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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. i. 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 (1.6, p. 4.) a rule to which, I believe, there are comparatively few
W. Proper signification of the term “System of Fissures”—Simultaneous Formation of Systems of Fissures.
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. 3. 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
Wol. VI. PART I. H
fissures would necessarily diverge in all directions from the central points, so that parallel systems such as above mentioned could not possibly be thus produced. It has moreover been shewn, (Art. 30.) that every system of parallel fissures in which no two consecutive fissures are remote from each other, must necessarily have had one simultaneous origin. Subsequent efforts of the subterranean forces may enlarge these fissures, and propagate some of them to the surface, converting incomplete into complete fissures, but it would seem essential, according to our view of the subject, that their positions in the lower portion of the mass, where their formation will commence, (Art. 36.) should be determined contemporaneously.
§. Formation of Riders—Earplanation of the Phenomena at the Intersections of Mineral Weins.
59. If two systems of fissures were formed by forces acting in the manner we have supposed on a mass without vertical or nearly vertical planes of less resistance, these systems would present to us cases of intersection only of nearly vertical fissures with horizontal beds, or with other vertical fissures at right angles to the intersecting ones. It is manifest, however, that the existence of planes of less resistance, combined with an irregularity of intensity in the elevatory force such as we have assumed, may produce some fissures irregular both in direction and inclination to the horizon, though the general phenomena may still present that distinct approximation to the laws we have indicated, which would be the necessary consequence of the great predominance of general over local causes. It is at the intersections of the two perpendicular systems of veins (metalliferous veins and cross courses) that the most important of the phenomena we are about to consider are found, while others occur at the intersections of veins of more irregular formation.
60. Before we proceed to examine these phenomena more particularly, we may notice one probable consequence of this occasional irregularity in the formation of veins, viz., the production of what are usually termed riders. If a fissure be propagated through a point in which two planes of less resistance meet, it is very possible that it may be propagated simultaneously along these planes. These diverging branches may continue separate, and present themselves at the surface as two distinct fissures, or they may meet again, and thus including a portion of the mass in which they are formed, produce the phenomenon above mentioned. If the insulation be perfect, the mass, if not too large, will of course fall, and may descend to any unknown depth; and possibly this may be one cause of the partial irregularities in the width of the fissures of mineral veins. If the insulation be imperfect, or the width of the mass be greater than that of the fissure immediately beneath it, it will be supported in its original position, or it may under other circumstances lodge at a certain depth below it. In either case if such a mass come within the sphere of the miner's observation, he terms it a rider, (Introd. II. u).
If the rider be originally supported as above suggested, till a sufficient quantity of matter shall have been deposited in the fissure, to afford a support to it independent of its contact with the walls, and the fissure be then increased in width by any renewed action similar to that which originally produced it, the rider may present itself to us supported by the vein-stuff, in a state of perfect insulation from the solid mass on either side of the vein”.
61. In the phenomena attending the intersections of veins, described in the Introduction (II. o, t, p,) the broken veins are generally supposed to have been originally continuous, and to have been broken by a relative movement of the portions of the mass on opposite sides of the unbroken vein. Adopting this hypothesis, we have not the smallest difficulty in accounting for the appearance represented in the figures, p. 6. (Introd. II. p.) since our elevatory force must necessarily produce in many cases that relative elevation of different sides of a fissure, which at once accounts for the phenomena in question. The other two cases above
* This perfect insulation of riders has been recently urged as an objection of the most serious weight against the mechanical origin of veins. It appears to me, on the contrary, to be an almost necessary consequence of the causes we are considering acting on a mass constituted like the crust of the earth.