those of Africa-in a word, in every quarter of the globe, were referred to one and the same period. The burden of proof was not supposed to rest with those who insisted on the identity in age of all these groups-their identity in mineral composition was thought sufficient. It was in vain to urge as an objection the improbability of the hypothesis which implies that all the moving waters on the globe were once simultaneously charged with sediment of a red colour.

But the rashness of pretending to identify, in age, all the red sandstones and marls in question, has at length been sufficiently exposed, by the discovery that, even in Europe, they belong decidedly to many different epochs. The investigations of De Verneuil in Spain have shown that the red sandstone and red marl, containing the rock-salt of Cardona in Catalonia, belong to the Middle Eocene or Nummulitic period. It is also known that certain red marls and variegated sandstones in Auvergne which are undistinguishable in mineral composition from the New Red Sandstone of English geologists, are nevertheless of the same older tertiary period : and, lastly, the gypseous red marl of Aix, in Provence, formerly supposed to be a marine secondary group, is now acknowledged to be a tertiary freshwater formation. In Nova Scotia one great deposit of red marl, sandstone, and gypsum, precisely resembling in mineral character the New Red' of England, occurs as a member of the Carboniferous group, and in the United States near the Falls of Niagara, a similar formation constitutes a subdivision of the Upper Silurian series.*

Nor was the nomenclature commonly adopted in geology without its influence in perpetuating the erroneous doctrine of universal formations. Such names, for example, as Chalk, Green Sand, Oolite, Red Marl, Coal, and others, were given to some of the principal fossiliferous groups in consequence of mineral peculiarities which happened to characterise them in the countries where they were first studied. When geologists had at length shown, by means of fossils and the order of superposition, that other strata, entirely dissimilar in colour, texture, and composition, were of contemporaneous

* See Lyell's Travels in N. America, ch. 2. and 25.

date, it was thought convenient still to retain the old names. That these were often inappropriate was admitted; but the student was taught to understand them in no other than a chronological sense; so that the Chalk might be a grey quartzose sandstone devoid of calcareous matter, as near Dresden, or a hard, compact, and sometimes flaggy limestone, as in parts of the Alps, or a brown sandstone or green marl, as in New Jersey, U. S. In like manner, the Green Sand, it was said, is often represented by limestone and other mineral masses entirely devoid of green grains. So the Oolitic texture was declared to be rather an exception than otherwise to the general rule in rocks of the Oolitic period, and to be found in strata both of older and newer date; and it often became necessary to affirm that no particle of carbonaceous matter could be detected in districts where the true Coal series abounded. In spite of every precaution the habitual use of this language could scarcely fail to instil into the mind of the pupil an idea that chalk, coal, salt, red marl, or the Oolitic structure were far more widely characteristic of the rocks of a given age than was really the case.

There is still another cause of deception, disposing us to ascribe a more limited range to the newer sedimentary formations as compared to the older, namely, the very general concealment of the newer strata beneath the waters of lakes and seas, and the wide exposure above waters of the more ancient. The Chalk, for example, now seen stretching for thousands of miles over different parts of Europe, has become visible to us by the effect, not of one, but of many distinct series of subterranean movements. Time has been required, and a succession of geological periods, to raise it above the waves in so many regions; and if calcareous rocks of the middle and upper tertiary periods have been formed, as homogeneous in mineral composition throughout equally extensive regions, it may require convulsions as numerous as all those which have occurred since the origin of the Chalk to bring them up within the sphere of human observation. Hence the rocks of more modern periods may appear partial, as compared to those of remoter eras, not because of any original inferiority in their extent, but because there has not

been sufficient time since their origin for the development of a great series of elevatory movements.

In regard, however, to one of the most important characteristics of sedimentary rocks, their organic remains, many naturalists of high authority have maintained that the same species of fossils are more widely distributed through formations of high antiquity than in those of more modern date, and that distinct zoological and botanical provinces, as they are called, which form so striking a feature in the living creation, were not established at remote eras. Thus the plants of the Coal, the shells, and trilobites of the Silurian rocks, and the ammonites of the Oolite, have been supposed to have a wider geographical range than any living species of plants, crustaceans, or mollusks. This opinion seems in certain cases to be well founded, especially in relation to the plants of the Carboniferous epoch, owing partly to greater uniformity of climate, and partly, as Professor Heer has suggested, to the fact that almost all the plants-including even large treesof that period, were cryptogamous: so that their minute spores might be carried by the wind for indefinite distances, as are now the spores of ferns, mosses, and lichens. But a recent comparison of the fossils of North American rocks with those of corresponding ages in the European series, has proved that the terrestrial vegetation of the Carboniferous epoch is an exception to the general rule, and that the fauna and flora of the earth at successive periods, from the oldest Silurian to the newest Tertiary, was as diversified as now. The shells, corals, and other classes of organic remains demonstrate the fact that the earth might then have been divided into separate zoological provinces, in a manner analogous to that observed in the geographical distribution of species now living.

117

CHAPTER VII.

ON THE SUPPOSED FORMER INTENSITY OF THE IGNEOUS

FORCES.

VOLCANIC ACTION AT SUCCESSIVE GEOLOGICAL PERIODS-PLUTONIC ROCKS OF DIFFERENT AGES GRADUAL DEVELOPMENT OF SUBTERRANEAN MOVEMENTSFAULTS-DOCTRINE OF THE SUDDEN UPHEAVAL OF PARALLEL MOUNTAINCHAINS OBJECTIONS TO THE PROOF OF THE SUDDENNESS OF THE UPHEAVAL, AND THE CONTEMPORANEOUSNESS OF PARALLEL CHAINS-TRAINS OF ACTIVE VOLCANOS NOT PARALLEL-AS LARGE TRACTS OF LAND ARE RISING OR SINKING SLOWLY, SO NARROW ZONES OF LAND MAY BE PUSHED UP GRADUALLY TO GREAT HEIGHTS-BENDING OF STRATA BY LATERAL PRESSURE-ADEQUACY OF THE VOLCANIC POWER TO EFFECT THIS WITHOUT PAROXYSMAL CONVULSIONS.

WHEN reasoning on the intensity of volcanic action at former periods, as well as on the power of moving water, geologists have been ever prone to represent Nature as having been prodigal of violence and parsimonious of time. Now, although it is less easy to determine the relative ages of the volcanic than of the fossiliferous formations, it is undeniable that igneous rocks have been produced at all geological periods, or as often as we find distinct deposits marked by peculiar animal and vegetable remains. It can be shown that rocks commonly called trappean have been injected into fissures, and ejected at the surface, both before and during the deposition of the Laurentian, Cambrian, Silurian, and Carboniferous series (see Table, page 139), and at the time when the Permian or Magnesian Limestone, and when the Upper New Red Sandstone were formed, or when the Lias, Oolite, Green Sand, Chalk, and the several tertiary groups newer than the chalk, originated in succession. Nor is this all; distinct volcanic products may be referred to the subordinate divisions of each period, such as the Carboniferous, as in the county of Fife, in Scotland, where certain masses of contemporaneous trap are associated

with the Lower, others with the Upper Coal-measures. And if one of these masses is more minutely examined, we find it to consist of the products of a great many successive outbursts, by which scoria and lava were again and again emitted, and afterwards consolidated, then fissured, and finally traversed by melted matter constituting what are called dykes.* As we enlarge, therefore, our knowledge of the ancient rocks formed by subterranean heat, we find ourselves compelled to regard them as the aggregate effects of innumerable eruptions, each of which may have been comparable in violence to those now experienced in volcanic regions.

It may indeed be said that we have as yet no data for estimating the relative volume of matter simultaneously in a state of fusion at two given periods, as if we were to compare the columnar basalt of Staffa and its environs with the lava poured out in Iceland in 1783; but for this very reason it would be rash and unphilosophical to assume an excess of ancient as contrasted with modern outpourings of melted matter at particular periods of time. It would be still more presumptuous to take for granted that the more deep-seated effects of subterranean heat surpassed at remote eras the corresponding effects of internal heat in our own times. Certain porphyries and granites, and all the rocks commonly called plutonic, are now generally supposed to have resulted from the slow cooling of materials fused and solidified under great pressure; and we cannot doubt that beneath existing volcanos there are large spaces filled with melted stone, which must for centuries remain in an incandescent state, and then cool and become hard and crystalline when the subterranean heat shall be exhausted. That lakes of lava are continuous for hundreds of miles beneath the Chilian Andes, seems established by observations made in the year 1835.‡

Now, wherever the fluid contents of such reservoirs are poured out successively from craters in the open air, or at the bottom of the sea, the matter so ejected may afford evidence by its arrangement of having originated at different

* See Elements of Geology, 6th ed. chap. 30 to 32, inclusive.

See below, Icelandic eruptions,

ch. 27.

28.

See below, Chilian earthquake, ch.