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k. The inclination of the plane of the fissure to a vertical plane, which is frequently termed by the miners of the more northern districts the hade of the vein, and by the Cornish miners its underlie, is very uncertain, amounting not unfrequently to as much perhaps as 20", generally, however, to considerably less, though in particular cases to considerably more. It will sometimes vary at different depths along the same vertical line, so that in some instances, when the hade is small, it will be in one direction in the upper, and in the contrary direction in the lower part of the vein. Upon the whole, however, the hade is not very great, and tolerably regular in each vein”. o
a. Masses of the adjoining rock, more or less perfectly detached from it, are frequently found imbedded in the matter which occupies the fissuref.
v. Apparent or real displacements in the position of a vein are frequently observed at its intersection with another vein, or with some particular stratified bed, which is generally found to be a bed of moist slimy clay. These intersections are of various kinds.
o. First, that of a vertical or nearly vertical vein, with a clay bed horizontal, or nearly so. The displacements in this case are shewn in the figures annexed, which represent vertical sections perpendicular to the plane of the vein.
It is manifest that here either the part of the vein above the stratum cd has been moved, or that below it, or both, if the two portions were ever in the same plane.
- * The underlie of the Cornish lodes is frequently greater, I conceive, than in our other mining districts. It may possibly also be more irregular. + These insulated masses are frequently termed by miners, Riders.
r. Secondly, we may have the intersection of two vertical veins, the planes of which are inclined to each other at any given angle. In such case it frequently happens, that while the continuity of one vein is preserved that of the other is broken, apparently by a relative displacement of the portions on opposite sides of the unbroken vein. This kind of displacement is exhibited in the annexed figures, which represent horizontal sections.
p. Thirdly, we may have the intersection of veins the planes of which are inclined, but at different angles, to a vertical plane. If such veins be near enough to each other, their intersection will take place sufficiently near the surface to be within the limits of observation, and if they meet the horizontal surface in parallel lines their line of intersection will be horizontal. If the subjoined figures represent vertical
sections perpendicular to this line, the displacements observed will be such as they exhibit.
These phenomena of faults, and mineral veins, are those which appear to approximate the most distinctly to well defined laws, and therefore afford the best means of testing the truth of any theory of elevation. The following phenomena also bear equally on the investigations contained in this paper, though their characters are in general much less distinct than those of the phenomena already cited.
III. Anticlinal and Synclinal Lines.
When two or more anticlinal lines, with the corresponding synclinal ones, are found in the same geological district”, their general directions frequently approximate to parallelism with each other+.
IV. Longitudinal Valleys.
a. Along the flanks of elevated ranges, longitudinal valleys are not unfrequently found running nearly parallel to the general axis of elevation f.
8. The partial elevations along the sides of an elevated range have usually these escarpments presented towards the central ridge|.
* I mean by a geological district, any tract of country throughout which the phenomena may be regarded as following the same laws without discontinuity.
+ If we take two planes coinciding at any proposed point of an anticlinal line, with the portions of the surface of a stratified bed on opposite sides of that line, these planes of stratification will intersect in a straight line not necessarily horizontal; and the direction of the anticlinal line at the proposed point will be determined by the azimuth of a vertical plane drawn through this intersection, or the direction of the intersection of this vertical plane with the horizon. Again, if through the proposed point we draw vertical planes respectively perpendicular to the two planes of stratification above mentioned, their respective intersections with them will be the lines of greatest inclination of the strata, and consequently the azimuths of these vertical planes will determine the directions of the dip. The angles between these two latter vertical planes, and the one before mentioned as determining the direction of the anticlinal line, will not generally be equal; they will become so only when the inclination of the planes of stratification on either side of the line is the same; i. e. the directions of the dip on opposite sides of an anticlinal line at any proposed point of it will not generally make equal angles with that of the line itself, unless the dip on opposite sides be the same. There is however an exception to this rule, when the direction of the dip on each side of the anticlinal line is perpendicular to it. This will occur when the two planes of stratification first mentioned intersect in a horizontal line.
f Saussure, Voyages dans les Alpes, Vol. I. Chap. x.
|| Traité de Geognosie, by D'Aubuisson, Vol. I, §. 24, p. 82.; and Saussure, Voyage dans
les Alpes, Vol. III. Chap. x. This rule is probably very general,
V. Transverse Valleys.
Deep valleys are sometimes found of which the directions are nearly at right angles to that of the general elevation*.
VI. Dykes or Weins, and Horizontal Beds of Trap.
a. The dykes are usually found in nearly vertical planes, and, when they occur in the vicinity of each other, with a general tendency to parallelism.
(3 Extensive beds of trap are found apparently interstratified with the stratified rocks.
VII. Granite Veins.
The form of a vein of this kind is frequently very different from that of mineral or trap-veins, as above described, inasmuch as a section of it does not generally approximate in the same degree to rectilinearity+.
These approximations to general laws have been, I believe, very generally recognized by geologists, and more especially in faults and mineral veins, in almost all cases in which these phenomena exist throughout districts of considerable eatent; and this appears unquestionably to justify the notion, that they are not to be referred to partial causes, but to some cause general at least with reference to the district throughout which the same laws are observed to hold without breach of continuity. Local and accidental causes may in some cases act with sufficient energy to obliterate all traces of general laws in phenomena such as those above mentioned; but still this will manifestly not invalidate our inference with respect to those districts in which such laws have been clearly recognized. We may moreover observe, that the law of approximate parallelism which equally characterizes the phenomena of anticlinal lines, faults, and mineral veins, affords, à priori, a strong probability that they are all assignable to the same general cause. We may also further remark, that if, with the previous conviction that the stratified beds have been deposited from water, and with a knowledge of the physical impossibility of beds of uniform thickness being so deposited except on planes but little inclined to the horizon, if, I say, under these circumstances, we examine many of the phenomena above mentioned, it seems impossible not to be struck with the idea of their being referrible to the action of some powerful elevatory force acting beneath the superficial crust of the globe, and thus producing those elevations and dislocations which we now witness. And, accordingly, such is the almost universal impression on the minds of geologists.
* These valleys may frequently be due in great measure to the effects of erosion. In some instances, however, they appear to have been obviously formed by the elevation of the strata on either side of them. The valley of the Wye, in Derbyshire, offers a beautiful example of this kind of formation.
t Trap veins sometimes assume the tortuous form of a granite vein. See M’Culloch's Description of the Western Islands of Scotland. Vol. III. Pl. xxxiii.
It appears, then, that we are arrived at that stage of geological science in which we are able to recognize certain well defined geological phenomena, distinctly approximating to geometrical laws; and we have also a distinct mechanical cause to which geologists, with almost one consent, have agreed in considering them to be assignable. The next step we are therefore called upon to take is obvious—it is to institute an investigation, founded on mechanical and physical principles, of the necessary relations which may exist between our observed phenomena and the general cause to which we attribute them. This investigation I have attempted, and now beg to lay it before the Society. I hope the nature of it will be deemed a justification of my introduction of a new term into the science, that of Physical Geology.
I have conducted the investigation by the methods supplied by mathematical analysis. I am aware, however, that to some persons the application of these methods to geological problems may appear like an affectation of an accuracy which the nature of the subject may not be conceived to admit of; but from this opinion I dissent entirely. We have, as I have before remarked, observed phenomena approximating to well-defined laws, and which we are prepared to regard as the effects of an assigned and definite cause; and to shew that this hypothetical
Vol. VI. PART I. B