ _Avicula socialis._
_b._ Side view of same.]
The abundance of the heads and stems of lily encrinites, _Encrinus
liliiformis_ (or _Encrinites moniliformis_), show the slow manner in which
some beds of this limestone have been formed in clear sea-water.
[Illustration: Fig. 323. Voltzia. Bunter-sandstein.
_a._ _Voltzia heterophylla._ (Syn. _Voltzia brevifolia_.)
_b._ portion of same magnified to show fructification. Sulzbad.]
_The Bunter-sandstein_ consists of various coloured sandstones,
dolomites, and red-clays, with some beds, especially in the Hartz, of
calcareous pisolite or roe-stone, the whole sometimes attaining a
thickness of more than 1000 feet. The sandstone of the Vosges, according
to Von Meyer, is proved, by the presence of _Labyrinthodon_, to belong
to this lowest member of the Triassic group. At Sulzbad (or
Soultz-les-bains), near Strasburg, on the flanks of the Vosges, many
plants have been obtained from the "bunter," especially conifers of the
extinct genus _Voltzia_, peculiar to this period, in which even the
fructification has been preserved. (See fig. 323.)
Out of thirty species of ferns, cycads, conifers, and other plants,
enumerated by M. Ad. Brongniart, in 1849, as coming from the "gres
bigarre," or Bunter, not one is common to the Keuper.[288-A]
The footprints of a reptile (_Labyrinthodon_) have been observed on the
clays of this member of the Trias, near Hildburghausen, in Saxony,
impressed on the upper surface of the beds, and standing out as casts in
relief from the under sides of incumbent slabs of sandstone. To these I
shall again allude in the sequel; they attest, as well as the accompanying
ripple-marks, and the cracks which traverse the clays, the gradual
formation in shallow water, and sometimes between high and low water, of
the beds of this formation.
_Triassic group in England._
In England the Lias is succeeded by conformable strata of red and green
marl, or clay. There intervenes, however, both in the neighbourhood of
Axmouth, in Devonshire, and in the cliffs of Westbury and Aust, in
Gloucestershire, on the banks of the Severn, a dark-coloured stratum, well
known by the name of the "bone-bed." It abounds in the remains of saurians
and fish, and was formerly classed as the lowest bed of the Lias; but Sir
P. Egerton has shown that it should be referred to the Upper New Red
Sandstone, for it contains an assemblage of fossil fish which are either
peculiar to this stratum, or belong to species well known in the
Muschelkalk of Germany. These fish belong to the genera _Acrodus_,
_Hybodus_, _Gyrolepis_, and _Saurichthys_.
Among those common to the English bone-bed and the Muschelkalk of Germany
are _Hybodus plicatilis_ (fig. 324.), _Saurichthys apicalis_ (fig. 325.),
_Gyrolepis tenuistriatus_ (fig. 326.), and _G. Albertii_. Remains of
saurians have also been found in the bone-bed, and plates of an _Encrinus_.
[Illustration: Fig. 324. _Hybodus plicatilis._ Teeth. Bone-bed,
Aust and Axmouth.]
[Illustration: Fig. 325. _Saurichthys apicalis._ Tooth; nat. size,
and magnified. Axmouth.]
[Illustration: Fig. 326. _Gyrolepis tenuistriatus._ Scale; nat. size,
and magnified. Axmouth.]
The strata of red and green marl, which follow the bone-bed in the
descending order at Axmouth and Aust, are destitute of organic remains;
as is the case, for the most part, in the corresponding beds in almost
every part of England. But fossils have lately been found at a few
localities in sandstones of this formation, in Worcestershire and
Warwickshire, and among them the bivalve shell called _Posidonia
minuta_, Goldf., before mentioned (fig. 321. p. 288.).
The upper member of the English "New Red" containing this shell, in
those parts of England, is, according to Messrs. Murchison and
Strickland, 600 feet thick, and consists chiefly of red marl or
slate, with a band of sandstone. Spines of _Hybodus_, called
_ichthyodorulites_, teeth of fishes, and footprints of reptiles, with
remains of a saurian called _Rhyncosaurus_, were observed by the same
geologists in these strata.[290-A]
In Cheshire and Lancashire the gypseous and saliferous red shales and loams
of the Trias are between 1000 and 1500 feet thick. In some places
lenticular masses of rock-salt are interpolated between the argillaceous
beds, the origin of which will be spoken of in the sequel.
[Illustration: Fig. 327. Single footstep of _Chirotherium_. Bunter
Sandstein, Saxony; one eighth of nat. size.]
[Illustration: Fig. 328. Line of footsteps on slab of sandstone.
Hildburghausen, in Saxony.]
The lower division or English representative of the "Bunter" attains a
thickness of 600 feet in the counties last mentioned. Besides red and green
shales and red sandstones, it comprises much soft white quartzose
sandstone, in which the trunks of silicified trees have been met with at
Allesley Hill, near Coventry. Several of them were a foot and a half in
diameter, and some yards in length, decidedly of coniferous wood, and
showing rings of annual growth.[290-B] Impressions, also, of the footsteps
of animals have been detected in Lancashire and Cheshire in this formation.
Some of the most remarkable occur a few miles from Liverpool, in the
whitish quartzose sandstone of Storton Hill, on the west side of the
Mersey. They bear a close resemblance to tracks first observed in a member
of the Upper New Red Sandstone, at the village of Hesseberg, near
Hildburghausen, in Saxony, to which I have already alluded. For many years
these footprints have been referred to a large unknown quadruped,
provisionally named _Chirotherium_ by Professor Kaup, because the marks
both of the fore and hind feet resembled impressions made by a human hand.
(See fig. 327.) The footmarks at Hesseberg are partly concave and partly in
relief; the former, or the depressions, are seen upon the upper surface of
the sandstone slabs, but those in relief are only upon the lower surfaces,
being in fact natural casts, formed in the subjacent footprints as in
moulds. The larger impressions, which seem to be those of the hind foot,
are generally 8 inches in length, and 5 in width, and one was 12 inches
long. Near each large footstep, and at a regular distance (about an inch
and a half), before it, a smaller print of a fore foot, 4 inches long and
3 inches wide, occurs. The footsteps follow each other in pairs, each pair
in the same line, at intervals of 14 inches from pair to pair. The large as
well as the small steps show the great toes alternately on the right and
left side; each step makes the print of five toes, the first or great toe
being bent inwards like a thumb. Though the fore and hind foot differ so
much in size, they are nearly similar in form.
The similar footmarks afterwards observed in a rock of corresponding age at
Storton Hill, were imprinted on five thin beds of clay, superimposed one
upon the other in the same quarry, and separated by beds of sandstone. On
the lower surface of the sandstone strata, the solid casts of each
impression are salient, in high relief, and afford models of the feet,
toes, and claws of the animals which trod on the clay.
As neither in Germany nor in England any bones or teeth had been met with
in the same identical strata as the footsteps, anatomists indulged, for
several years, in various conjectures respecting the mysterious animals
from which they might have been derived. Professor Kaup suggested that the
unknown quadruped might have been allied to the _Marsupialia_; for in the
kangaroo the first toe of the fore foot is in a similar manner set
obliquely to the others, like a thumb, and the disproportion between the
fore and hind feet is also very great. But M. Link conceived that some of
the four species of animals of which the tracks had been found in Saxony
might have been gigantic _Batrachians_; and Dr. Buckland designated some of
the footsteps as those of a small web-footed animal, probably crocodilean.
In the course of these discussions several naturalists of Liverpool, in
their report on the Storton quarries, declared their opinion that each of
the thin seams of clay in which the sandstone casts were moulded had formed
successively a surface above water, over which the _Chirotherium_ and other
animals walked, leaving impressions of their footsteps, and that each layer
had been afterwards submerged by a sinking down of the surface, so that a
new beach was formed at low water above the former, on which other tracks
were then made. The repeated occurrence of ripple-marks at various heights
and depths in the red sandstone of Cheshire had been explained in the same
manner. It was also remarked that impressions of such depth and clearness
could only have been made by animals walking on the land, as their weight
would have been insufficient to make them sink so deeply in yielding clay
under water. They must therefore have been air-breathers.
When the inquiry had been brought to this point, the reptilian remains
discovered in the Trias, both of Germany and England, were carefully
examined by Mr. Owen. He found, after a microscopic investigation of the
teeth from the German sandstone called Keuper, and from the sandstone of
Warwick and Leamington, that neither of them could be referred to true
saurians, although they had been named _Mastodonsaurus_ and _Phytosaurus_
by Jaeger (fig. 329.). It appeared that they were of the _Batrachian_ order,
and attested the former existence of frogs of gigantic dimensions in
comparison with any now living. Both the Continental and English fossil
teeth exhibited a most complicated texture, differing from that previously
observed in any reptile, whether recent or extinct, but most nearly
analogous to the _Ichthyosaurus_. A section of one of these teeth exhibits
a series of irregular folds, resembling the labyrinthic windings of the
surface of the brain; and from this character Mr. Owen has proposed the
name _Labyrinthodon_ for the new genus. By his permission, the annexed
representation (fig. 330.) of part of one is given from his "Odontography,"
plate 64. A. The entire length of this tooth is supposed to have been about
three inches and a half, and the breadth at the base one inch and a half.
[Illustration: Fig. 329. Tooth of _Labyrinthodon_; nat. size.
Warwick sandstone.]
[Illustration: Fig. 330. Transverse section of tooth of _Labyrinthodon
Jaegeri_, Owen (_Mastodonsaurus Jaegeri_, Meyer); nat. size, and
a segment magnified.
_a._ Pulp cavity, from which the processes of pulp and dentine radiate.]
When Mr. Owen had satisfied himself, from an inspection of the cranium,
jaws, and teeth, that a gigantic _Batrachian_ had existed at the period of
the Trias or Upper New Red Sandstone, he soon found, from the examination
of various bones derived from the same formation, that he could define
three species of _Labyrinthodon_, and that in this genus the hind
extremities were much larger than the anterior ones. This circumstance,
coupled with the fact of the _Labyrinthodon_ having existed at the period
when the _Chirotherian_ footsteps were made, was the first step towards the
identification of those tracks with the newly discovered _Batrachian_. It
was at the same time observed that the footmarks of _Chirotherium_ were
more like those of toads than of any other living animal; and, lastly,
that the size of the three species of _Labyrinthodon_ corresponded with the
size of three different kinds of footprints which had already been supposed
to belong to three distinct _Chirotheria_. It was moreover inferred, with
confidence, that the _Labyrinthodon_ was an _air-breathing_ reptile from
the structure of the nasal cavity, in which the posterior outlets were at
the back part of the mouth, instead of being directly under the anterior or
external nostrils. It must have respired air after the manner of saurians,
and may therefore have imprinted on the shore those footsteps, which, as we
have seen, could not have originated from an animal walking under water.
It is true that the structure of the foot is still wanting, and that a
more connected and complete skeleton is required for demonstration; but
the circumstantial evidence above stated is strong enough to produce
the conviction that the _Chirotherium_ and _Labyrinthodon_ are one
and the same.
In order to show the manner in which one of these formidable _Batrachians_
may have impressed the mark of its feet upon the shore, Mr. Owen has
attempted a restoration, of which a reduced copy is annexed.
[Illustration: Fig. 331. _Labyrinthodon pachygnathus_, Owen.]
The only bones of this species at present known are those of the head, the
pelvis, and part of the scapula, which are shown by stronger lines in the
above figure. There is reason for believing that the head was not smooth
externally, but protected by bony scutella.
_Origin of Red Sandstone and Rock Salt._
We have seen that, in various parts of the world, red and mottled clays,
and sandstones, of several distinct geological epochs, are found associated
with salt, gypsum, magnesian limestone, or with one or all of these
substances. There is, therefore, in all likelihood, a general cause for
such a coincidence. Nevertheless, we must not forget that there are dense
masses of red and variegated sandstones and clays, thousands of feet in
thickness, and of vast horizontal extent, wholly devoid of saliferous or
gypseous matter. There are also deposits of gypsum and of muriate of soda,
as in the blue clay formation of Sicily, without any accompanying red
sandstone or red clay.
To account for deposits of red mud and red sand, we have simply to suppose
the disintegration of ordinary crystalline or metamorphic schists. Thus, in
the eastern Grampians of Scotland, as, for example, in the north of
Forfarshire, the mountains of gneiss, mica-schist, and clay-slate, are
overspread with alluvium, derived from the disintegration of those rocks;
and the mass of detritus is stained by oxide of iron, of precisely the same
colour as the Old Red Sandstone of the adjoining Lowlands. Now this
alluvium merely requires to be swept down to the sea, or into a lake, to
form strata of red sandstone and red marl, precisely like the mass of the
"Old Red" or New Red systems of England, or those tertiary deposits of
Auvergne (see p. 182.), before described, which are in lithological
characters quite undistinguishable. The pebbles of gneiss in the Eocene red
sandstone of Auvergne point clearly to the rocks from which it has been
derived. The red colouring matter may, as in the Grampians, have been
furnished by the decomposition of hornblende, or mica, which contain oxide
of iron in large quantity.
It is a general fact, and one not yet accounted for, that scarcely any
fossil remains are preserved in stratified rocks in which this oxide of
iron abounds; and when we find fossils in the New or Old Red Sandstone in
England, it is in the grey, and usually calcareous beds, that they occur.
The gypsum and saline matter, occasionally interstratified with such red
clays and sandstones of various ages, primary, secondary, and tertiary,
have been thought by some geologists to be of volcanic origin. Submarine
and subaerial exhalations often occur in regions of earthquakes and
volcanos far from points of actual eruption, and charged with sulphur,
sulphuric salts, and with common salt or muriate of soda. In a word, they
are vents by which all the products which issue in a state of sublimation
from the craters of active volcanos, obtain a passage from the interior of
the earth to the surface. That such gaseous emanations and mineral springs,
impregnated with the ingredients before enumerated, and often intensely
heated, continue to flow out unaltered in composition and temperature for
ages, is well known. But before we can decide on their real instrumentality
in producing in the course of ages beds of gypsum, salt, and dolomite, we
require to know what are the chemical changes actually in progress in seas
where this volcanic agency is at work.
Yet the origin of rock-salt is a problem of so much interest in
theoretical geology as to demand a full discussion of another hypothesis
advanced on the subject; namely, that which attributes the precipitation
of the salt to evaporation, whether of inland lakes or of lagoons
communicating with the ocean.
At Northwich, in Cheshire, two beds of salt, in great part unmixed with
earthy matter, attain the extraordinary thickness of 90 and even 100 feet.
The upper surface of the highest bed is very uneven, forming cones and
irregular figures. Between the two masses there intervenes a bed of
indurated clay, traversed with veins of salt. The highest bed thins off
towards the south-west, losing 15 feet in thickness in the course of a
mile.[295-A] The horizontal extent of these particular masses in Cheshire
and Lancashire is not exactly known; but the area, containing saliferous
clays and sandstones, is supposed to exceed 150 miles in diameter, while
the total thickness of the trias in the same region is estimated by Mr.
Ormerod at more than 1700 feet. Ripple-marked sandstones, and the
footprints of animals, before described, are observed at so many levels
that we may safely assume the whole area to have undergone a slow and
gradual depression during the formation of the Red Sandstone. The evidence
of such a movement, wholly independent of the presence of salt itself, is
very important in reference to the theory under consideration.
In the "Principles of Geology" (chap. 28.), I published a map, furnished to
me by the late Sir Alexander Burnes, of that singular flat region called
the Runn of Cutch, near the delta of the Indus, which is 7000 square miles
in area, or equal in extent to about one-fourth of Ireland. It is neither
land nor sea, but is dry during a part of every year, and again covered by
salt water during the monsoons. Some parts of it are liable, after long
intervals, to be overflowed by river-water. Its surface supports no grass,
but is encrusted over, here and there, by a layer of salt, about an inch in
depth, caused by the evaporation of sea-water. Certain tracts have been
converted into dry land by upheaval during earthquakes since the
commencement of the present century, and, in other directions, the
boundaries of the Runn have been enlarged by subsidence. That successive
layers of salt might be thrown down, one upon the other, over thousands of
square miles, in such a region, is undeniable. The supply of brine from the
ocean would be as inexhaustible as the supply of heat from the sun to cause
evaporation. The only assumption required to enable us to explain a great
thickness of salt in such as area is, the continuance, for an indefinite
period, of a subsiding movement, the country preserving all the time a
general approach to horizontality. Pure salt could only be formed in the
central parts of basins, where no sand could be drifted by the wind, or
sediment be brought by currents. Should the sinking of the ground be
accelerated, so as to let in the sea freely, and deepen the water, a
temporary suspension of the precipitation of salt would be the only result.
On the other hand, if the area should dry up, ripple-marked sands and the
footprints of animals might be formed, where salt had previously
accumulated. According to this view the thickness of the salt, as well as
of the accompanying beds of mud and sand, becomes a mere question of time,
or requires simply a repetition of similar operations.
Mr. Hugh Miller, in an able discussion of this question, refers to Dr.
Frederick Parrot's account, in his journey to Ararat (1836), of the salt
lakes of Asia. In several of these lakes west of the river Manech, "the
water, during the hottest season of the year, is covered on its surface
with a crust of salt nearly an inch thick, which is collected with shovels
into boats. The crystallization of the salt is effected by rapid
evaporation from the sun's heat and the supersaturation of the water with
muriate of soda; the lake being so shallow that the little boats trail on
the bottom and leave a furrow behind them, so that the lake must be
regarded as a wide pan of enormous superficial extent, in which the brine
can easily reach the degree of concentration required."
Another traveller, Major Harris, in his "Highlands of Ethiopia," describes
a salt lake, called the Bahr Assal, near the Abyssinian frontier, which
once formed the prolongation of the Gulf of Tadjara, but was afterwards cut
off from the gulf by a broad bar of lava or of land upraised by an
earthquake. "Fed by no rivers, and exposed in a burning climate to the
unmitigated rays of the sun, it has shrunk into an elliptical basin, seven
miles in its transverse axis, half filled with smooth water of the deepest
caerulian hue, and half with a solid sheet of glittering snow-white salt,
the offspring of evaporation." "If," says Mr. Hugh Miller, "we suppose,
instead of a barrier of lava, that sand-bars were raised by the surf on a
flat arenaceous coast during a slow and equable sinking of the surface, the
waters of the outer gulf might occasionally topple over the bar, and supply
fresh brine when the first stock had been exhausted by evaporation.[296-A]
We may add that the permanent impregnation of the waters of a large shallow
basin with salt, beyond the proportion which is usual in the ocean, would
cause it to be uninhabitable by mollusca or fish, as is the case in the
Dead Sea, and the muriate of soda might remain in excess, even though it
were occasionally replenished by irruptions of the sea. Should the saline
deposit be eventually submerged, it might, as we have seen from the example
of the Runn of Cutch, be covered by a freshwater formation containing
fluviatile organic remains; and in this way the apparent anomaly of beds of
sea-salt and clays devoid of marine fossils, alternating with others of
freshwater origin, may be explained.
Dr. G. Buist, in a recent communication to the Bombay Geographical Society
(vol. ix.), has asked how it happens that the Red Sea should not exceed the
open ocean in saltness, by more than 1/10th per cent. The Red Sea receives
no supply of water from any quarter save through the Straits of
Babelmandeb; and there is not a single river or rivulet flowing into it
from a circuit of 4000 miles of shore. The countries around are all
excessively sterile and arid, and composed, for the most part, of burning
deserts. From the ascertained evaporation in the sea itself, Dr. Buist
computes that nearly 8 feet of pure water must be carried off from the
whole of its surface annually, this being probably equivalent to 1/100th
part of its whole volume. The Red Sea, therefore, ought to have 1 per cent.
added annually to its saline contents; and as these constitute 4 per cent.
by weight, or 2-1/2 per cent. in volume of its entire mass, it ought,
assuming the average depth to be 800 feet, which is supposed to be far
beyond the truth, to have been converted into one solid salt formation in
less than 3000 years.[297-A] Does the Red Sea receive a supply of water
from the ocean, through the narrow Straits of Babelmandeb, sufficient to
balance the loss by evaporation? And is there an undercurrent of heavier
saline water annually flowing outwards? If not, in what manner is the
excess of salt disposed of? An investigation of this subject by our
nautical surveyors may perhaps aid the geologist in framing a true theory
of the origin of rock-salt.
_On the New Red Sandstone of the valley of the Connecticut River
in the United States._
In a depression of the granitic or hypogene rocks in the States of
Massachusetts and Connecticut, strata of red sandstone, shale, and
conglomerate are found occupying an area more than 150 miles in length from
north to south, and about 5 to 10 miles in breadth, the beds dipping to the
eastward at angles varying from 5 to 50 degrees. The extreme inclination of
50 degrees is rare, and only observed in the neighbourhood of masses of
trap which have been intruded into the red sandstone while it was forming,
or before the newer parts of the deposit had been completed. Having
examined this series of rocks in many places, I feel satisfied that they
were formed in shallow water, and for the most part near the shore, and
that some of the beds were from time to time raised above the level of the
water, and laid dry, while a newer series, composed of similar sediment,
was forming. The red flags of thin-bedded sandstone are often
ripple-marked, and exhibit on their under sides casts of cracks formed in
the underlying red and green shales. These last must have shrunk by drying
before the sand was spread over them. On some shales of the finest texture
impressions of rain drops may be seen, and casts of them in the incumbent
argillaceous sandstones. Having observed similar markings produced by
showers, of which the precise date was known, on the recent red mud of the
Bay of Fundy, and casts in relief of the same, on layers of dried mud
thrown down by subsequent tides, I feel no doubt in regard to the origin of
some of the ancient Connecticut impressions. I have also seen on the
mud-flats of the Bay of Fundy the footmarks of birds (_Tringa minuta_),
which daily run along the borders of that estuary at low water, and which I
have described in my Travels.[297-B] Similar layers of red mud, now
hardened and compressed into shale, are laid open on the banks of the
Connecticut, and retain faithfully the impressions and casts of the feet of
numerous birds and reptiles which walked over them at the time when they
were deposited, probably in the Triassic Period.
According to Professor Hitchcock, the footprints of no less than thirty-two
species of bipeds, and twelve of quadrupeds, have been already detected in
these rocks. Thirty of these are believed to be those of birds, four of
lizards, two of chelonians, and six of batrachians. The tracks have been
found in more than twenty places, scattered through an extent of nearly 80
miles from north to south, and they are repeated through a succession of
beds attaining at some points a thickness of more than 1000 feet, which may
have been thousands of years in forming.[298-A]
[Illustration: Fig. 332. Footprints of a bird. Turner's Falls, Valley of
the Connecticut. (See Dr. Deane, Mem. of Amer. Acad. vol. iv. 1849.)]
As considerable scepticism is naturally entertained in regard to the nature
of the evidence derived from footprints, it may be well to enumerate some
facts respecting them on which the faith of the geologist may rest. When I
visited the United States in 1842, more than 2000 impressions had been
observed by Professor Hitchcock, in the district alluded to, and all of
them were indented on the upper surface of the layers, while the
corresponding casts, standing out in relief, were always on the lower
surfaces or planes of the strata. If we follow a single line of marks we
find them uniform in size, and nearly uniform in distance from each other,
the toes of two successive footprints, turning alternately right and left
(see fig. 332.). Such single lines indicate a biped; and there is generally
such a deviation from a straight line, in any three successive prints, as
we remark in the tracks left by birds. There is also a striking relation
between the distance separating two footprints in one series and the size
of the impressions; in other words, an obvious proportion between the
length of the stride and the dimension of the creature which walked over
the mud. If the marks are small, they may be half an inch asunder; if
gigantic, as, for example, where the toes are 20 inches long, they are
occasionally 4 feet and a half apart. The bipedal impressions are for the
most part trifid, and show the same number of joints as exist in the feet
of living tridactylous birds. Now such birds have three phalangeal bones
for the inner toe, four for the middle and five for the outer one (see fig.
332.); but the impression of the terminal joint is that of the nail only.
The fossil footprints exhibit regularly, where the joints are seen, the
same number; and we see in each continuous line of tracks the three-jointed
and five-jointed toes placed alternately outwards, first on the one side
and then on the other. It is not often that the matrix has been fine enough
to retain impressions of the integument or skin of the foot; but in one
fine specimen found at Turner's Falls on the Connecticut, by Dr. Deane,
these markings are well preserved, and have been recognized by Mr. Owen as
resembling the skin of the ostrich, and not that of reptiles.[298-B] Much
care is required to ascertain the precise layer of a laminated rock on
which an animal has walked, because the impression usually extends
downwards through several laminae; and if the upper layer originally trodden
upon is wanting, one or more joints, or even in some cases an entire toe,
which sank less deep into the soft ground, may disappear, and yet the
remainder of the footprint be well defined.
The size of several of the fossil impressions of the Connecticut red
sandstone so far exceeds that of any living ostrich, that naturalists at
first were extremely adverse to the opinion of their having been made by
birds, until the bones and almost entire skeleton of the _Dinornis_ and of
other feathered giants of New Zealand were discovered. Their dimensions
have at least destroyed the force of this particular objection. The
magnitude of the impressions of the feet of a heavy animal, which has
walked on soft mud, increases for some distance below the surface
originally trodden upon. In order, therefore, to guard against
exaggeration, the casts rather than the mould are relied on. These casts
show that some of the fossil birds had feet four times as large as the
ostrich, but not perhaps larger than the _Dinornis_.
Some of the quadrupedal footprints which accompany those of birds are
analogous to European _Chirotheria_, and with a similar disproportion
between the hind and fore feet. Others resemble that remarkable reptile,
the _Rhyncosaurus_ of the English Trias, a creature having some relation
in its osteology both to chelonians and birds. Other imprints, again,
are like those of turtles.
Among the supposed bipedal tracks, a single distinct example only has been
observed of feet in which there are four toes directed forwards. In this
case a series of four footprints is seen, each 22 inches long and 12 wide,
with joints much resembling those in the toes of birds. Professor Agassiz
has suggested that it might have belonged to a gigantic bipedal batrachian;
but the evidence on this subject is too defective to warrant such a bold
conjecture, and if we were to give the reins to our imagination, we might
as well conceive a bird having four toes projecting forwards as a huge
two-legged frog. Nor should we forget that some quadrupeds place the hind
foot so precisely on the spot just quitted by the fore foot, as to produce
a single line of imprints like a biped.
No bones have as yet been met with, whether of reptiles or birds, in the
rocks of the Connecticut, but there are numerous coprolites; and an
ingenious argument has been derived by Mr. Dana, from the analysis of these
bodies, and the proportion they contain of uric acid, phosphate of lime,
carbonate of lime, and organic matter, to show that, like guano, they are
the droppings of birds, rather than of reptiles.[299-A]
Mr. Darwin, in his "Journal of a Voyage in the Beagle," informs us that the
"South American ostriches, although they live on vegetable matter, such as
roots and grass, are repeatedly seen at Bahia Blanca (lat. 39 deg. S.), on the
coast of Buenos Ayres, coming down at low water to the extensive mud-banks
which are then dry, for the sake, as the Gauchos say, of feeding on small
fish." They readily take to the water, and have been seen at the bay of San
Blas, and at Port Valdez, in Patagonia, swimming from island to
island.[300-A] It is therefore evident, that in our times a South American
mud-bank might be trodden simultaneously by ostriches, alligators,
tortoises, and frogs; and the impressions left, in the nineteenth century,
by the feet of these various tribes of animals, would not differ from each
other more entirely than do those attributed to birds, saurians,
chelonians, and batrachians, in the rocks of the Connecticut.
To determine the exact age of the red sandstone and shale containing these
ancient footprints in the United States, is not possible at present. No
fossil shells have yet been found in the deposit, nor plants in a
determinable state. The fossil fish are numerous and very perfect; but they
are of a peculiar type, which was originally referred to the genus
_Palaeoniscus_, but has since, with propriety, been ascribed, by Sir Philip
Egerton, to a new genus. To this he has given the name of _Ischypterus_,
from the great size and strength of the fulcral rays of the dorsal fin
(from +ischys+; strength, and +pteron+, a fin). They differ from
_Palaeoniscus_, as Mr. Redfield first pointed out, by having the vertebral
column prolonged to a more limited extent into the upper lobe of the tail,
or, in the language of M. Agassiz, they are less heterocercal. The teeth
also, according to Sir P. Egerton, who, in 1844, examined for me a fine
series of specimens which I procured at Durham, Connecticut, differ from
those of _Palaeoniscus_ in being strong and conical.
That the sandstones containing these fish are of older date than the
strata containing coal, before described (p. 284.) as occurring near
Richmond in Virginia, is highly probable. These were shown to be as old
at least as the oolite and lias. The higher antiquity of the Connecticut
beds cannot be proved by direct superposition, but may be presumed from
the general structure of the country. That structure proves them to be
newer than the movements to which the Appalachian or Alleghany chain
owes its flexures, and this chain includes the ancient coal formation
among its contorted rocks. The unconformable position of this _New Red_
with ornithichnites on the edges of the inclined primary or paleozoic
rocks of the Appalachians is seen at 4. of the section, fig. 379. p.
327. The absence of fish with decidedly heterocercal tails may afford an
argument against the Permian age of the formation; and the opinion that
the red sandstone is triassic, seems, on the whole, the best that we can
embrace in the present state of our knowledge.
FOOTNOTES:
[286-A] Buckland, Bridgew. Treat., vol. ii. p. 38.
[287-A] Monog. des Bunten Sandsteins.
[288-A] Tableau des Genres de Veg. Fos., Dict. Univ. 1849.
[290-A] Geol. Trans., Second Series, vol. v.
[290-B] Buckland, Proc. Geol. Soc. vol. ii. p. 439.; and Murchison and
Strickland Geol. Trans., Second Ser., vol. v. p. 347.
[295-A] Ormerod, Quart. Geol. Journ. 1848, vol. iv. p. 277.
[296-A] Hugh Miller, First Impressions of England, 1847, pp. 183. 214.
[297-A] Buist, Trans. of Bombay Geograph. Soc. 1850, vol. ix. p. 38.
[297-B] Travels in North America, vol. ii. p. 168.
[298-A] Hitchcock, Mem. of Amer. Acad. New Ser., vol. iii. p. 129.
[298-B] This specimen is now in Dr. Mantell's museum.
[299-A] Amer. Journ. of Sci. vol. xlviii. p. 46.
[300-A] Journal of Voyage of Beagle, &c. 2d edition, p. 89. 1845.