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referrible to some cause totally distinct from the action of extraneous forces on the general mass. This, however, forms no argument against our theory, as applied to those cases in which the phenomena present to us features entirely different from those just mentioned, and in perfect accordance with our theoretical deductions.
With the causes which may have superinduced the jointed structure in rocks, I have at present no concern, except so far as it might possibly be influenced by the action of extraneous forces. It has been shewn, however, (Art. 32), that such forces could only tend to produce systems of fissures crossing each other at right angles, whereas regular systems of joints appear to meet each other frequently at acute angles, and consequently, must necessarily have been owing to some different cause. I do not therefore conceive that any general tension of the mass produced by extension from elevation, or contraction in the course of solidification, can have had any material effect on the formation of joints. It is probably, I think, to be referred entirely to some kind of internal molecular action.
Though the law of approximate parallelism has long been recognized by geologists as characterizing mineral veins, faults, &c., I am not aware that any attempt has hitherto been made to deduce this important law from the causes to which these phenomena have been referred. In the preceding investigations, however, I have shewn, that under certain simple conditions, such a law is the necessary consequence of a general elevatory force acting in the manner I have supposed; and I have moreover shewn, that this law is entirely inconsistent with the partial action of such a force; because an elevatory force acting thus partially at a particular point, would necessarily produce fissures diverging from that point, so that in a general elevated range produced by the elevation of different portions in succession, there could be no general system of parallel fissures. This deduction appears to me perfectly conclusive as to the respective claims of two theories, one of which should assign the phenomena of elevation, in which the law of parallelism is observable, to the partial, and the other to the general action of an elevatory force, the terms general, and partial being taken in the sense in which I have heretofore used them, (see p. 1.) It must not, however, be supposed that our theory would lead us to the conclusion, that the whole elevation of any elevated range must have been communicated to it at once. It requires only that the first movement should have been general, and sufficient to produce at least the commencement of the systems of fissures, by which the range may subsequently be characterized, (Art. 58). Elevations, partial or general, may afterwards take place without producing other fissures following any law different from that of the preceding ones.
In the present state of geological theory, this deduction will not, I conceive, be deemed unimportant. It forms no part, however, of my present purpose to examine the merits of the different theories of elevation, which have been propounded by geologists; nor have I entered into these investigations in the spirit of advocacy of any peculiar and preconceived notions. My object has been simply to develope the necessary or probable consequences of certain definite hypothetical causes, and to compare them with those results which appear to be at present best established by observation; but, at the same time, leaving the theory of elevation founded upon our hypotheses, open to that refutation, or more complete verification, which must arise from the comparison of the results of more extended and accurate geological research with those of theory, deduced not by vague and indeterminate methods, from assumptions still more vague and indeterminate, but by accurate methods, from hypotheses the most simple and definite, which the nature of the subject will admit of.
In our own country the elevated range extending from Derbyshire to Northumberland, seems peculiarly calculated to afford us an opportunity of comparing the results of observation with those of the theory we have been investigating. On the slightest inspection of a map of this portion of the island, the direction of the central line of eleWol. VI. PART I. L
vation is indicated to us by the sources of the rivers, which pursue their courses from it respectively to the eastern and western coasts. This line appears to be almost straight, running nearly north from its southern extremity to the valley of the Eden, where the well defined ridge of Cross Fell commences, in a direction almost north-west and south-east. On the eastern side of this range, the different formations succeed each other with a general regularity in the order of their superposition, which would appear to indicate the absence of any comparatively irregular action of the elevatory forces in that region; and the existence of extensive mining and coal districts along this range, afford the surest means of ascertaining with accuracy the exact positions of the fissures and lines of dislocations which exist in it. Hitherto these phenomena have not, however, been made the objects of sufficiently careful examination, and if these observations should have the effect of leading to a more detailed investigation of them, one object of my entering into these researches will be accomplished. According to our theory the mineral veins in the southern part of the range above mentioned ought to run east and west, while in the Cross Fell part we should expect them to assume a direction more nearly north-east and south-west. From my own observation I have ascertained that in the mining district in Derbyshire, the phenomena are in this respect as well as in others strikingly accordant with theory, and I have reason to believe that in the coal district lying along the eastern boundary of that country they will be found so likewise. I hope, however, shortly to bring the details of this district under the notice of geologists.
The northern and southern portions of this range present us also with the important and interesting phenomena of extensive horizontal beds of trap, (the toadstone of Derbyshire, and the whinsill of the north) apparently interstratified with the sedimentary rocks with which they are associated. In the preceding investigations, I have entered with considerable detail into the subject of the formation of such beds, from the conviction that the notion of injection with reference to them has been carried by some geologists much too far, and that conclusions have been adopted without a due regard to the necessary effects on the contiguous beds, of that enormous hydrostatic pressure, which the process of injection of an extensive horizontal bed would necessarily call into action. That the toadstone of Derbyshire is not an injected bed, admits, I think, of the most indubitable proof from observation; and if the interstratification of the whinsill of the north, with comparatively thin beds of limestone and shale, be as regular as it is represented to be, I should have no hesitation in coming to the same conclusion with respect to that bed, for the reasons which have been heretofore mentioned, (Art. 77).
In the preceding investigations, I have spoken of the law of parallelism only as recognized in phenomena of faults, mineral veins, &c., comprized within narrow boundaries as compared with those to which it has been attempted to extend it, in the theory of Elie de Beaumont. It is very possible, however, that the physical causes to which I have referred this law, may have had a far more extensive operation than that I have ventured to assign to them. The parallelism of two mountain chains might thus be accounted for as simply as that of two neighbouring anticlinal lines; but it is obvious, that the more remote they should be from each other, the less would be the probability of the fissures to which our theory would refer them, belonging to the same system, and the less satisfactory would our solution become.
I have been anxious to avoid, for the present, any speculations respecting the interior constitution of our globe, beyond what is comprized in the simple assumptions on which these investigations have been founded; we may, however, include in those assumptions, the hypothesis of the elevatory forces having acted in different cases at different depths. The application of our theory, alluded to in the preceding paragraph, would perhaps require the hypothesis of these forces having acted at a much greater depth in such instances, than in those where the resulting phenomena are on a much smaller scale; and we may observe, that if the formation of the fissures should commence very far beneath the surface, it is extremely probable that very few would become complete fissures (see Art. 39), or would ever reach nearly to the surface, in comparison with those which would do so in cases where these fissures should originate at a much smaller depth. The complete fissures would consequently be distant from each other and very large, and all the phenomena of elevation resulting from them might be expected to be of proportionate magnitude. I have no intention, however, of insisting on this extended application of our theory, but merely to indicate its possible extension (should established geological facts appear hereafter to require it) to account for phenomena on a much larger scale than those to which I have considered it essential to refer in the preceding investigations.