LYELL'S GEOLOGY!
Is would be difficult to name any other department of science which has undergone such decided improvement, within the last twenty years, as the very interesting branch of physical investigation which forms the subject of the volume before us. Until within a few years, this department of knowledge, which is so immediately connected with the mineral riches of our country, was mystified by the theories of a few dogmatists, who took upon themselves to decide that the various strata which compose the crust of the earth ought not to be investigated, without assuming that we must limit the antiquity of our planet to the wra supposed to be assigned by the Hebrew historian. It was taught ex cathedra, that Cosmogony was a necessary adjunct of Geology ; and but for * Principles of Geology; being an attempt to explain the former Changes of the Earth's Surface, by reference to causes now in operation. By C. Lyell, Esq. F.R.S. in volumes. Vol. I. London, 1830. the establishment of the London Geological Society, which res.- cued this important and beautiful field of research from the hands of schoolnien we should never to this day have witnessed the substitution of practical observation and the registry of facts, in lieu of crude hypotheses invented to support some preconceived theory. Mr. LYELL is evidently one of the new school ; and, so far as we have had an opportunity of looking over his first volume, a worthy as well as a distinguished disciple of a school of geology which has now taken the read throughout Europe. In order to arrive at practical utility—to point out the thick- ness and inclination of the different mineral strata, and thus pre- vent a vast sacrifice of capital and labour in mining operations, it is requisite that an immense number of observations should be made in situ, instead of determining, in the cabinet, as was form- erly the case, how the order of superposition and inclination ought to be in the bowels of the earth. It is this c011ection and compa- rison of facts and observations, which distinguishes the volume before us. In one respect the work is not put together in the best possible form ; it is somewhat too discursive for the geological stu- dent ; but it is perhaps more likely to become popular from that very defect. After an historical review of the progress of Geology down to the present period, and some just reflections on the inutility of in- dulgino.b in mere hypotheses concerning the original nucleus of our planet, or the formation of the lower strata, called primary rocks, the author observes— • " it would be idle to controvert, by reference to modern analogies, the conjectures of those who think they can ascend in their retrospect to the origin of our system. Let us, therefore, consider what changes the crust of the globe suffered after the consolidation of that ancient series of rocks to which we have adverted. Now, there is evidence that, before our secondary strata were formed, those of older date (from the old red sand- stone to the coal inclusive) were fractured and contorted, and often thrown into vertical positions. We cannot enter here into the geological details by which it is demonstrable, that at an epoch extremely remote, some parts of the carboniferous series were lifted above the level of the sea, others sunk to greater depths beneath it, and the former, being no longer protected by a covering of water, were partially destroyed by torrents and the waves of the sea, and supplied matter for newer horizontal beds. These were arranged on the truncated edges of the submarine portions of the more ancient series, and the fragments included in the more modern conglomerates still retain their fossil shells and corals, so as to enable us to determine the parent rocks from whence they were derived. t By such remodelling of the surface the small islands of the first period increased in size, and new land was introduced into northern regions, consisting partly of primary and volcanic rocks, and partly of the newly raised carboniferous strata. Among other proofs that earthquakes were then governed by the same laws which now regulate the subterranean forces, we find that they were restrained within limited areas, so that the site of Germany was not agitated while that of some parts of England was convulsed. The older rocks, therefore, remained in some cases undisturbed at the bottom of the ancient ocean, and in this case the strata of the succeeding epoch were de- posited upon them in conformable position. By referenceto groups largely developed on the continent, but which, are some of them entirely wanting, and others feebly represented in our own country,vve find that the apparent interruption in the chain of events between the formation of our coal and the has arises merely from local deficiency in the suite of geological monu-
ments. During the great interval which separated the formation of these groups, new species of animals and plants made their appearance, and in their turn became extinct ; volcanos broke out, and were at length exhausted ; rocks were destroyed in one region, and others accumulated elsewhere, while, in the mean time, the geographical condition of the
northern hemisphere suffered material modifications. Yet the sea still ex- tended over the greater part of the area now occupied by the lands which
we inhabit, and was even of considerable depth in many localities where our highest mountain-chains now rise. The vegetation, during a part at least of this new period (from the has to the chalk inclusive), appears to have approached to that of the larger islands of the equatorial zone. These islands appear to have been drained by rivers of considerable size, which were inhabited by crocodiles and gigantic oviparous reptiles, both herbivorous and carnivorous, belonging for the most part to extinct ge- nera. Of the contemporary inhabitants of the land, we have as yet ac- quired but scanty information ; but we know that there were flying rep- tiles, insects, and small insectivorous manunifera, allied to the opossum.
In further confirmation of the opinion that countries of considerable ex- tent now rose above the sea in the temperate zone, we may mention the
discovery of a large estuary formation in the south-west of England, of higher antiquity than the chalk, containing terrestrial plants and fresh- water testacea, tortoises, and large reptiles,—in a word, such an assem- blage as the Delta of the Ganges, or a large river in a hot climate, might be expected to produce."
Mr. LYELL is of opinion that a considerable lapse of ages inter- vened between the deposition of those rocks which constitute the secondary series, from the lower sandstone and mountain lime- stone to the more recent chalk, and the period when the tertiary beds were deposited; or else, that some grand convulsion of the earth took place, by which the relative position of the earth in its orbit became deranged, and its temperature materially altered. Whether these important changes were brought about through the instrumentality of a comet entering the earth's orbit, or by i• Thus, for example, on the banks of the Avon, in the Bristol coal.field, the dolor. mitic conglomerate, a rock of an age intermediate between the carboniferous series and the Has, rests on the truncated edges of the coal and mountain limestone, and contains rolled and angular fragments of the latter, in which are seen the character- istic mountain-limestone fossils. For accurate sections illustrating the disturb- ances which rocks of the carboniferous series underwent before the newer the, sand- stone was formed, the reader should consult the admirable memoir on the South- western Coal district ofEngland, by Dr. Buckland and Mr. Conybeare, Geol. Trans., vol. i., second series. r In many parts of Germany, the newer red sandstone, and other rocks of about the same age, lie in conformable strata on the coal. In some districts, as in the Thuringerwald, among others, there is an immense series of formations intervening between the coal and the lias; one of these groups, called the muschelkallestein, which seems to have no existence in England, is of great thickness and full of organic remains. See Professor Sedgwick's Memoir on the Geological relations and internal structure of the klagnesian Limestone, &c. Geol. Trans., second series, vol. Me part 1, p. 321. the force of volcanic agency, it is impossible to determine, and
unimportant to inquire. • Alluding to the generally-received opinion of the interior of the earth being still in a state of incandescence, and that the entire• strata may be considered only as a nut-shell or crust covering this mass of ignited matter, but which is supposed to undergo a gra- dual refrigeration in proportion to the distance from the centre, the author remarks-
" But, without entering into farther discussion on the merits of the hypothesis of gradual refrigeration, let us hope that experiments will con- tinue to be made, to ascertain whether there be internal heat in the globe, and what laws may govern its distribution. When its existence has been incontrovertibly established, it will be time to inquire whether it be sub- ject to secular variations. Should these also be confirmed, we may begin to indulge speculations respecting the cause, but let us not hastily assume that it has reference to the original formation of the planet, with which it might he as unconnected as with its final dissolution. ' In the mean time, we know that great changes in the external configuration of the earth's crust have at various times taken place, and we may affirm that they must have produced some effect on climate. The extent of their in- fluence ought, therefore, to form a primary object of inquiry, more especially as there seems an obvious coincidence between the wras at -which the principal accessions of land in high latitudes were made, and the successive periods when the diminution of temperature was most
decided."
Mr. LYELL differs also from preceding geologists with regard to the progressive order that has been supposed to exist in the organic remains found in the lower series of the secondary rocks, and those of the more recent strata. But we think the evidence lie brings forward is not sufficient to support the premises. The most interesting and most valuable portion of Mr. LYELL'S labours, is the great mass of facts he has collected together rela- tive to the changes that have taken place in remote ages, and are in still constant operation, through the agency of running water, in excavating valfies and transporting the alluvium to the mouths of large rivers ; and also by the destructive action of the ocean in carrying away large masses of land on certain portions of the coast, and depositing the materials in some adjacent district. Without the incontestible evidence which is here brought forward, we could hardly have credited to what extent changes on the earth's surface, and particularly on the sea-coast, have been brought about by the mechanical agency of running water. This part of the subject acquires peculiar interest in an island like Great Britain, where the work of marine dilapidation both on the eastern and the southern coast is going on to a very considerable extent. On the other hand, in many districts, from the joint action of currents in transporting and depositing alluvial soil, many large tracts of land have been, and many more might be wrested from the dominion of the sea, at the mouths of rivers and estuaries, if greater attention were paid to this important branch
of the science. •
But the extent of diluvial agency in this country bears a very slight proportion to the changes produced by the joint action Of water and volcanic agency in other parts of the world. With the following extract, connected with the changes going on in the val- lies of the great rivers in North America, we must conclude this very slight notice of a valuable book.
" The frequent fluctuations in the direction of river-courses, and the activity exerted by running water in various parts of the basin of the Mississippi, are partly, perhaps, to be ascribed to the cooperation of sub- terranean movements, which alter from time to time the relative levels of various parts of the surface. So late as the year 1812, the whole valley, from the mouth of the Ohio to that of the St. Francis, including a front of three hundred miles, was convulsed to such a degree, as to create new islands in the river, and lakes in the alluvial plain, some of which were twenty miles in extent. We shall allude to this event when we treat of earth- quakes, but may state here, that they happened exactly at the same time as the fatal convulsions of Caraccas; and the district shaken was nearly five degrees of latitude farther removed from the great centre of volcanic disturbance, than the basin of the Red River, to which we before alluded.* When countries are liable to be so extensively and permanently affected by earthquakes, speculations concerning changes in their hydrographical features must not be made without regard to the igneous as well as the aqueous causes of change. It is scarcely necessary to observe, that the inequalities produced even by one shock, might render the study of the alluvial plain of the Mississippi, at some future period, most perplexing to a geologist who should reason on the distribution of transported materials, without being aware that the configuration of the country had varied materially during the time when the excavating or removing power of the river was greatest. The region convulsed in 1812, of which New Madrid was the centre, exceeded in length the whole basin of the Thames, and the shocks were connected with active volcanos more distant from New Madrid than are the extinct craters of the Eyfel or of Auverme from London. If, therefore, during the innumerable eruptions which formerly broke forth in succession in the parts of Europe last alluded to, the basin of the principal river of our island was frequently agitated, and the rela- tive levels of its several parts altered (an hypothesis in perfect accordance with modern analogy), the difficulties of some theorists might, perhaps, be removed ; and they might no longer feel themselves under the necessity of resorting to catastrophes out of the ordinary course of Nature, when they endeavour to explain the alluvial phenomena of that district."
* Darby mentions beds of marine shells on the banks of Red River, which seem to indicate that Lower Louisiana is of recent formation : its elevation' perhaps above the sea, may have been due to the same series of earthquakes whichcontinue; to agitate equatorial America.
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