The changes in the gritstone group just described are plotted to scale in the Sections in Fig. 28. If this be compared with the fellow woodcut for the other side of the Penine anticlinal, given on page 85 of the Memoir on the country round Stockport, Macclesfield, Congleton, and Leek, it will be seen that both show distinctly a great decrease in the thickness of the whole as we go southwards. Joints. The gritstones, especially the Chatsworth Grit, are well suited for observations on the laws which govern the direction of joints. As an instance we will take the Chatsworth Grit of Stanage-Edge (see p. 59). On the map, Fig. 29, page 108, the position and directions of a number of measured joints are laid down, the rays of each star-like figure showing the bearing of the joints observed at the centre of the star. We see from this map that the joints have roughly a tendency to arrange themselves parallel to two fixed straight lines, but this will be rendered still more clear by the following artifice. If from the centre of a fixed circle we draw lines parallel to the directions of the joints, and if, where there are several joints having the same bearing, we make the corresponding spoke proportional in breadth to the number of these joints, we shall obtain the diagram in Fig. 30, page 109. In the map, which shows both the bearing and geographical position of the joints, it is somewhat difficult to carry the eye from one set to another; but by referring them all to one centre in the diagram, we see more clearly the relations which these compass-bearings have to one another, and can deduce the laws, if there be any, which govern them. The diagram now shows us that the joints arrange themselves in two sets, and show a tendency to cluster round two lines bearing roughly north-east and south-east, which are about the directions of the dip and strike of the rock. The first set are less numerous, but keep closer to the mean bearing than the second. While in the first case, however, the largest number of joints which have the same direction is four, pointing E. 60° N., in the second case we have six joints, ranging E. 40° S. The mean direction of one set is E. 46° N., of the second, E. 43° S.; these are shown by the two arrows outside the circle. The time at the disposal of the officers of the Geological Survey does not allow of matters like these being entered into in detail; but it is hoped that these slight hints may lead local observers to take up the subject more fully. A like class of observations, to which the same remark will apply, is a record of the directions of the dip of planes of current-bedding. Dr. Sorby, who first drew attention to the subject, has already done excellent service in this line, and it is to be hoped that he will publish in detail the results of his observations; if he, or anyone with time to spare, will carry on and complete what is already well begun, much light will be thrown on the question of the method of the formation of the Carboniferous sandstones. Contrast between the lie of the Rocks on opposite sides of the Penine Anticlinal. This is very strongly marked, and, when a sufficiently extensive set of observations have been brought together, they will doubtless throw light on the mechanics of the upheaval of the range. To this end we now contribute our quota from the present district. Where the anticlinal is most marked the dip is much steeper on the west than on the east, as is well seen north of Staley bridge. Elsewhere the beds on the west are thrown into a number of troughs and arches, and are broken by many large faults, so that at the first glance it does not strike us that there is any prevailing dip one way more than another, and a general easterly rise is at least inferred only from the broad fact that in going towards the centre of the range we pass upon the whole from higher to lower beds. On the opposite side, however, every long transverse section shows a gentle and tolerably constant dip to the east, the rolls and faults being smaller and less numerous than on the west. On the west the strike is north and south, and the great faults with the axes of the folds run markedly in the same direction; witness the "Anticlinal" and "Red Rock " faults, the Goyt Trough, the Rudyerd Basin, and the Coal-field of The Potteries. On the east the strike is west of north and east of south, and the faults, troughs, and saddles have either the same trend or run east and west; the Ashover anticlinal is an instance of the first class, and the Rivelin anticlinal, with the Crawshaw and Rood Hill basins, which lie just outside the country described in this Memoir, of the second. At the same time, there are important north and south faults, as the Chatsworth fault, the Trinity Chapel fault, and the Ambergate fault; these, however, have none of them a large horizontal range. In short, on the west the trend of the great geological features is everywhere nearly north and south, while on the east we find them running sometimes north and south, sometimes east and west, and sometimes taking an intermediate course. We venture on the following explanation of these facts. some Professor E. Hull* has shown that it is highly likely that the Carboniferous Rocks of the north-midland counties had their present lie given them by two separate upheavals, to which he has given the names of the Pendle and Penine Systems. The earlier in date he supposes to have been the Pendle System, which bent the rocks into a number of troughs and arches, whose axes ranged approximately in an easterly and westerly direction. Then followed the Penine upheaval, acting along northerly and southerly lines. Now the excess of contortion and disturbance, and the greater steepness of dip, which we find on the western side of the Penine Chain, seem to show that the force, whatever it was, which brought about the later system of disturbance, had its starting point on the west, and that its effects became less and less felt as they were transmitted eastwards. On what then is now the western side of the Penine anticlinal it may well be that the later or Penine upheaval may have been strong enough to wipe out completely all traces of the earlier system of easterly and westerly folds, and to give to the rocks their present northerly and southerly trend. On the east of the Penine Range, however, where the later upheaval acted with less intensity, the two systems were more evenly matched, and the lie of rocks will sometimes follow one; sometimes the other, and sometimes share in both directions, as local circumstances may determine. Rivers and Valleys.-A word must be said touching the eccentric behaviour of rivers, which seem in many cases to have, as it were, purposely gone out of their way to find difficulties, and picked out the hardest rocks and the most formidable ridges to cut through. The valleys may be divided into three classes. 1st. Those which run more or less across the strike, cutting through escarpments and whatever stands in their way with a total disregard to the present configuration of the surface. 2nd. Those which run parallel to the strike. 3rd. The lesser gorges, feeders of both classes. Of the first class there are many instances, none perhaps more marked than the valley of the Derwent at Matlock. The river has flowed from Rowsley along a broad shale-valley, and about a mile before reaching Matlock strikes up against the limestone, which rises like a wall across *On the Relative Ages of the Leading Physical Features and Lines of Elevation of the Carboniferous District of Lancashire and Yorkshire. Quart. Journ. Geol. Soc., vol. xxiv., p. 323. 1868. its path from the broad flat of the alluvial plain. A stream of water set going down such a valley would, one would think, be turned aside by such a block in its path, and, bending a little to the east, would run on round the outskirts of the limestone in the soft shale. But in this wall of rock there is a breach, only one, and that so narrow that it had to be enlarged by blasting to admit the turnpike road into the valley, and through this rift the Derwent enters and flows in a deep, steep-sided gorge down to Cromford, where it again comes forth into a broad shalevalley. We could not find a better illustration of Colonel Greenwood's doctrine, that "as sure as there are alternations of hard and soft strata in the course of a river or valley, so sure will there be alternations of gorge and alluvial flat." But now comes the question what made the breach and gorge? One school says, when the limestone stood up as a rock in the sea the waves cut the passage through, which the river afterwards appropriated to its own use ;* another school that a river has done it all; that once the wall of limestone did not exist, the shale on the northern side reaching up to or far above its top, and that in this comparative flat some accidental depression determined the course of a stream, which cut a channel step by step deeper and deeper; that, where the valley thus formed ran through soft shale it was widened out by the help of rain and tributary brooks, but that in a hard rock it kept more nearly the trench-like character with which it began. Many more such cases might be pointed out, such as the entrance of the Dove into the limestone at Beresford Hall, and of the Manifold into the same rock at Apes Tor near Ecton, where the contrast between the shale flat and the limestone wall, and the breach in the latter, are most striking. On either theory it is likely, and on the latter certain, that these transverse valleys are the oldest of the three classes. The largest case of a transverse valley is that formed by the river Wye and the portion of the river Derwent below its junction with the former. This hollow, though here and there running parallel to the strike and at the foot of an escarpment, as a rule pays no regard to dip, comparative hardness of beds, or present shape of the surface, but cuts across everything that comes in its way; we therefore think that this is the oldest and was once the main line of drainage, and that the upper part of the Derwent, which partakes more of the character of the second class of valleys, was formed later, and was at first only a feeder of the main stream. We may also notice that the brooks between Chapel-en-le-Frith and Glossop, on the western side of the great watershed, have all cut through the ridge formed by the easterly outcrop of the Millstone Grits of the Goyt Trough, and so found their way into the Goyt or Etherow, while, if they had been guided by the present surface configuration of the ground, it seems more likely that they would have flowed, some to the north, and some to the south, along the valley at the foot of the ridge, and the latter would have gone down the Doveholes valley into the Wye. In the case of Glossop Dale and the Hayfield Brook there are faults ranging along these cross valleys, but there are at least three other valleys, quite equal to these in size, which are free from any disturbance. The second class of valleys, those which run parallel to the strike and often lie between two escarpments, have been mentioned in the introductory sketch of the general shape of the grit country, and are illus * These words are left in the form in which they appeared in the first edition of the Memoir (1869), but it should be added that this view of the origin of such gorges is rarely, if ever, maintained now.-A. S. trated in Fig. 1. Among other writers Messrs. Foster and Topley have explained how these valleys may grow into existence as feeders of the transverse valleys;* and a conjectural explanation of the method of their formation in the present country was put forward in the Memoir on the country round Stockport, Macclesfield, Congleton, and Leek (p. 87). The clean sandstone dip-slopes of these valleys are very noticeable, and observations in the present district have tended to strengthen our belief in the explanation quoted. One of the best illustrations is found in the neighbourhood of Strines (north-east part of map 81 N.E.). A transverse feeder of the Bradfield Dyke there cuts across the Upper Kinder Scout and Chatsworth Grits; two of its tributaries, one running along Hollindale, and the other at the westerly foot of Strines Edge, belong to the second class of valleys. Both brooks flow for long distances just on the top of the Upper Kinder Scout Grit, with a steep slope of the overlying shales for their eastern bank; the lower part of this slope is a cliff with a raw, freshly cut face, and at the bottom are heaps of newly fallen shale, which the stream is grinding down and carrying away; and we see clearly the process in action by which the side of the valley is being worked back, and the valley widened to the east, exactly after the fashion described in the passage referred to above. When a valley of the second class has grown into sufficient importance, rivulets will begin to trickle down into it, especially on the steep or escarpment flank. These are always cutting their way back into the hill, and thus in the course of time become transverse valleys of considerable size, and will have in turn feeders of their own of the second class; and by such a branching system, taking its rise originally from one great transverse valley, and ever spreading and pushing out new arms further and wider over the land, the moulding and carving of the present surface may be supposed, without any extravagant assumption or any great degree of improbability, to have in great measure been brought about. This Landslips. Many cases have been noticed in the course of the Memoir, at the foot of hill-slopes formed of soft shales and capped by massive sandstones. Frost, springs, and underground streams soften and carry away the shales, and huge masses of the sandstone, thus deprived of their support, break off along joints, and slide or topple down into the hollow below. This happens especially in the case where the dip is down into the valley, for then the rain which percolates through the porous sandstone is checked in its course downwards on reaching the impervious shale, and flows out along the plane of junction of the two; and so this floor, whose slope already gives the sandstone a tendency to slide, is made slippery, and the friction and cohesion, which alone keep the rock in place, are lessened or destroyed, and a fall ensues. process seems to have gone on till equilibrium was established, and many of these slips and the hills above them must have been for a very long time in state of perfect rest. Instances are known, however, where such ground has been tampered with by artificial cuttings, and a renewal of the movement has followed; and any small change in the natural conditions of the country would certainly have the same effect. Landslips also of large size have happened within this district in the memory of man, and many cases of the wearing away of hills, as at Mam Tor, are now going on every winter or rainy season; so that, in spite of the outwardly settled look of the face of the land, we are assured that the same causes are at work and the same effects resulting from them as in times long gone by. * Quart. Journ. Geol. Soc., vol. xxi., p. 443. U 18863. H Other points of connexion between the shape of the ground and its geological structure have been already noticed, or are too obvious to call for detailed description. The basin-shaped lie of the beds in outliers is well illustrated by the case of the Peak. It is doubtless to this accident that we owe the preservation of the capping of grit to the hill; had the dip been from, instead of towards, the centre, that is, had the beds been domed instead of being basin-shaped, landslips would long ago have brought down a large portion of the rock, which now reposes without any risk from this source at least. Landslips have happened where, for a short distance, the dip has the right direction, and would have been found all round the outlier, had a corresponding dip obtained elsewhere. But, though safe on this score, the grit-capping is being slowly carried away by other means, and the time may come when this splendid outlier shall be reckoned among things that have passed away. The brooks that gash the escarpment are always undermining the cliffs of rock at the top and bringing down masses of grit to grind small and carry away, and, by thus working their way back into the table-land, will split up its now unbroken flat into a number of separate outliers, each fated in its turn to be cut up by new rivulets, and at last perhaps to be carried away altogether. In this way we can conceive that the Kinder Scout Grit has been cleaned bit by bit off the plateau of Shale Grit that surrounds the Peak, and that the few patches that do remain have only contrived to outlive for a time the wholesale removal of the rock in virtue of the basin-shaped arrangement of their beds. As an exactly opposite case to the Peak we may take the ridge at Coumbs Wood two miles S.S.E. of Matlock. This ridge is capped by Shale Grit, and an anticlinal ranges along it, the beds dipping on both sides away from the highest line. On each side the whole of the sandstone has slipped away in huge masses into the valley below; and it is only along the crest of the saddle, where the beds lie flat, and have no tendency to slide, that a narrow strip of the rock remains. The effect of a bed of shale in the limestone, in acting as a greased platform on which the beds slide, has been exemplified in the landslip of Crich Hill, described on p. 83. (See also p. 42, on the landslip of Alport Tower, and p. 53, on landslips near Ashopton). Warm springs.-A large number of warm springs issue from the limestone of Derbyshire, and have been used for medicinal purposes from time immemorial. The best known at the present day are those of Buxton and Matlock, but we have records of warm water having been observed also at Bradwell, Fyam, Stoney Middleton, Wirksworth, and Crich. The origin of the warm water has been ably discussed by Dr. Darwin,* of whose arguments the following is an abstract :-The warmth of the water has been supposed to be due to the decomposition of iron pyrites in the clays associated with, or resulting from the decomposition of, the toadstone. But though warm water has been found in such beds, yet no smell or taste attended it. Cold water, moreover, was found oftener than warm. It is more probable that the warmth is due to the great depth of origin of the springs. For in the first place the temperature has remained unaltered for centuries, and is the same both in summer and winter; in the second place, so dry a summer as that of 1780, when all the cold springs were dried up or greatly diminished, had no perceptible effect upon the warm springs. In 1780 Dr. Darwin saw the sources of two of the springs at Matlock opened, 200 yards above their usual exit. The upper one issued from * In a letter dated February 5th, 1788, and quoted by Pilkington in a View of the Present State of Derbyshire, p. 256. |