صور الصفحة
PDF
النشر الإلكتروني

measure the hades), the angle DCB (Fig. 1.) between their intersections with the horizontal surface, and the length of the line CC", which evidently measures the throw of the unbroken vein AB, produced by the supposed movement. To express the horizontal displacement of the vein in terms of these quantities, suppose a sphere described with center C in the previous diagram (2), {or in the following one in which the same letters denote the same points as in (2)}, and any radius so as to

D

α δ

B

C"

form the spherical triangle abc, by its intersections with the planes of the veins and the horizontal plane. Let

α =

angle bac, the inclination of the plane DCC" of the broken vein to the horizon.

Babc, the inclination of the unbroken vein to the horizon.

[blocks in formation]

bc
h

DCB the angle between the intersections of the veins with

[blocks in formation]

Then shall we have

cot = cot a. sin ß cosec. 8+ cos ß. cot d;

and the apparent horizontal displacement C'C"

[merged small][ocr errors][merged small]

The quantities C'C", a, ẞ and can generally be obtained with very considerable accuracy, as may h also, when the mass in which the veins

are formed is distinctly stratified. In such cases therefore, by comparing our observed and computed values of C'C", we might obtain very accurate tests of the truth of the explanation which has been given of these phenomena.

66. The value of the explanation which has been given above of the phenomena we are now considering, consists in the substitution of vertical for horizontal movements, and therefore depends on the approximate verticality of the unbroken vein, parallel to the plane of which the motion is assumed to take place. It not unfrequently happens, however, that a horizontal displacement of a vertical vein takes place at the thin horizontal beds of moist clay, of which so considerable a number is found interstratified with the mountain limestone. The slimy nature of these beds undoubtedly affords a great facility for a relative movement of the masses respectively above and below them; and therefore where the displacement is small, there seems no difficulty in accounting for it on the supposition of this relative motion. In other cases a more probable cause may be found in the following considerations.

67. In the annexed figure let cd represent a thin stratum of clay,

[blocks in formation]

of such a nature as to give a considerable facility to a relative horizontal motion of the masses above and below it, and suppose a fissure to have been propagated upwards by the action of horizontal tensions, from D to C. If there were no cohesion whatever between the upper and lower divisions of the mass, it is manifest that the position of DC would not in any degree influence the position of a fissure C'E, which might be produced in the same manner and at the same time in the upper portion of the mass, and consequently the point C' would then

be determined by the constitution of the upper mass, or some circumstance not immediately depending on the position of DC. In such a case, therefore, there might be an apparent horizontal shift of any magnitude. If, however, a certain force, arising from cohesion and friction, should oppose a relative horizontal movement of the upper and lower portions of the mass, a limit will be imposed on the extent of the apparent shift, for it is obvious that this force must be called into action in the formation of C'E, (in the progressive formation upwards of the fissure) by the opposite motions of the upper surface of the lower mass, and lower one of the upper mass between C and C', and in no other part. Consequently, if the resistance at C' to the formation of a fissure in the upper mass, together with the lateral force just mentioned, be greater than the resistance to the continuation of the fissure from C towards E', the former fissure cannot be formed in preference to the latter, and thus a limit will be imposed on the distance CC'. It is easy, however, to conceive, from the known constitution of the beds which appear to give rise to phenomena of this kind, that this distance may be sufficient to account very easily for all such appearances of displacement as we are now considering.

68. If we conceive the figure in page 64 to represent a horizontal instead of a vertical section of the mass, and cd to represent a fissure, then, if a fissure DCC'E be propagated across it, it is manifest that considerations exactly similar to the above would enable us to account for the apparent displacement CC' in this as well as in the former case, and it appears highly probable that such appearances may have been not unfrequently thus produced. We may also observe, that if the fissure cd has not been completely filled, and its sides again cemented together, the movements of the masses on opposite sides of it will be in a certain degree independent of each other, so that a fissure DC propagated so as to meet cd at C, might be continued on the other side of cd, from a point C' quite remote from C. In such case DC would appear to terminate at C, and this, in fact, (DC being a small, and cd a large vein) is not of unfrequent occurrence.

69. There is also another manner somewhat different from the above, in which an apparent displacement of a fissure may be produced. It

[blocks in formation]

has been already shewn (Art. 17), that if a fissure in its progressive formation meet with any line of less resistance, it will under certain conditions be propagated along it for a certain distance, and then resume its original direction. If AB (Fig. 3, p. 61) be a line of less resistance, ECC"D' would represent a horizontal section of the fissure formed in the manner just supposed, and thus presenting the apparent displacement C'C".

It must be remarked, however, that an apparent displacement due to this cause must necessarily be such as represented in the figure just referred to, viz. on the side of the obtuse angle EC"C", or D'C"C", and not on that of the acute angle ec'c", or d'c'c' (Fig. 4, p. 61); and we may also observe, that neither this cause, nor that pointed out in the previous article, appear sufficient to account for the fact, which has been frequently recognized, of two or more adjoining veins being apparently displaced, or heaved, to the same extent and in the same direction by the same cross course. We see no reason why the apparent displacements of two such veins should be related in either of these particulars, when produced by the cause indicated in Art. 68; and if produced by that mentioned in the preceding paragraph, though the apparent displacements would necessarily be in the same direction, there seems to be no reason why they should be of the same extent. When the heaves, therefore, of adjoining veins appear to be related to each other both in extent and direction, the above two causes do not appear to offer an adequate explanation of the phenomena.

70. It was a notion first propagated, I believe, by Werner, and subsequently adopted by many other geologists and miners, that when two veins meet each other, of which one is heaved, and the other unbroken, the formation of the latter must necessarily have been posterior to that of the former. The theory of elevation, however, which we have been discussing, will not authorize this conclusion. If we assume the modes of producing apparent displacements considered in Arts. 68 and 69, it is evident that we must adopt a rule exactly the reverse of the one just stated; and if we suppose the displacements to be real, it is manifest from what has been advanced in this and

the previous section, that the formation of one or both of the fissures may have been either contemporaneous with, or anterior to that movement of the mass which produced these displacements; and consequently the existence of the heave in the one or the other of two intersecting veins, can afford no test of their relative ages. In cases, however, where several veins are found to have been heaved in the immediate vicinity of each other (as in some of the Cornish veins) indications may be obtained of their relative ages from the phenomena they exhibit, assuming them to have been produced in the manner just supposed.

71. It has been stated (Introd. n, p. 4), that the fissure of a vein is frequently almost entirely closed in passing through a thin stratum of clay. This fact may, I conceive, be easily accounted for from the greater extensibility, and less elasticity of this stratum, as compared with the masses with which it is interstratified. The former quality would allow it to remain unbroken, with an extension which the general mass could not but yield to, or if broken, it would from the latter property have little tendency to recede to its original extent.

72. It is not my intention to enter into any discussion on the mode in which the fissures of mineral veins have been filled*; but I would remark, that the frequent occurrence of the fact above mentioned seems equally unfavorable to the hypothesis of this process having taken place by superficial agency, or by any species of injection from beneath. The difficulty, however, assumes a far more formidable character when considered with reference to the toadstone of Derbyshire, which, as I have already stated (Introd. 11. n.), produces the same effect, in nearly destroying the continuity of the fissure, as the clay beds above mentioned. But in this case, instead of a bed of a few inches in thickness, we find a bed of toadstone of from ten to forty fathoms, through which the vein can sometimes be traced only by mere threads of calcareous spar. How then can we conceive the upper part of such a fissure

*I do not here allude merely to the process by which the mineral vein properly so called, (see p. 2.) has been deposited, but that by which the whole fissure may have been filled with the vein stuff which now occupies it. The fissure may be several feet wide, while the mineral vein is not an inch in width.

« السابقةمتابعة »