« AnteriorContinuar »
the great and fundamental problem of theoretical geology is to assign adequate causes for the change of level which must have taken place. To solve this difficulty numerous and very plausible hypotheses have been proposed by very emi. nent men. It is in relation to this enquiry, that it becomes peculiarly interesting to observe the phænomena of the inclinations and contortions of different strata, and to endea, voar to examaine how far that position which they have resulted from original formation, and how far from subsequent convulsions of the earth, producing a derangement in the order in which its materials were disposed.
In conducting such an inquiry, one circumstance cannot fail to strike the observer; that is, when beds like the conglomerate before mentioned, recomposed as it were, from the fragments and detritus of older rocks, and which must have existed previously to their consolidation in the state of loose gravel, occur among vertical or highly inclined strata, we may conclude with absolute certainty that this inclined position cannot have been original, but must have resulted from subsequent disturbance : for it is obviously physically impossible to support an aggregation of loose gravel in verti. cal, or nearly vertical, planes. An argument of nearly similar force will apply, where among inclined strata, we find (as is often the case) thin beds, distinguished from the others by thin peculiar organic remains, interposed : for we cannot imagine any combination of circumstances under which (previously to the consolidation of the matrix containing them) the detached joints of encrinites, or the loose shells of testaceæ, or the scattered pinnulæ of ferns, should have disposed themselves in thin vertical layers.
Instances of this kind are bowever very frequent, and the action of some kind of disturbance or convulsion, is consequently clearly evinced. The same inference is also deducible from the consideration of what are called faults, a sort of dislocation, as it were, of a stratum : these are breaks or fissures cutting across a mass of strata aocompanied by a sinking or depression of the portion of that mass on one side of the break, often amounting to many hundred feet. These faults are commonly met with in our coal mines.
The phenomena of what are generically designated trap rocks, and of which the basaltic columns are a familiar instance, seem to prove in a manner almost equally clear with the last mentioned instances, the action of some powerful convulsive force. Their origin has been the subject of much dispute, and our authors, without expressing any judgment on the merits of the various arguments which have been ad
duced, content themselves by stating that the weight of geological authorities decidedly preponderates at present in favour of the igneous origin of these rocks. They seem to have been protruded, if not formed, by some powerful heaving up of the lower strata by volcanic agency.
Our authors examine the Wernerian hypothesis, of a gradual deposition of strata, from materials held in solution by the primitive ocean, but shew clearly that it is insufficient to account for the actual appearances which the obviously contorted and disturbed strata of different regions present.
The subject of the formation of vallies next occupies their attention. This process, they shew, must have been carried on by the agency of powerful aqueous carrents : by these means, in the majority of instances, must vallies have been entirely excavated, and in all cases greatly modified in their form, depth, &c. It is not, as they have clearly proved, to the operation of single streams that these effects can be attributed, but that large sheets of water sweeping over the face of an extensive tract at once, can alone account for the phenomena. Almost all the vallies of the weald of Kent, Surrey, and Sussex, present this combination of circumstances, as do many others of those which traverse the chalk range in various parts of the island : and a circuit of a few miles round Bristol alone affords no less than ten instances of the same kind, The denudation of strata in many places, and the occurrence of detached groups of the superior strata so washed off, are facts to be attributed to the same agency. The surfaces of the strata appear to have been exposed partially, at least, more than once to the action of these denuding causes : and even at very early periods, while many of the more recent beds were as yet only in the process of being deposited. Over all the strata the effects of a more recent process of this kind may be clearly traced. To this general covering of water, the name of Diluvium has been given. By this name it is intended to distinguish the effects of this more general revolution from those produced by more partial causes now in action, such as torrents, inundations, &c.; to the relies of these the name Alluvium has been applied. To the operation of the former cause, is attributed the formation of extensive beds of rolled gravel, composed of fragments from almost all rocks, as also the transportation of large insulated masses, often found at considerable distances from their parent mountains, and with the intervention of wide and deep vallies. The most rational explanation which geologists give of these appearances is, that the masses in question were transported by dilavian
action before it excavated the intervening vallies. Instances of this kind occur in the blocks of granite transported from the summits of the Alps to a high level on the opposite side of the valley of the lake of Geneva; and in similar insulated masses scattered over the plains of Germany, which may be traced up to the Scandinavian hills on the other side of the Baltic.
Into the further details of the arguments, founded on observation of the position and confirmation of strata, our present limits forbid us to enter. We will only observe, in general, that the authors of the work before us are constantly distinguished by their candour as well as clearness, in stating the various opinions which have been beld on these points, with out being unduly biassed for or against any particular system. They give a very perspicuous as well as comprehensive view of all the general appearances which the surface of our globe presents, which can in any way tend to throw light on the probable causes which have operated in producing its present condition. And in the exposition of those causes, and their probable mode of operation, these excellent writers have all along displayed the same judicious spirit of philosophizing, accompanied by many instructive and interesting remarks. In the introduction they give only a general account of the phenomena, reserving the particular instances to be described in the subsequent details of local geology to which they belong.
The most fruitful source of geological controversy, has been the question of the igneous, or aqueous origin of rocks. And whilst two parties have each exclusively maintained the operation of one cause, and others the operation of both in different instances, few seem sufficiently to have attended to the very close connexion which the investigations of the late Dr. Clarke bave shewn to subsist, between igneous and aqueous phenomena ; and the consequent probability, if noc certainty, of the joint action of both, in the production of the different formations. Volcanic action is not necessarily confined to an explosion of ignited matter : it often produces eruptions of mud and water, and is always preceded by an absorption of water from all neighbouring reservoirs : and we cannot help expressing our opinion that the curious spe: culations on the "gas, blow-pipe,” as they have already done much in advancing our knowledge of volcanic action, will ultimately tend to a much more complete explanation of geological phenomena than any of the theories hitherto proposed..
Our authors have with great propriety devoted the concluding part of their introduction to some remarks on the connexion subsisting between the deductions of geology and the truihs of natural and revealed religion. They introduce the subject with the following observations, which we consider excellent.
“ And here we cannot conclude this rapid sketch of the general bearings of geological science, without some allusion (imperfect as from our limits it must nécessarily be) to those highest interests which the eager attacks of an half-informed scepticism, and sometimes also the injudicious defences of those whose sincerity of intention ill supplied the want of a precise acquaintance with the phenomena under consideration, have seemed to involve in the discussions of this branch of physics. With respect to the former class, the characteristic to which we have just alluded, their impatience, namely, to avail themselves of the immature results of an inperfect knowledge, opposed as it is, in every respect, to that persevering and reflective spirit of enquiry which marks genuine philosophy, and can alone lead to the ultimate discovery of truth, inust create a reasonable suspicion of their opinions; for no sooner bas any new discovery, whatever might have been its subject, occurred, (whether it was a fragment of Indian chronology, or an Egyptian zodiac, or the mechanism of the universe, or that of living bodies ; or, lastly, some new fact relating to the structure of the earth) than the first aspect under which some minds have seemed anxious to view it has been, whether it would not furnish some new weapon against revelation. Whether such a mode of proceeding was more likely to arise from a genuine desire to remove prejudice and bigotry, or rather was itself the fruit of a prejudiced and bigoted eagerness to propagate peculiar opinions, we do not feel called upon to decide,"
Our authors then proceed to remark that the establishment of physical truths is not the proper province of revelation; whatever connection we may find between them can only be considered incidental, and is confined to the case of such single facts as happen to be mentioned in relation to the history of the divine dispensations to man, which it is the grand object of revelation to explain. However, then, such a connection may be discovered, in some few instances, it is to natural theology that the science of the earth's structure will contribute the most valuable arguments. In this department of religious enquiry, as our authors very justly observe,
..“ The great problem is to trace the Author of Nature, in his works, and our interest in the evidences thus furnished, is materially (as we have seen) kept alive by their being made the matter of gradual and successive discovery ; so that the mind is continually presented with fresh proofs, extending as its general knowledge extends."
This judicious remark, we could wish were more generally made, as a conviction of its truth could not fail to repress that basty spirit of unfounded generalization which is too commonly the parent of infidelity; which leads men to condemn the whole, because they misunderstand a part, and to think the proofs insufficient, because they have seen only a small portion of them. Our authors, after a few preliminary observations on this part of the subject, have given, as containing the best view of the argument with which they are acquainted, a long extract from Professor Buckland's valuable inaugural lecture. The proofs of design which the structure of the globe affords, though less obvious to ordinary notice than those exhibited by the animal and vegetable world, are, nevertheless, plainly discernible, and capable of demonstration. Among those we find adverted to, are in the first place the inclined position of the strata, by which a variety of soils and mineral treasures are afforded to different countries, as well as access to the latter facilitated. Next the mechanism of springs, and supply of water are considered. Another valuable contrivance is, that nearly all the materials of which the surface of the globe is composed, afford by their decomposition, a soil fit for the nourishment of vegetables.
In another point of view geology furnishes arguments to natural theology, in pointing out the existence of a period antecedent to the habitable state of the earth, and becoming familiarized with the idea of a beginning and first creation of the existences around us, the hypothesis of an eternal succession of causes is destroyed, the existence of a Creator established, and we receive from the previous proofs of design a more forcible conviction of the agency of an intelligent and all-powerful Being, the maker and upholder of all things.
After thus adverting to the connection between geology and natural religion, the progress of enquiry leads to similar remarks with respect to revelation. In relation to this part of the subject, only two points can be implicated in the discussions of Geology, the Noachian deluge, and the antiquity of the earth. With respect to the first of these topics the arguments afforded by Geology are most strong and decisive; though as in many kindred instances, arguments have been often brought forward by unskilful and injudicious defenders of the truth, grounded on facts which had not in reality any connection with the question. Those, however, which are afforded by a real and accurate acquaintance with the phenomena of the earth's surface, are here stated with considerable force and conciseness. The authors have again