ON THE BENDING OF BEDS NEAR VEINS.

BY DAVID BURNS, C.E., F.G.S.

The origin of mineral veins, and the sources and method of arrangement of their contents, are still subjects shrouded in considerable obscurity. The present paper is an effort to glean from the strata in which veins occur some fragments of knowledge of the physical conditions under which they were formed. When veins traverse beds of varying resistance and of moderate thickness, as they frequently do in the North of England, the strata are thrust upwards or downwards, more or less, on one or both sides, as they approach the vein, and especially in its immediate vicinity. The well-known alternations of the north country Lead-measures allow such disturbances to be accurately noted, although, unfortunately, not being of commercial importance, there is not as much exact knowledge of the behaviour of strata, near to even well-known veins, as might be desirable; but there is sufficient for a general treatment of the subject, and if once its importance is fully appreciated, accurate observations will multiply.

Fig. 1, Plate VI., is an instance of disturbance of a very simple character. The strata are only disturbed very locally, and that along the line of the fault. There has been upward fluid pressure of a very contracted but intense character, which has been able to force itself through the strata and so to escape. Such a vein is an elongated crater, and the mineral left in it but the sweepings of a chimney, so to speak, that once reeked with riches. The formation of the arch might give a little relief laterally to subsiding strata, and might be, to some extent, a wrinkle caused by increasing horizontal thrust. Nor is this a condition to be ignored. If a stratum be taken 100 miles long, lying perfectly horizontal, and suppose it to be depressed one mile-a small change of level in comparison with those frequently assumed by geologists-and the radius of the earth being reckoned at 3,962 miles, then 3,962 miles 3,961 miles :: 100 miles: 99-974 miles; that is, this length of strata will have 026 miles or 45 yards less room in which to lie, and it must, therefore, be inclined at an angle of 1° 17', or be crumpled up to the extent of 45 yards per 100 miles. There can be little doubt that the shrinking of the earth's radius, and the dip of sedimentary strata, are mutually accommodating conditions. If an accurate horizontal section of any particular sedimentary bed in, say, the Carboniferous formation, could be made in any direct line for 100 miles, and the total length of the pieces so measured were taken, then their sum would exceed 100 miles, in the same proportion as the diameter of the earth, at the close of the Carboniferous period, exceeded its present diameter. This estimate would probably be under the truth, owing to increasing density of the rock with time and pressure, and, it might be, to the secular cooling of even the most superficial strata of the earth, since Carboniferous times.

Fig. 2, Plate VI., represents the cross section of a fault, showing what will usually be considered the normal hade and the usual flexure of the strata. The

foundations of B are supposed to have given way somehow while A stood fast, and so a break occurred, and the B section slid down along the face of the A section, dragging down, by sheer friction, the edges of the strata a, b, c, on the A side, and dragging up, by the same agency, those strata on the B side. Such is the aspect most frequently presented by faults, and such is the explanation put forth. Now, if the strata B were free to move horizontally in the direction C, this explanation might be considered fairly satisfactory; but they are not. As already seen, the strata, by getting nearer the earth's centre, get into more contracted quarters, but, ignoring this circumstance, as being of little effect in the case of faults of moderate throw, the hade makes a vertical motion of the strata B impossible. Indeed, if B were to fall down vertically, simply by reason of its weight, the hade of the fault would be instantly reversed, and a fault like that shown in Fig. 5, Plate VI., would be the consequence, the rising beds in A, or the falling beds in B, finding relief from horizontal thrust by sliding on each other in the manner indicated, it will be noticed that each stratum a, b, and c overlaps its continuation on the other side of the fault, that is, a and a, b and b', c and c' severally overlap each other. But, instead of supposing B to fall down vertically, let us suppose it continues to be supported at the point C (Fig. 2, Plate VI.). A is stationary as before, and now, if the foundations of B give way, the strata B will fall down, revolving roughly round a centre, C, and tracing ares to which the fault will be tangential. According to this view, the mass B would, at the time the fault was forming, partake of the nature of a floating body, and of the descending ram of an hydraulic engine. The yielding fluid under B need not have escaped through the fault shown. It may have been forced from one underground chamber to another, and hence to the day. The writer has frequently found, and believes it to be a rule to which there are but few exceptions, that whereas on one side of a considerable fault the strata are pretty sound, on the other side they are broken up by minor and often parallel faults, and otherwise bent and distorted. The side so broken is the one that has tottered on the abyss of chaos, and the one within which metalliferous riches may be most confidently expected.

But although the hade shown in Fig. 2, Plate VI., is general, it is not the invariable rule, as the hade is sometimes observed reversed as in Fig. 5, Plate VI. Indeed, the writer believes the latter figure represents more correctly than any of the others a fault pure and simple, namely, a dislocation between two neighbouring areas, caused by a difference of upward pressure in these areas giving rise to a relatively vertical motion, as influenced by the disturbing forces, and a relatively horizontal motion, to make room for the fault, and it might be horizontal thrust experienced independent of the disturbance which gave rise to the fault.

But there is still another variety of vein which, as far as I am aware, has been a puzzle to geologists up to the present time. It is shown in Fig. 3, Plate VI. The peculiarity is that on the downthrow side the beds dip to the fault, and on the upthrow side rise to the fault. This clearly could not be the result of friction, for as has been already seen, the rubbing causes flexures in the opposite directions. It has been undeniably due to intense local pressure. Let us suppose a crystalline

stratum as c in Fig. 4, Plate VI., and that it is assailed from below by intense fluid pressure. This fluid ultimately breaks e across, and the probability is that the fracture will not be quite vertical. Let us suppose it takes the direction of the line om. The fluid then insinuates itself into the crack, and as all fluid pressure is normal to the surface on which it acts, forces, represented by the arrows, come into play, and as a result the stratum on the side A is thrust up, and on B it is thrust down into the form shown by the dotted lines in the figure. If now we suppose the crystalline bed c to be followed by a thinly laminated bed of shale (b) this would naturally tear in the direction of pressure put upon it by the upward rising edge of c, namely, in the line ms, and each lamina would communicate its pressure to the next, a little to the left of the point at which it received it. Probably at this stage the shale on the B side would also be thrust upwards against the next resisting bed; but no sooner would the fluid reach the next strong and resisting stratum a than a fracture would be created in somewhat the direction mo, and the same sequence of events would be repeated, a would now thrust down the soft and yielding shale, thereby breaking it up and forming the "donk," which now characterises veins within it, and going far to close the vein at this point.

The succession of strata put in the diagrams is that most usual in the Leadmeasures of the north, and it explains several oft-observed features in the veins therein.

If this explanation be a true one it becomes no mystery why veins sometimes take a run along the top of a hard bed, as along the top of c towards B, as the crushing down of e would almost invite it in that direction. But probably the rush of the fluid through these strata was nearly instantaneous, and in the majority of cases the flow would continue in the main in the direction once instituted.

It is found in these veins that they are nearest the vertical in the hard beds, as limestone and sandstone, and nearest the horizontal in the shales. This is what would naturally occur in a vein as supposed. In repeated attempts to shear sealingwax by applying, as it were, force upwards at A and downwards at B, the writer found that in at least two-thirds of the cases the fracture inclined slightly in the direction of mo, thus tending to confirm this hypothesis, and refute the theory that the hade and throw of veins is due to a direct thrust upwards or downwards on either hand. The pressure on the walls of these veins would practically be uniform throughout, and hence the shales would be subjected to the same pressure as the limestones. But it would be the more crystalline rocks that would present the distinct sloping fracture necessary to give rise to the twisting action, and hence there would be a tendency for each hard bed to crush back the yielding shale below it, and, to a certain extent, close the vein at that point, the fluid contents readily accommodating themselves, even when under enormous pressure, to the various resistances of the different strata. This is one reason at least why in the limestone we find the veins widest and the minerals most plentiful, and in the shales the veins narrowest and poorest. Whereas the fault with the reversed hade is the true fault, so the vein in the strata with reversed bends is the truest mineral vein.

The writer has noticed in plotting horizontal sections that much of the reported throw of veins was compensated for by the flexures in the strata, and that, taking two points some distance away from the vein on either side, the strata were in the same plane, either level or that of the ordinary dip of the country, as they would have been had there been no veins at all.

That molten or gaseous minerals have escaped to be condensed in the air, or in an overlying ocean, seems to be proved by the deposits worked at Commern in Rhenish Prussia, and at other points in Germany and elsewhere.

The presence of galena and other mineral matter disseminated through the rocks has been considered evidence that veins have derived their contents by infiltration from the country rock, but it may with equal probability be taken as evidence of the escape of mineral matter at the time of the formation of some other vein and of the rocks in question.

In the thick limestones of the North of England horizontal layers of mineral occasionally lie alongside of the veins, and are known as "flats." These would most naturally occur on the downthrow side of such veins as have the reversed flexure, as on that side the partings between the posts of the limestone would be opened and more likely to catch the upward rush of mineral.

The peculiar value of the phenomena presented in Fig. 3, Plate VI., is that there is no ambiguity as to the cause of the bending of the strata. In most speculations as to the origin of veins, we have generally two or more theories which account with almost equal probability for them. In this case, however, there is a series of veins that were formed by a fluid of some kind bursting upward under immense pressure, and which could not have been formed in any other way. There may have been in many cases subsequent dislocations along this line of fracture altering the throw and complicating the phenomena, but where the flexure shown in Fig. 3, Plate VI., now exists, or can be shown to have existed, the vein has been formed by the upward intrusion through the strata of some fluid substance under enormous pressure. Whether the fluid was a gas or a liquid the writer is aware of no evidence to show. But one or other, or both, must have been there. Moreover, these fluids have not been free to rush to the outer air or ocean as they probably were in the cases illustrated by Fig. 1, Plate VI., but have been confined so long as to distort the strata, as shown in Fig. 3, Plate VI., and seal them in that position. They may have escaped later and been replaced by other mineral matter. But since there is conclusive evidence of foreign matter having been thrust up through the strata, and that there is found in these veins such matter distributed very much as would be expected from the causes shown, it is difficult to avoid the conclusion that in these veins, at least, the theory of sublimation is absolutely proved.

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Professor LEBOUR said there was one point on which he would like to ask Mr. Burns's opinion; it was in regard to the dip of beds towards "necks" or traps." The same thing occurred in the case of whin dykes, the dip being towards them, and

not away from them as one might have imagined if the veins had been formed by the rising of liquid matter from below. This was not the same thing as Mr. Burns referred to; he mentioned veins filled in a different manner, but this applied to faults filled with igneous rocks, and in that case it was imagined that the bending towards the igneous matter was caused by the dragging down, or cooling, or probably after the intrusion of the matter. It was interesting to point out that the first time this was noticed in print was by Mr. Arthur Balfour, in the Geological Magazine.

Mr. D. BURNS said he was not previously aware of this phenomenon; he had never noticed it, and was rather at a loss to account for it. It occurred to him, on the spur of the moment, that it might have been caused by the upheaval of the whole country, so to speak, the stratified beds only being upheaved while the others remained stationary. If the whole country was being raised up, a trap resting on a foundation which was not raised at all might cause this bending.

Mr. THOS. BELL (H.M. Inspector of Mines)—In what part of the country do you find that dipping of the seams?

Professor LEBOUR said the particular neck he was speaking of was on the Haddingtonshire coast near Dunbar; but he had seen the phenomenon at whin dykes in a few places in Northumberland. For instance, in the three dykes near the North Tyne up the Borswick Burn it is shown very distinctly.

Mr. THOS. BELL said he had traced the whin dyke from Shotton Colliery as far as Middleton-in-Teesdale, and he had not found anything of the kind indicated by Prof. Lebour. He had worked up to it in perhaps a dozen places at Thornley, Cassop, Shincliffe, and Houghall, and in all cases the beds were perfectly level.

Mr. BAILES said that at North Fenham Colliery there was a distinct dip of the beds on each side of the whin dyke, and this dip to the dyke was very distinctly visible.

The CHAIRMAN proposed a vote of thanks to Mr. Burns for his very interesting paper. Without altogether agreeing with all the views expressed, and which were no doubt perfectly correct as regards the lead-mining districts, he thought they must look in the coal-mining district for a different explanation. As a general rule their faults were like Fig. 3 on Plate VI., rising to a dip trouble and dipping to a rise trouble, and as the faults were sometimes "risers" and sometimes "dippers," they must look for some more general disturbance than a fluid acting in the fissure. Mr. J. B. SIMPSON seconded the vote of thanks, and it was cordially adopted. Mr. D. BURNS acknowledged the compliment, and thanked the members for the very patient hearing which they had given him. He was sorry the paper did not cover the whole of the ground; there were still many points to be considered.

The following paper by Mr. Edward Halse on "Some Banket Deposits of the Gold Coast, West Africa," was taken as read :