A Manual of Elementary Geology Part 45

According to Professor Hitchc.o.c.k, 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]

[Ill.u.s.tration: 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 Hitchc.o.c.k, 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 a.n.a.logous 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 Aga.s.siz 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 a.n.a.lysis 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 gra.s.s, are repeatedly seen at Bahia Blanca (lat. 39 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 sh.e.l.ls 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. Aga.s.siz, 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 tria.s.sic, 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] Hitchc.o.c.k, 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.

CHAPTER XXIII.

PERMIAN OR MAGNESIAN LIMESTONE GROUP.

Fossils of Magnesian Limestone and Lower New Red distinct from the Tria.s.sic--Term Permian--English and German equivalents--Marine sh.e.l.ls and corals of English Magnesian limestone--Palaeoniscus and other fish of the marl slate--Thecodont Saurians of dolomitic conglomerate of Bristol--Zechstein and Rothliegendes of Thuringia--Permian Flora--Its generic affinity to the carboniferous--Psaronites or tree-ferns.

When the use of the term "Poikilitic" was explained in the last chapter, I stated, that in some parts of England it is scarcely possible to separate the red marls and sandstones so called (originally named "the New Red"), into two distinct geological systems. Nevertheless, the progress of investigation, and a careful comparison of English rocks between the lias and the coal with those occupying a similar geological position in Germany and Russia, has enabled geologists to divide the Poikilitic formation; and has even shown that the lowermost of the two divisions is more closely connected, by its fossil remains, with the carboniferous group than with the trias. If, therefore, we are to draw a line between the secondary and primary fossiliferous strata, as between the tertiary and secondary, it must run through the middle of what was once called the "New Red," or Poikilitic group. The inferior half of this group will rank as Primary or Paleozoic, while its upper member will form the base of the Secondary series. For the lower, or Magnesian Limestone division of English geologists, Sir R. Murchison has proposed the name of Permian, from Perm, a Russian government where these strata are more extensively developed than elsewhere, occupying an area twice the size of France, and containing an abundant and varied suite of fossils.

Mr. King, in his valuable monograph, recently published, of the Permian fossils of England, has given a table of the following six members of the Permian system of the north of England, with what he conceives to be the corresponding formations in Thuringia.[301-A]

North of England. Thuringia.

1. Crystalline or concretionary, 1. Stinkstein.

and non-crystalline limestone. 2. Brecciated and pseudo-brecciated 2. Rauchwacke.

limestone. 3. Fossiliferous limestone. 3. Dolomit, or Upper Zechstein.

4. Compact limestone. 4. Zechstein, or Lower Zechstein.

5. Marl-slate. 5. Mergel-schiefer, or Kupferschiefer.

6. Inferior sandstones of various 6. Rothliegendes.

colours.

I shall proceed, therefore, to treat briefly of these subdivisions, beginning with the highest, and referring the reader, for a fuller description of the lithological character of the whole group, as it occurs in the north of England, to a valuable memoir by Professor Sedgwick, published in 1835.[302-A]

_Crystalline or concretionary limestone_ (No. 1.).--This formation is seen upon the coast of Durham and Yorkshire, between the Wear and the Tees.

Among its characteristic fossils are _Schizodus Schlotheimi_ (fig. 333.) and _Mytilus septifer_ (fig. 335.).

[Ill.u.s.tration: Fig. 333. _Schizodus Schlotheimi_, Geinitz. Syn. _Axinus obscurus_, Sow. Crystalline limestone, Permian.]

[Ill.u.s.tration: Fig. 334. _Schizodus truncatus_, King; to show hinge. Permian.]

[Ill.u.s.tration: Fig. 335. _Mytilus septifer_, King. Syn. _Modiola ac.u.minata_, James Sow. Permian crystalline limestone.]

These sh.e.l.ls occur at Hartlepool and Sunderland, where the rock a.s.sumes an oolitic and botryoidal character. Some of the beds in this division are ripple-marked; and Mr. King imagines that the absence of corals and the character of the sh.e.l.ls indicate shallow water. In some parts of the coast of Durham, where the rock is not crystalline, it contains as much as forty-four per cent. of carbonate of magnesia, mixed with carbonate of lime. In other places,--for it is extremely variable in structure,--it consists chiefly of carbonate of lime, and has concreted into globular and hemispherical ma.s.ses, varying from the size of a marble to that of a cannon-ball, and radiating from the centre. Occasionally earthy and pulverulent beds pa.s.s into compact limestone or hard granular dolomite. The stratification is very irregular, in some places well-defined, in others obliterated by the concretionary action which has re-arranged the materials of the rocks subsequently to their original deposition. Examples of this are seen at Pontefract and Ripon in Yorkshire.

_The brecciated limestone_ (No. 2.) contains no fragments of foreign rocks, but seems composed of the breaking-up of the Permian limestone itself, about the time of its consolidation. Some of the angular ma.s.ses in Tynemouth Cliff are 2 feet in diameter. This breccia is considered by Professor Sedgwick as one of the forms of the preceding limestone, No.

1., rather than as regularly underlying it. The fragments are angular and never water-worn, and appear to have been re-cemented on the spot where they were formed. It is, therefore, suggested that they may have been due to those internal movements of the ma.s.s which produced the concretionary structure; but the subject is very obscure, and after studying the phenomenon in the Marston Rocks, on the coast of Durham, I found it impossible to form any positive opinion on the subject. The well-known brecciated limestones of the Pyrenees appeared to me to present the nearest a.n.a.logy, but on a much smaller scale.

_The fossiliferous limestone_ (No. 3.) is regarded by Mr. King as a deep-water formation, from the numerous delicate corals which it includes.

One of these, _Fenestella retiformis_ (fig. 336.), is a very variable species, and has received many different names. It sometimes attains a large size, measuring 8 inches in width. The same zoophyte is also found abundantly in the Permian of Germany.

[Ill.u.s.tration: Fig. 336. Fenestella.

_a._ _Fenestella retiformis_, Schlot.

Syn. _Gorgonia infundibuliformis_, Goldf.; _Retepora fl.u.s.tracea_, Phillips.

_b._ Part of the same highly magnified.