beyond it, giving evidence of a very great fault, which has suddenly depressed the whole of the first limestone and the lower part of the shale, and buried them out of sight to the S. of this fault. W. of this place, however, and over all the S.W. corner of the district, as well as in the limestone district of Staffordshire, no trace of the toadstone has ever been detected. This circumstance is probably due to the absence of any considerable faults. All the S.W. portion of the limestone district has been so acted upon by the disturbing forces as rather to be bent and twisted into great ridges and hollows than actually snapped asunder; or where broken this, there has not been any great elevation or depression caused at the line of fault, so much as a change of dip or inclination of the beds. Even the Dove does not cut deeply into the limestone, as the beds dip, upon the whole, towards it on either hand; and thus the first tendency to the formation of the valley was a downward curve of the limestone itself.*

However great the apparent irregularity and confusion to which the faults give rise, it is found, on carefully tracing them out and laying them down on a map, that they have themselves very great regularity, and evidently follow certain general laws. All the principal faults run for considerable distances, sometimes several miles, in directly straight lines, and have others crossing them at right angles.t The one set in Derbyshire run N. and S. and the others, of course, E. and W. When a fault running from the S. in one direction is crossed by another at right angles, the angle of rock included between the two is frequently lifted up, with a steep face on either side; or when two faults running parallel to each other are crossed by another, the included piece is either elevated or depressed at the cross fault, and the other end affected in an opposite direction, and thus the piece made to sway, as it were, on a central axis. Priestcliff Lowe is an example of this latter case. A valley is frequently bounded by a fault on either hand, the two running parallel to each other; but it is very remarkable that these faults seldom coincide with the sides of the valley, but run at one or two hundred yards distance from it, the whole mass between them having been let down together, and thus, probably, a depression caused which gave the first tendency to the

* The river frequently cuts through beds having the same dips on either side, but the position of the strata in the great mass of the hills I believe to be that mentioned in the text.

For by far the greatest part of the information respecting the faults and veins of Derbyshire, I am indebted to Mr. Hopkins's pamphlet before mentioned.

formation of the valley. Of this circumstance, Monk's Dale, the valley of the Wye in what are called Miller's and Mousal Dales, Lathkill Dale, and Bousal Dale, are admirable examples. One of the most interesting circumstances respecting the faults is their connection with the mineral veins. All the principal lead veins in Derbyshire are connected with faults, which run parallel to them at a little distance; and as a general rule the vein is on the upcast side of the fault. The direction of the veins, moreover, is the same with that of the faults, one set running N. and S. the other E. and W.While, however, the principal faults run N. and S. the most regular and continuous veins have an E. and W. direction. Some of the E. and W. veins have been traced for many miles without any real termination having been reached. Of these, the principal are the Yokecliff, running through Wirksworth; the Longrake, whose line passes S. of Haddon Hall and Mony Ash, and which has been traced into Staffordshire; the Mochsha vein, S. of Bakewell; the Deep Rake of Longstone Edge; and the High Rake, the Moss Rake, and the Dritton vein, between Tideswell and Castleton. The E. and W. veins are generally comparatively narrow, but preserve their width throughout their course, and contain a great quantity of ore. The N. and S. veins, on the contrary, are more irregular, sometimes widening to three or four yards, and then closing, and apparently, perhaps, ending, again to open out in another part of their course. Their contents, too, are very irregular, being sometimes very rich, and at others containing nothing but spar, or not unfrequently fragments of rock and broken rubbish, in which merely detached pieces of ore can be discovered. To all these evidences of mechanical origin it may be added that a vein is itself sometimes a fault, the beds on opposite sides of it being shifted from their original common level.

All these facts accord very well with the theoretical results deduced by Mr. Hopkins* by mathematical analysis, from the examination of the problem "What would be the effect of the action of an elevating expansive force upon a homogeneous mass of rock, of indefinite thickness and longitudinal extent, and bounded laterally by parallel lines.” He has shown that this effect would be the production of great longitudinal fissures running parallel to each other in straight lines, and having others at right angles to them. The direction of these systems of fissures would be determined by the directions of the principal tensions of the mass. That the principal fissures would be at

* See his paper in the Transactions of the Cambridge Philosophical Society.

right angles to the principal tension, and parallel to the axis of elevation. That the fissures, likewise, would not begin at the surface, but at some point beneath it; and that those which are parallel must be necessarily contemporaneous in their origin. That subsequent movements would be likely to convert the longitudinal fissures into faults, producing considerable relative displacement of the beds on either side of them, and might cause the fissure to be very irregular in its width at different points; that the width of the transverse fissures, on the contrary, would be likely to be more regular, and the relative movement of the beds on either side of them to be less.

Now, applying these general results to the particular instance of Derbyshire, we know the axis of elevation of this district to be a north and south one, because it belongs to an elevated ridge which stretches away to the N. as far as the borders of Scotland; and we have seen that the chief faults and most irregular veins have a N. and S. direction, while the E. and W. veins, on the contrary, are remarkably regular. From all these circumstances it is clear that faults and veins are both due to a mechanical cause; that they are, in fact, both fissures produced by the same cause and at the same time, during the general elevation of the district. That a fault is a fissure generally devoid of minerals, on opposite sides of which the beds are relatively displaced; that a vein is a fissure producing generally little displacement, but filled with minerals in a state of greater or less purity and crystallisation. In what way these mineral substances gained their present position, we must probably have recourse to the chemist to resolve. That they were not placed there by mechanical causes, as was at one time supposed, is certain; because in Derbyshire the toadstone almost invariably continues across the vein, completely cutting off the connection between the parts above and below. If, then, the veins had been mechanically filled from above, they ought to be empty below the toadstone; if from below, the part above the toadstone would contain no minerals. It is evident, moreover, from the veins of this and other districts, that the quantity and quality of the minerals in a vein very greatly depend upon the kind of rock which composes its sides, making it probable that these minerals have been, in some way, segregated from the adjoining rock into their present position. This subject is, however, at present beset with difficulties. We come now to the next superior rock,

THE LIMESTONE SHALE.

The E. and W. sides of the limestone district are bounded by a narrow valley worn out in the soft beds of the shale, which on the one side reposes regularly on the limestone, and on the other rises half way up the hills, and supports the millstone grit.-(See sections 1 and 2). This valley varies in width from 3m. to 2m. according to the greater or less extent to which denuding forces have acted upon it and its gritstone covering. Occasionally, however, there is no valley, but a hill of shale rising abruptly from the hill of limestone on which it rests; or it is not unfrequent to find a hill of shale rising out of the valley, and protected by a cupping of millstone grit. It, of course, every where partakes of the inclination of the limestone below it, and must be affected by the same or similar faults. Where, however, the solid beds of the limestone have been snapped clean asunder by the dislocating force, the softer beds of the shale are often bent and contorted in the most singular manner. In these lateral tracts, however, the shale seldom shows anything of remarkable interest. In the part N. of Castleton, the most remarkable spot in the shale, is the well-known hill called Mam Torr, or the Shivering Mountain, which is a hill of shale resting upon limestone, and being very precipitous, especially on one side, the action of the weather has continually undermined and worn away the loose and crumbling materials, until a great natural excavation has been formed, to which each successive winter makes additions, while the ruin lies scattered at its foot. The line of hills between Castleton and Eedale consists chiefly of this formation, whence it stretches away to the N. with many minor corrugations and changes of dip, but on the whole nearly horizontal, forming, as it were, the base of the country for the support of the masses of gritstone. The valleys all cut down deeply into the shale, frequently exposing faces that exhibit its alternation with, and gradual passage into, the millstone grit. If we go to the S. of the limestone district, we find the shale spreading out, and occupying nearly all the country between Wirksworth, Derby, and Ashbourne, north of the boundary of the new red sandstone. Over this tract its position is remarkable, as it is bent up and down, in every direction, into great curves; but the scarcity of natural or artificial sections renders it rather difficult to make out. On the E. it may be seen going down from Wirksworth, below Ashley Hay and Shottle, to Milford, dipping every where to the E. under the gritstone. At Milford its dip is

VOL. IX., NO. XXV.

2

rather N.E. as may be seen by the examination of the tunnel of the North Midland Railway; and it no doubt preserves the same dip, as it crosses the Derwent, and runs below Little Eaton to Breadsall. In Breadsall brook it may be seen greatly disturbed, no less than ten changes of dip occurring within about 4m. the angles of inclination varying from 20° to a complete perpendicular. At Breadsall it becomes concealed by the new red sandstone. Crossing to the W. side of the valley that comes down from Wirksworth to Duffield, the shale may occasionally be seen dipping rapidly to the W. more especially about Ideridge Hay, and thence to Wirksworth. In the small valley that runs down from below Wirksworth to Hopton, the shale dips to the S. and the effect of this depression to the W. and S. is, to bring in a patch of gritstone resting upon it about Kirk Jreton. From underneath this, however, it soon rises again with a dip towards the N. and this is continued till the lower beds appear at Flower Lillies, near Turnditch, all beyond this is obscured by new red sandstone and diluvium; but we get the upper beds again, where it alternates with the gritstone, within the boundaries of the new red sandstone, at the back of the Royal Oak at Langley.* Here, likewise, it dips to the N.E. at an angle of 15°; so that one or two changes, and probably some faults, must be concealed by the overlying beds between here and Turnditch. On tracing the shale to the W. we find it between Atlow and Kniveton, rising up into a round hill of very considerable elevation, called Atlow Winn, at the S. end of which, at a place called Agnis Meadow, beds the same as those of Turnditch are worked for their limestone, and the most extraordinary twistings and contortions of the strata are exposed. Even the solid beds of limestone are bent into regular curves, and bear the appearance of arched masonry. Its general dip here appears to be towards the S., whence it shortly gets concealed by the new red sandstone. A quarry, apparently in beds similar to these, was formerly worked at Wild Park, near Brailsford, but no face is now exposed to give us any information as to the position of the strata. From Agnis Meadow it lies pretty level all the way to Ashbourne, but at Ashbourne Green it dips 30° N.E. and immediately beyond is concealed by new red sandstone.

* In speaking of the "red and white sandstone," in the former part of this paper, I believed, from some detached specimens I had seen, and from its position within the borders of that formation, that this quarry belonged to it. On seeing the quarry itself, however, it was immediately evident that the beds belonged to the upper part of the limestone shale, or lower part of the millstone grit. Horizontal new red sandstone may be seen in the road close behind it.