A Manual of Elementary Geology Part 44

[Ill.u.s.tration: 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 Tria.s.sic 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 inc.u.mbent 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.

_Tria.s.sic 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 Devons.h.i.+re, and in the cliffs of Westbury and Aust, in Gloucesters.h.i.+re, 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 cla.s.sed 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 a.s.semblage 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_.

[Ill.u.s.tration: Fig. 324. _Hybodus plicatilis._ Teeth. Bone-bed, Aust and Axmouth.]

[Ill.u.s.tration: Fig. 325. _Saurichthys apicalis._ Tooth; nat. size, and magnified. Axmouth.]

[Ill.u.s.tration: 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 dest.i.tute 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 Worcesters.h.i.+re and Warwicks.h.i.+re, and among them the bivalve sh.e.l.l called _Posidonia minuta_, Goldf., before mentioned (fig. 321. p. 288.).

The upper member of the English "New Red" containing this sh.e.l.l, 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 Ches.h.i.+re and Lancas.h.i.+re the gypseous and saliferous red shales and loams of the Trias are between 1000 and 1500 feet thick. In some places lenticular ma.s.ses of rock-salt are interpolated between the argillaceous beds, the origin of which will be spoken of in the sequel.

[Ill.u.s.tration: Fig. 327. Single footstep of _Chirotherium_. Bunter Sandstein, Saxony; one eighth of nat. size.]

[Ill.u.s.tration: 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 Lancas.h.i.+re and Ches.h.i.+re 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 Ches.h.i.+re 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 Jager (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 a.n.a.logous 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.

[Ill.u.s.tration: Fig. 329. Tooth of _Labyrinthodon_; nat. size.

Warwick sandstone.]

[Ill.u.s.tration: 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 circ.u.mstance, 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 sh.o.r.e 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 circ.u.mstantial 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 sh.o.r.e, Mr. Owen has attempted a restoration, of which a reduced copy is annexed.

[Ill.u.s.tration: 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 a.s.sociated 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 ma.s.ses 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 Forfars.h.i.+re, the mountains of gneiss, mica-schist, and clay-slate, are overspread with alluvium, derived from the disintegration of those rocks; and the ma.s.s 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 ma.s.s 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 quant.i.ty.

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 pa.s.sage 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 Ches.h.i.+re, 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 ma.s.ses 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 ma.s.ses in Ches.h.i.+re and Lancas.h.i.+re 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 a.s.sume 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 gra.s.s, 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 a.s.sumption 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 acc.u.mulated. 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 repet.i.tion 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 a.s.sal, 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 sh.o.r.e. 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 const.i.tute 4 per cent.

by weight, or 2-1/2 per cent. in volume of its entire ma.s.s, it ought, a.s.suming 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 Ma.s.sachusetts 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 ma.s.ses 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 sh.o.r.e, 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 inc.u.mbent 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 Tria.s.sic Period.