if the elevatory force act at C with a considerably greater intensity than at M, it will communicate to the mass CC'NM, together with its general upward motion, a rotatory one, of which the axis will be horizontal and perpendicular to the transverse boundaries. This motion will tend to depress the extremity M, particularly if CM be of sufficient length. No such cause will exist in the adjoining mass AA'NM to lower its extremity N; and moreover it may be remarked, that this mass once elevated is more likely to be supported by the debris produced by a convulsive movement such as we are supposing, and therefore its extremity N will be less likely to subside than the adjoining extremity of the contiguous mass. From these causes it would seem highly probable that these two portions of the general mass should assume the relative positions above represented. A partial elevation and escarpment may thus be produced in accordance with the general fact stated in the Introduction, (IV. B. p. 7.)

We may also observe, that the fault thus formed at N must very generally possess the character mentioned in the Introduction, (1. y. p. 2.)

54. In the diagram, page 46, DEFG may represent a section parallel to the general axis of elevation of the portion of the mass which we have supposed, in the preceding article, to be subsequently elevated in a greater degree than the portions contiguous to it on either side, as represented in the diagram of the following page. If we conceive the portion also of which the section is FD'E'G' (p. 46.) to be raised in the same manner, it is obvious that a transverse valley will thus be formed between these two partial elevations, such as described in the Introduction. (v. p. 8.)

55. A section of one of our partial elevations above mentioned, by a vertical plane parallel to the axis of the general elevation and the longitudinal fissures, will now present an appearance (taking the phenomena as far as we have yet investigated them) similar to that of the annexed diagram, in which DEFG represents the portion of the mass defined by the same letters in the diagram of page 46. The broken

line edgh, supposed to be originally horizontal, indicates the faults along DE and FG. We may easily conceive, however, a further modification of the phenomena from any irregularity in the action of the elevating force, or in the resistance opposed to it, in adjoining portions of the mass on opposite sides of any one of the incomplete transverse

fissures, similar to that which we have assumed to produce the faults DE, FG, at complete fissures; for if this inequality of action on two such portions of the mass be sufficient, it may evidently convert the incomplete fissure into a complete one, provided the fissure extend near enough to the surface to weaken the mass so much as to render it unable to counteract the tendency of this unequal action, to give a greater elevation to the portion on one side of the fissure than to that on the other. In such case a fault would almost necessarily be produced, but probably smaller than that which would be produced by the same cause at a complete fissure. In either case, however, the fault may of course be of any magnitude, depending on the intensity of the action producing it.

If then we conceive the phenomena represented in the preceding diagram to be thus modified, and the superficial elevations to have been partially removed by denudation, the actual phenomena may be represented as in the annexed section. The broken line abcdefghi is as

h

before, supposed to have been originally continuous and horizontal, or

if the mass be stratified, to represent a line of stratification. cd and gh are faults; the differences of elevation ab, de, fg, are supposed too small to be so designated. Small relative elevations of this kind constitute what is frequently termed the throw of the vein. (Introd. II. . p. 4.)

56. It is important to observe the different effects which will be produced on the form of the longitudinal and transverse fissures by the movements above described. It has been shewn (Art. 38.) that a fissure immediately after its formation, and before any subsequent movement of the mass has taken place, must offer a certain approximation to uniformity of width; but an inspection of the diagram in page 51, will make it appear very evident, that this subsequent movement must in general destroy, in great measure, this character in the longitudinal fissures, since it must almost necessarily close them in some parts and open them considerably in others; while a movement similar to that described in Art. 53, and represented in the figure, page 54, will not necessarily produce any derangement in this respect in a perfectly uniform fissure, because the motion of one wall of the fissure is parallel, or nearly so, to the other. We should expect therefore, as a necessary consequence of this view of the subject, a much nearer approximation to uniformity of width in the transverse, than in the longitudinal fissures. This is strikingly in accordance with what has been stated in the Introduction (1. 0. p. 4.) a rule to which, I believe, there are comparatively few exceptions.

§. Proper signification of the term " System of Fissures".

Formation of Systems of Fissures.

-Simultaneous

57. I have hitherto spoken of systems of parallel fissures, as if the parallelism of the fissures constituted the essential characteristic of each system; and in the case we have been considering of an elevation of indefinite length, and of which the axis is rectilinear, this parallelism will characterize the two systems at right angles to each other, and which I have designated as longitudinal and transverse. If, however, the axis of the general elevation of indefinite length be not in a right line, the fissures of the longitudinal system (assuming them to be produced in

the manner I have indicated,) will be still parallel to this axis (in the sense in which one curve line may be said to be parallel to another) and every fissure of the transverse system will be perpendicular to each fissure of the former system at the points of their intersections, and consequently the fissures in this transverse system will not be parallel. Again, if we suppose the superficies of our elevated mass to be of finite length, and to be bounded for instance by a line approximating to the form of an elongated ellipse, the directions of the fissures in the transverse system, as we approach towards either extremity of the elevated range, will gradually change from perpendicularity with the major axis (the axis of elevation) till they become parallel to it, at the extremities of the ellipse, always preserving their approximate coincidence with the directions of the lines of greatest inclination of the general surface of the mass. The fissures of the other system will be approximately perpendicular to these lines. In this case then, the two systems will be no longer characterized by any constant relations which their directions bear to that of the axis of elevation, and therefore the terms longitudinal and transverse will cease to designate them so correctly as in other cases; and still more is this the case, where the elevation approximates to the conical form, in which all the fissures analogous to those we have termed transverse, diverge from the vertex of the cone. I have not, however, thought it necessary to supersede these terms by others, since they are very generally applicable with great propriety. It is highly important, however, as respects the application of this theory of elevation, to distinguish these two systems carefully from each other. It has been pointed out (Art. 56) how much the transverse fissures exceed the others in regularity of formation, and it seems not improbable, that this fact may be in some way connected with that of their containing mineral veins, so much more continuous than those found in the more irregular fissures of the other system, (Introd. II. 8. p. 3.) The most general rule will probably be, whatever be the form of the elevated mass, that the direction of a transverse fissure approximates to that of the dip of the strata, (supposing the mass stratified) the direction of a longitudinal one, consequently, approximating to that of the strike of the stratified beds. It should be observed, however, that the present form of the elevated mass may in some cases differ

materially from that which was originally given to it, by the movement to which the formation of the principal fissures must be referred. The rule would probably be more applicable immediately after this first elevation, than after the modifications in the position of the mass, which may possibly have been produced by subsequent ones.

It will be observed that the law of parallelism, which characterizes alike the phenomena of anticlinal lines, faults, mineral veins, &c., is to be traced, according to the view we are taking of the subject, to the same origin; viz. the formation of the two great systems of fissures, which have been shewn to be, under certain simple conditions, the necessary effects of the elevatory force to which they have been referred. The term parallelism, therefore, when used as characterizing systems of any of the above phenomena, must be equally regarded as subject in its interpretation to the exceptions or modifications pointed out in the last paragraph. In fact, if the extent of the mass be comparatively small, and its boundary irregular, this property would cease altogether to characterize the phenomena. If the elevated mass be of great superficial extent, partial irregularities in its boundary will have no appreciable effect on the directions of the fissures; and though two remote fissures of the same system might, in such case, (as appears from the preceding paragraph), be inclined at any angle to each other, any two adjoining fissures would in general be approximately parallel. The law of parallelism, however, in the strict acceptation of the term, could only hold through the whole extent of the elevated mass, in the case above considered of a rectilinear elevation of indefinite length. In other cases, the law must be subject to the modifications indicated above,

58. If the approximate accuracy of our assumptions be allowed, as applied to the crust of the globe, it appears, from our investigations, that an elevated range characterized by continuous systems of longitudinal and transverse fissures, referrible to the causes to which we have been assigning such phenomena, could not be produced by successive elevations of different points, by the partial action of an elevatory force. It has been shewn (Art. 46) that in such elevations