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I. Researches in Physical Geology. By WILLIAM HoPKINS, M.A., Fellow of the Cambridge Philosophical Society, and of the Geological Society, and Mathematical Lecturer of St Peter's College, Cambridge.
NotwitHSTANDING the appearances of irregularity and confusion in the formation of the crust of our globe which are presented to the eye in the contemplation of its external features, geologists have been able in numerous instances to detect, in the arrangement and position of its stratified masses, distinct approximations to geometrical laws. In the phenomena of anticlinal lines, faults, fissures, mineral veins, &c., such laws are easily recognized; and though, when we consider how large a portion of the surface of the earth remains geologically unexplored, it may appear premature to assert that these are perfectly general laws, yet, founding our reasoning on our knowledge, and not on our ignorance, and feeling that confidence which we are entitled to feel in the universality of the laws and operations of nature, we shall, I conceive, be justified, if not in the absolute conclusion, at least in the presumption, that the laws already observed in phenomena such as those above mentioned will be found, by the wider extension and increased accuracy of geological research, to be the approximative general laws of those phenomena.
If the legitimacy of this inference be allowed, we are necessarily led to the conclusion, that the phenomena alluded to are referrible not to the particular and irregular action of merely local causes, but to the more widely diffused action of some simple cause, general in its nature with respect to every part of the globe, and general in its action at least with respect to the whole of each district throughout Vol. VI. PART I. A
which the phenomena are observed to approximate, without interruption to the same geometrical laws. Between these phenomena and their actual causes necessary relations must of course exist; and in this paper I purpose to examine how far such relations do exist between our observed phenomena and a certain general cause to which they may be attributed. But in the first place it will be necessary to state distinctly the nature of the phenomena to which I refer, though without entering into more detail than may be necessary for my immediate object.
a. In districts where faults abound, two distinct systems are usually found, in each of which the faults approximate to parallelism” with each other.
£3. The common direction of one of these systems is approximately perpendicular to that of the other.
'y. The plane in which the dislocation at a fault has taken place is frequently somewhat inclined to the vertical; and it appears that the side of the fault on which the strata are most elevated, is more frequently that towards which the plane of dislocation inclines from a vertical through the lowest point of a section of the fault, by a vertical plane transverse to the plane of dislocation f.
II. Mineral Weins.
A distinct idea of a mineral vein is perhaps most easily formed by conceiving a vertical fissure, varying in width from a few inches to a few feet, to have been formed, extending downwards from the surface, and to have been subsequently filled up with matter in the midst of which the ore which properly constitutes the mineral vein f is deposited, sometimes in a regular vertical layer, and sometimes in irregular and detached masses. I shall therefore occasionally, without wishing to prejudge the question of the formation of veins, speak of the fissures in which they are deposited.
* This term must in certain cases be taken in a modified sense, as will be explained hereafter, whatever may be the phenomena to which it is applied.
+ See Encyclopedia Metropolitana, Art. Geology, p. 541.
† This is termed by miners in the Northern districts a Rake-Wein. In Cornwall the whole substance contained in the fissure is called a Lode.
a. The direction of the intersection of a vein with a horizontal plane usually approximates to rectilinearity. It is not meant that every short portion of this intersection forms a straight line, but, when considered with reference to its whole extent, these variations are not for the most part considerable.
6. In every mining district the largest and most important veins are divided into two distinct groups, in each of which a very decided approximation to parallelism is observable, and of which the directions are nearly perpendicular to each other.
y. When the veins occur in stratified masses, the direction of one of these systems usually coincides with that of the general dip of the strata, the other being consequently perpendicular to that direction*.
3. A large proportion of the most productive mineral veins are found in the former of these systems. The latter (frequently termed by the miner cross courses) carry ore very irregularly.
e. It seems doubtful whether any actual limits of a fissure containing a mineral vein were ever arrived at by the miner, though the division of a large fissure into several small ones not unfrequently seems to indicate a near approach to such a limit in the direction of its length. I know of no case, however, in which such indications have been observed of an approach to both extremities of a large vein. It is probable that their linear extent is frequently much greater than has yet been, or in many cases ever can be, observed. In numberless instances they have been traced for four or five miles in the mining districts of this country, and in some cases to the distance of eight or ten miles.
* I first observed this relation between the general direction of the mineral veins and that of the dip of the strata in the mining district of Derbyshire. I find on enquiry that the same relation holds in the Alston-moor district, and in Flintshire. In Cornwall also, when the lodes are in stratified rock, I apprehend this is generally the case, assuming the killas formation in the immediate vicinity of the granite to be stratified. -
Ç. Their depth appears to be uniformly greater than that to which man has been able to penetrate.
m. The width of the fissures in that system of the two above mentioned which contains the most productive and the most continuous mineral veins, varies in general from a few inches to about 12 feet. In the same vein the width will frequently vary, and sometimes suddenly along the same vertical line. In passing through a horizontal bed of clay the fissure will be sometimes almost entirely closed; and the toadstone of Derbyshire produces the same effect, frequently closing the fissure so effectually that it can only be traced through it by means of small ramifying veins of calcareous spar. The average width however does not appear at all to diminish as we descend”. The strata through which the fissure penetrates generally form well defined though uneven walls bounding it on either side, and perfectly firm and solid, except where the strata themselves cease to be so.
6. The width of the cross courses is frequently greater than that above stated, and generally much more irregular.
1. The fact of the strata in one wall of a fissure being higher than the same strata in the opposite one, has been recognized by all miners in some parts of almost every vein of consequence that has been explored, when existing in a distinctly stratified mass. This difference in general does not exceed a few feet, though it has not unfrequently been found to be many fathoms, in which case the vein of course coincides with a fault. This is sometimes termed by miners the throw t of the vein.
* In the mining district about Alston-moor there appears to be a few exceptions to this rule, as well as to the assertion of the preceding paragraph ((), in what are termed gash veins. These are comparatively wide at the top, and become gradually narrower as they descend, till they appear to terminate. (See Forster's account of this district, p. 186.) They are probably rents the formation of which began at the surface, but are hardly worthy of notice as exceptions to our general rules.
t A thron is in fact a small fault.