extremity to the other, and therefore the transmitted pressure at one extremity would nearly equal that impressed at the other. On the contrary, if the space between the lines ab and a,b,, represent the section of the bed, it is manifest that the smallest number of straight lines which could be drawn entirely within this space, so as to form a continued but broken line between a and b, would be considerable, and that consequently the loss of transmitted pressure would be considerable. The magnitude of the impressed pressure at a is limited by the power belonging to the incumbent mass of resisting dislocation there; and when the loss of pressure by transmission is so great, that there is no longer sufficient force to cleave the mass into which the injected matter is penetrating, the horizontal injection will cease. I think it very probable that the limits thus imposed on the extent of possible injection, in the case of a thin bed like that just described, may be much narrower than some geologists seem to have conceived.

§. Effect of Joints in determining the Directions of Fissures.

I have stated (Introd. p. 11), that the investigations of Sect. I., are not to be considered as applicable to a mass in which the jointed structure should prevail generally, because the cohesion of the mass being in great measure, or altogether destroyed along the joints, the fissures resulting from any external force, would of course be formed along them. If, however, there should be two systems of joints existing previously to the action of the elevatory forces, in directions respectively parallel and perpendicular to the general axis of elevation, it is evident that the systems of fissures produced by this force, as well as all the phenomena resulting from them, would be exactly the same as those already described. If the direction of these systems should be only approximately parallel, and perpendicular to the axis of elevation, the same would still be true as respects the distinctive characters of longitudinal and transverse fissures, (see Art. 56). If, however, the directions of these two systems of joints should not have approximately these relations to that of the axis of elevation, or should not be nearly at right angles to each other, systems of fissures will result different from those which

we have already described as the consequence of a general elevatory force.

Since the existence of joints in rocks appears to be very general, it becomes a matter of interest to enquire what effect they may possibly have had in determining the positions of the lines of dislocation, which we at present observe in the crust of the globe, as already described. Our present limited knowledge of the extent of joints, horizontally and vertically, and of their relative directions, will not enable us to return any direct and definite answer to this enquiry. We may however, observe, (and the observation is important as respects the applicability of this theory) that in those districts where the directions of faults, mineral veins, cross courses, &c., bear those relations to a well defined axis of elevation, which would exist according to these theoretical views, and which observation, so far as it has proceeded, has shewn to hold very generally, it would appear absurd to assign those directions to the influence of joints, unless some cause can also be assigned why the elevatory force should act in such a manner as to give to the axis of elevation, a direction bearing a necessary relation to that of any previously existing system of joints. As it appears almost impossible to conceive any such cause, we may, I think, without hesitation, in the cases above-mentioned, reject the hypothesis of any extensive influence of a jointed structure upon the phenomena in question. Should a general coincidence be hereafter observed in the directions of joints, and those lines of dislocation which follow the laws before mentioned, it would seem far more probable, that the former had been influenced by the latter, than the latter by the former phenomena.

In asserting the generality of the laws above mentioned, it must not be supposed that we are assuming the absence of all exceptions, or that the directions of mineral veins may not, in some instances, have been determined by causes different from those we have been considering. This, I think, has been unquestionably the case in the veins or lodes of St Austle moor, in Cornwall, where we recognize systems of lodes forming acute angles with each other, and obviously

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 ele

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