Popular Science Monthly/Volume 3/September 1873/Old Continents
OF THE ENGLISH GEOLOGICAL SURVEY.
FOR many years the stratified formations in general were described in manuals of geology as of marine origin, with the exception perhaps of part of the Coal measures, and more unequivocally of the Purbeck and Wealden beds, and the fresh-water strata of parts of the Eocene and Miocene series. Even now the Old Red Sandstone, as distinct from the marine Devonian rocks, is only occasionally and hesitatingly allowed to have a fresh-water origin, in spite of the statement made by Mr. Godwin-Austen long ago, that it was deposited in lakes.
My present object is to prove that, in the British Islands, all the great formations of a red color, and which are partly of Palæozoic, and partly of Mesozoic or Secondary age, were deposited in large inland lakes, fresh or salt; and if this can be established, then there was a long continental epoch in this part of the world comparable to, and as important in a physical point of view as any of, the great continents of the present day.
The Upper Silurian rocks of Shropshire, Herefordshire, Monmouthshire, and South Wales, are succeeded immediately by the Old Red Sandstone series, and there is no unconformity between them.
The teeming life of the Upper Silurian seas, in what is now "Wales and the adjoining counties, continued in full force right up to the narrow belt of passage-beds which marks the change from Silurian brown muddy sands into lower Old Red Sandstone. In these transition beds on the contrary, genera, species, and often individuals, are few in number and sometimes dwarfed in size, the marine life rapidly dwindles away, and in the very uppermost Silurian beds land-plants appear, consisting of small pieces of undetermined twigs and the spore-cases of Lycopodiaceæ (Pachytheca spherica). Above this horizon the strata become red.
The poverty in number and the frequent small size of the shells in the passage-beds indicate a change of conditions in the nature of the waters in which they lived; and the plants alluded to clearly point to the close neighborhood of a land, of which we have no direct signs, in the vast development of a purely marine fauna in lower portions of the Ludlow strata. In the Ludlow bone-beds the fish-remains, Onchus and Sphagodus, and the large numbers of marine Crustacea, almost entirely trilobitic in the Ludlow rocks, indicate a set of conditions very unlike those that prevailed when the passage-beds and the lower strata of the true Old Red Sandstone were deposited, in both of which new fish appear, trilobites are altogether absent, and are more or less replaced by Crustacea of the genera Pterygotus and Eurypterus, one of which, Eurypterus Symondsii, has only been found in the lower Old Red Sandstone. Neither are there any mollusca in the Old Red Sandstone, excepting where that formation passes at the top into the Carboniferous rocks. All these circumstances indicate changes of conditions which were, I believe, of a geographical kind, and connected with the appearance in the area of fresh water.
The circumstances which marked the passage of the uppermost Silurian rocks into Old Red Sandstone seem to me to have been the following: First, a shallowing of the sea, followed by a gradual alteration in the physical geography of the district, so that the area became changed into a series of mingled fresh and brackish lagoons, which finally, by continued terrestrial changes, were converted into a great fresh-water lake, or, if we take the whole of Britain and areas now sea-covered beyond, into a series of lakes. The occurrence of a few genera or even species of fish and Crustacea common to the salt, brackish, or fresh waters, does not prove that the passage-beds and those still higher are truly marine. At the present day, animals commonly supposed to be essentially marine are occasionally found inhabiting fresh water. In the inland fresh lakes of Newfoundland, seals, which never visit the sea, are common and breed freely. The same is the case in Lake Baikal, 1,280 feet above the sea-level, in Central Asia; and, though these facts bear but slightly on my present subject, seals being air-breathing Mammalia, yet in the broad mouth of the Amazon, far above the tidal influx of sea-water, marine mollusca and other kinds of life are found, and in some of the lakes in Sweden there are marine Crustacea. This may be easily accounted for in the same way that I now attempt to account for analogous peculiarities in the Old Red Sandstone. These Swedish lakes were submerged during the Glacial period, and remained as deep basins while the land was emerging, and, after its final emergence, the salt-waters of the lakes freshened so slowly that some of the creatures inhabiting them had time by degrees to adjust themselves to new and abnormal conditions.
In further illustration of the subject let us suppose a set of circumstances such as the following: By long-continued upheaval of the mouth of the Baltic (a process now going on), its waters, already brackish in the Gulfs of Bothnia and Finland, would eventually become fresh, and true lacustrine strata over that area would succeed and blend into the marine and brackish water-beds of earlier date. Something of this kind I conceive to have marked the transition from the Upper Silurian beds into the Old Red Sandstone. Again: if by changes in the physical geography of the area, of a continental kind, a portion of the Silurian sea got isolated from the main ocean, more or less like the Caspian and the Black Sea, then the ordinary marine conditions of the "passage-beds," accompanied by some of the life of the period, might be maintained for what, in common language, seems to us a long time. There is geological proof that the Black Sea was once united to the Caspian, the two forming one great brackish lake. Since they were disunited and the Bosporus opened, the Black Sea has, it may be inferred, been steadily freshening; and it is easy to conceive that, by the reclosing of the Bosporus (a comparatively small geographical change), it might in the course of time again be converted into a fresh lake. At present a great body of salt-water is constantly being poured out through the Bosporus, and its place taken by the fresh waters of the Danube and other rivers, while, owing to the uncongenial quality of the freshening sea, some of the Black-Sea shells are strangely distorted, as was shown by Edward Forbes.
Or, if we take the Caspian alone as an example, there we have a brackish inland sea which was once joined to the ocean, as proved by its molluscan fauna. Changes in physical geography have taken place of such a kind that the Caspian is now separated from the ocean, while its waters, gradually growing Salter by evaporation, are still inhabited by a poor and dwarfed marine molluscan fauna. If by increase of rainfall the Caspian became freshened, evaporation not being equal to the supply of water poured in by rivers, it would by-and-by, after reaching the point of overflow, be converted into a great fresh-water lake larger in extent than the whole area of Great Britain. Under these circumstances, in the Caspian area we should have a passage more or less gradual from marine to fresh-water conditions, such as I conceive to have marked the advent of the Old Red Sand-stone.
The total absence of marine shells, and the nature of the fossil fishes of the Old Red Sandstone, also help to prove its fresh-water origin, for we find the nearest living analogues of the fishes in the Polypterus of the rivers of Africa, the Ceratodus of Australia, and in less degree in the Lepidosteus of North America. In the upper beds of the formation there is distinct proof of fresh water, in shells of the genus Anodonta mingled with ferns and other land-plants.
One other sign of the inland character of these waters remains to be mentioned—I mean the red color of their strata. As a general rule, all the great ocean formations, such as the Silurian, Carboniferous Limestone and Jurassic series, are gray, blue, brown, yellow, or of some such color. The marls and sandstones of the Old Red series are red because each grain of sand or mud is incrusted with a thin pellicle of peroxide of iron. When this coloring-matter is discharged the rock becomes white, and the iron that induces the strong red color in the New Red Marl, which much resembles that of the Old Red series, is found to be under two per cent, of the whole. I cannot conceive how peroxide of iron could have been deposited from solution in a wide and deep sea by any possible process, but, if carbonate of iron were carried in solution into lakes, it might have been deposited as a peroxide through the oxidizing action of the air and the escape of the carbonic acid that held it in solution. It is well known that ferruginous mud and ores of iron are deposited in the lakes of Sweden at the present day. These are periodically dredged for economic purposes by the proprietors till the layer is exhausted, and after a sufficient interval they renew their dredging operations and new deposits are found. With a difference the case is somewhat analogous to the deposition of peroxide of iron that took place in the Old Red Sandstone waters. It is obvious that common pink mud might have been formed from the mechanical waste of red granite, gneiss, or other red rocks in which pink felspars are found, but such felspars are tinted all through with the coloring-matter, and such a tint is very different from the deep-red color that was produced by the encasing of each individual grain of sediment with a thin pellicle of peroxide of iron.
The proof that the Old Red Sandstone was deposited in inland lakes is strengthened by a similar case in well-known ancient inland sheets of water, as shown by the red marls of the Miocene lakes of Central France.
It is known that in Ireland and in Scotland the Old Red Sandstone consists of two divisions, upper and lower, the upper division lying quite unconformably on the lower. In South Wales there are symptoms of the same kind of unconformity, for the upper beds of the Old Red Sandstone gradually overlap the lower strata. But, on consideration, this last circumstance does not appear to present any real difficulty with regard to the physical conditions of the period. If the great hollow in which the Dead Sea lies were gradually to get filled with fresh water and silted up, 1,300 feet of strata might be added above the level of the present surface of the water, without taking into account the depth of the sea and the deposits that have already been formed; and the upper strata all round would overlap the lower, apparently much as the Old Red Sandstone strata do in Wales and the adjoining counties. If the Caspian and other parts of the Asiatic area of inland drainage got filled with water, the same general results would follow.
Neither does the decided unconformity between the Upper and Lower Old Red Sandstones both in Ireland and in Scotland present any insuperable difficulty as to the fresh-water origin of the strata. It indicates only great disturbance and denudation, and a long lapse of geological time unrepresented by strata between the disturbance and denudation of the older beds and the deposition of the newer. Here also we have a parallel case in times comparatively recent, for the fresh-water Miocene strata of Switzerland and the adjacent countries have been exceedingly disturbed, heaved up into mountains, and subjected to great denudation, while at a much later geological date—that of to-day—we have all the large fresh-water lakes that diversify the country north of the Alps in the same general area.
It is unnecessary to dilate on the well-known continental aspect of a large part of the Carboniferous strata which succeed the Old Red Sandstone, especially of the Coal-measures, which in the north of England and in Scotland are not confined to the upper parts of the series, but reach down among strata which elsewhere are only represented by the marine beds of the Carboniferous Limestone. The soils (underclays), forests, and peat-mosses of the period, now beds of coal; the sun-cracks, rain-pittings, bones, and footprints of Labyrinthodont Amphibia on mud now hardened into shale; the estuarine and fresh-water shells all point to vast marshes and great deltoid deposits, formed in a country which underwent many changes in its physical geography, and yet retained its identity throughout.
I will now discuss the conditions under which the British Permian strata were deposited. These rocks in their general characters very much resemble the Rothliegende, Kupferschiefer or Marl-slate, and Zechstein of the Thuringerwald and other parts of Germany, with this difference, that where the English Magnesian Limestone (Zechstein) is in force between Tynemouth and Nottingham, there are no red sandstones, marls, and conglomerates (Rothliegende), between the limestone and the Coal-measures, and in all the other parts of Britain where the red sandstones, etc., occur, there is only in two instances a little magnesian limestone lying, not at the top, but in the midst of, or interstratified with, the sandy and marly series.
The Permian marls, sandstones, conglomerates, and subangular breccias of Warwickshire, Staffordshire, Shropshire, Lancashire, North Wales, the Vale of Eden, and the south of Scotland, are all red, and, in fact, I nowhere recollect any important gray, yellow, or brown shales and sandstones among them. It is, however, foreign to my present purpose to discuss minor stratigraphical details, or any questions connected with English and Continental equivalent geological horizons of Permian age, nor is it necessary to do more than allude to the disturbances and denudations which preceded the unconformable deposition of our Permian strata, on all or any of the Palæozoic formations of earlier date. It is enough if I am able to show good reason for my belief that all of our Permian strata were deposited, not in the sea, but in the inland waters of lakes, which were probably mostly salt, but may possibly sometimes have been fresh or brackish.
As with the red strata of the Old Red Sandstone, so I consider that the red coloring-matter of the Permian sandstones and marls is due to the precipitation of peroxide of iron in a lake or lakes, in the manner already stated, and the nearly total absence of sea-shells, in by far the largest part of the areas occupied by the strata colored red, strongly points to this conclusion. There is other evidence bearing upon the question. The British plants of Permian age were mostly of genera common in the Coal-measures, though of different species. Among them there are Calamites and Lepidodendron, Walchia, Chondrites, Ullmania, Cardiocarpon, Alethopteris, Sphenopteris, Neuropteris, and many fragments of undetermined coniferous wood. This, however, forms no perfectly conclusive proof of the lacustrine origin of the strata, though it is not unlikely that land-plants, drifted by rivers, should have been water-logged and buried in the sediments of lakes.
The evidence derived from Reptilian remains more strongly points in the same direction. First we have the Labyrinthodont Amphibian, Dasyceps Bucklandi, from the Permian sandstones near Kenilworth; next the footprints mentioned by Prof. Harkness in the red sandstones of the Vale of Eden; and again, the numerous footprints in the sandstones of Corncockle Moor, in Dumfriesshire, long ago described by Sir William Jardine. All of these prints indicate that the Amphibia were accustomed to walk on damp surfaces of sand or mud open to the air, and the impressions left by their feet were afterward dried in the sun, before the waters flooded anew, overspread them with layers of sediment, in a manner that now annually takes place during the variations of the seasons on the broad flats of the Great Salt Lake of Utah and in other salt lakes. The occurrence of pseudomorphs of crystals of salt in the Permian beds of the Vale of Eden also helps to this conclusion, together with ripple-marks, sun-cracks, and rain-pittings, impressed on the beds. Crystals of common salt were not likely to have been deposited in an open sea, for, to form such crystals, concentration of chloride of sodium by evaporation is necessary. Deposits of gypsum, common in the Permian marls, could also only have been formed in inland waters by concentration, or on occasional surfaces of mud exposed to the sun and air, for no reasonable explanation can be offered of a process by means of which sulphate of lime can be deposited amid common mechanical sediments at the bottom of an open sea.
The question now arises how to account for the formation of the bands of magnesian limestone, sparingly intermingled with the red marls and sandstones of Lancashire and the Vale of Eden, and of that more important limestone district in the eastern half of the north of England, forming a long escarpment between Tynemouth and Nottingham. In these we have a true but restricted marine fauna, intermingled, however, with the relics of Amphibian and terrestrial life.
Let us broadly compare the marine life of the preceding epoch, that of the Carboniferous Limestone series, with the fossils of the Magnesian Limestone. The marine fauna of the Carboniferous Limestone of Britain contains about 1,500 species, most of which are mollusca (869), corals (124), echinodermata, crustacea (149), and fish (203). The Permian fauna feebly resembles that of the Carboniferous epoch, but, instead of the vast assemblage of many kinds of life found in the latter, the Magnesian Limestone of England only holds nine genera and 21 species of Brachiopoda, 16 genera and 31 species of Lamellibranchiata, 11 genera and 26 species of Gasteropoda, one Pteropod (Theca), and one Cephalopod (Nautilus). The whole comprises only 38 genera and 80 species, and all of these are dwarfed in size when compared with their Carboniferous congeners, when such there are.
I cannot easily account for this poverty of numbers and dwarfing of the forms, except on the hypothesis that the waters in which they lived were uncongenial to a true ocean fauna; and in this respect the general assemblage may be compared to the still more restricted marine faunas of the Caspian Sea and the Sea of Aral, or rather to that, a little more numerous and partly fossil, of the great Aralo-Caspian area of inland drainage, at a time when these inland brackish lakes formed part of a much larger body of water. Some of the fish of the Marl-slate have strong generic affinities with those of Carboniferous age, a number of which undoubtedly penetrated into the shallow estuarine lakes and salt lagoons of that period. Associated with the Permian mollusca we find the Labyrinthodont Amphibian Lepidotosaurus Duffii, together with Proterosaurus Speneri and P. Huxleyi, both of which were true Lacertilian land reptiles.
Besides the poverty of species and the small size of the Mollusca of the true Magnesian Limestone, the chemical composition of these strata seems to afford strong hints that they were formed in an inland salt lake, the sediments of which were partly deposited through the effect of solar evaporation. Broadly stated, the rock may be said to consist of a mixture of carbonate of lime and carbonate of magnesia in proportions more or less equal, mingled with a little silicious sand mechanically deposited. The solid dolomite still contains "about one-fifth per cent, of salts soluble in water, consisting of chlorides of sodium, magnesium, potassium, and calcium, and sulphate of lime. These must have been produced at the same time as the dolomite, and caught in some of the solution then present, which is thus indicated to have been of a briny character" (Sorby). But, instead of such deposits having been formed in open sea-water, I submit that this evidence, joined to the facts previously stated, leads me to believe that our Permian dolomite was formed in an inland salt lake, in which carbonates of lime and magnesia might have been deposited simultaneously. This deposition was chiefly the result of concentration of solutions caused by evaporation, the presence of carbonate of lime in the rock being partly due to organic agency, or the life and death of the molluscs that inhabit the waters. I cannot understand how deposits of carbonate of magnesia could have taken place in an open sea, where necessarily lime and magnesia only exist in solution in very small quantities in a vast bulk of water. In the open sea, indeed, the formation of all beds of limestone is produced simply by the secretion of carbonate of lime effected by molluscs, corals, and other organic agents, and I know of no animal that uses carbonate of magnesia to make its bones.
The very lithological character of some of the strata helps to lead to the same conclusion, for, when weathered, they are seen to consist of a number of thin layers curiously bent and convoluted, and approximately fitting into each other, like sheets of paper crumpled together, conveying the impression that they are somewhat tufaceous in character, or almost stalagmitic, if it be possible to suppose such deposits being formed under water. The curious concretionary and radiating structures common in the limestone are probably also connected with the chemical deposition of the sediments.
Arguments of the same kind apply to the magnesian limestones of Lancashire and the Vale of Eden, and the miserable marine fauna in some of these beds also indicates inland unhealthy waters, while the deposits of bedded gypsum so common in the marls of the series show that the latter could not have been deposited in the sea.
Taking all these circumstances into account, the poverty of the marine fauna, the terrestrial lizards, the Amphibia, and the land-plants, I cannot resist the conclusion that the Permian rocks of England were deposited in a lake or in a series of great inland continental lakes, brackish or salt; and, if this be true, it will equally apply to some other regions of Europe.
The strata that succeed the Permian formations in the geological scale are those included in the word Trias, on the continent of Europe. These consist of three subdivisions: first and lowest, the Bunter sandstone; second, the Muschelkalk; and third, the Keuper marl, or Marnes irisées. The Bunter sandstone on the Continent consists chiefly of red sandstones, with interstratified beds of red marl and thin bands of limestone, sometimes magnesian. These form the Grès bigarré of France. In these strata, near Strasbourg, about thirty species of land-plants are known, chiefly ferns, Calamites, Cycads, and Coniferæ, and with them remains of fish are found and Labyrinthodont Amphibia. In the same series there occur Lamellibranchiate marine mollusca of the genera Trigonia, Mya, Mytilus, and Posidonia, so few in number that they suggest the idea, not of the sea, but of an inland salt lake, especially when taken in connection with the Labyrinthodont Amphibia and the terrestrial plants.
The Muschelkalk, next in the series, is essentially marine. A partial submergence took place, and a large and varied fauna of Mesozoic type occupied the area previously covered by the lake deposits of the Bunter sandstone.
Above this comes the Keuper series, with Gypsum and dolomite, land-plants, fish, and Labyrinthodont remains, and a few genera and species of marine shells, again suggesting the idea of a set of conditions very different from those that prevailed when the Muschelkalk was formed.
These strata, as a whole, are the geological equivalents of the New Red Sandstone and Marl of England, with this difference—that the Muschelkalk is entirely absent in our country, and we only possess the New Red Sandstone (Bunter) and the New Red Marl (Keuper).
The kind of arguments already applied to part of the Permian strata may, with equal force, be used in relation to the New Red Sandstone and Marl of England. I have for long held that our New Red Sandstone was deposited in an inland lake, probably salt, and that our New Red Marl was certainly formed in a salt lake. Pseudo-morphous crystals of salt are common throughout the whole formation, which, besides, contains two great beds of rock-salt, each 80 or 100 feet thick, which could only have been deposited in a lake that had no outflow, and from which all the water poured into it by the rivers of the country was entirely got rid of by evaporation induced by solar heat. It has been proved by analyses that all spring and river waters contain chloride of sodium and other salts in solution, and in such a lake, by constant evaporation, salts must in time have become so concentrated that the water could hold no more in solution. This state of evaporation is now going on in the comparatively rainless areas of the Dead Sea, the Great Salt Lake of Utah, and in numerous lakes in Central Asia, though it is by no means asserted that in all of these positive deposition of salt has begun to take place. At length saline deposits began to be formed, which in the case of the New Keel Marl consisted chiefly of common salt. This is impossible in an ordinary ocean, for the salt in solution cannot there be sufficiently concentrated to permit of deposition.
Gypsum and other salts contained in the red marl may also have been formed in like manner, and, as in the Permian and Old Red formations, I consider that the peroxide of iron which stains both salt and marl may have been carried into the lakes in solution as carbonate of iron, to be afterward deposited as a peroxide.
The remains of plants found in the British Keuper beds also speak of a surrounding land, while the Crocodile (Stagonolepis), the Dinosauria (land reptiles), Lizards (one of them a true land lizard, Telerpeton), and six supposed species of Labyrinthodont Amphibia, all tell the same tale of land. Rain-prints and sun-cracks are not wanting to help in the argument; and while the fishes yield no conclusive proof, the well-known bivalve crustacean Estheria minuta might have lived in any kind of area occupied by salt-water, while the small Marsupial Mammal Microlestes antiquus speaks conclusively of land.
Taken as a whole, it seems to me that the nearest conception we can form, of part of the old continent in which the Permian and New Red strata were deposited, is, that it physically resembled the great area of inland drainage of Central Asia, in which, from the Caspian 3,000 miles to the eastward, almost all the lakes are salt in a region comparatively rainless, and in which the area occupied by inland salt or brackish waters was formerly much more extensive than at present.
And now let me endeavor to sum up the whole of the argument. If, as I believe, the Old Red Sandstone was deposited in a lake or lakes; if the Coal-measures, as witnessed by the great river-beds, estuarine shoals, and wide-spread terrestrial vegetation, show proof of a continental origin; if the Permian strata were formed in inland salt or brackish waters, and if the New Red beds had a similar origin—then from the close of the Uppermost Silurian formation down to the influx of the Rhœtic Sea, which brought the Keuper Marl period to an end, there existed over the north of Europe, and in other lands besides, a great continent throughout all that time, one main feature of which was the abundance of Reptilian and Amphibian life. This old continent was probably comparable in extent to any of the largest continents of the present day, and perhaps comparable in the length of its duration to all the time represented by all the Mesozoic strata from the close of the Triassic epoch down to the latest strata of the Chalk, and it may be even comprehending the additional time occupied in the formation of the Tertiary strata. But this latter part of the subject I propose to work out before long.
One other point remains. I have elsewhere attempted to prove, and the opinion is gaining ground in England, that this long continental epoch embraces at least two glacial episodes, as witnessed first by the bowlder-beds of the Old Red Sandstone of Scotland and the north of England, and secondly by the occurrence of similar deposits containing far-borne erratic blocks and ice-scratched stones, in a portion of that part of the Permian strata that is usually considered to represent the German Rothliegende. Should this be finally admitted, it may, on astronomical grounds, some day help us in the positive measurement of geological time.
Finally, let me rapidly pass in review what I think we know of later terrestrial, as opposed to marine epochs, in the British and neighboring areas of Europe. A wide-spred partial submergence brought the old continent to an end, and during the Liassic and Oolitic epoch (Jurassic) the Highlands of Scotland and other mountain-regions in the British Islands formed, with some other European Palæozoic rocks, groups of islands, round which, in warm seas, the Jurassic strata were deposited. These relics of an older continent, by deposition of newer strata and subsequent gradual upheaval, began to grow in extent, and at length formed the great continental area through which the mighty rivers flowed that deposited the strata of the Purbeck and Wealden series of England and the continent of Europe.
A larger submergence at length closed this broad local terrestrial epoch, and in those areas now occupied by Northern Europe (and much more besides), the sea, during the deposition of great part of the Chalk, attained a width and depth so great that probably only the tops of our British Palæozoic mountains stood above its level.
By subsequent elevation of the land, the fluvio-marine Eocene strata of Western Europe were formed, including in the term fluviomarine the whole English series, embracing the London Clay, which as shown by its plant-remains was deposited at, or not far from, the mouth of a great river, which in size, and in the manner of the occurrence of some of these plants, may be compared to the Ganges. With this latter continent there came in from some land, unknown as yet, a great and new terrestrial mammalian fauna wonderfully different from that which preceded it in Mesozoic times, and from that day to this the greater part of Europe has been essentially a continent, and in a large sense all its terrestrial faunas have been of modern type.
One shadowy continent still remains unnamed, far older than the oldest of those previously spoken of. What and where was the land from which the thick and wide deposits that form the Silurian strata of Europe were derived? For all sedimentary strata, however thick and extended in area, represent the degradation of an equal amount of older rocks wherewith to form them. Probably, like the America