tion. If the unstratified masses at Cloud's Hill owe their form to the action of heat, it is difficult, if not impossible, to conceive how this heat could have changed internal portions of the limestone, without affecting the surrounding strata. In Devonshire, and elsewhere, hills of mountain limestone may be seen in which the stratification of the entire mass is obliterated or nearly so; but there can be no difficulty in this case, indeed, it may be said that we do not know that the limestone was ever stratified, though the probabilities are greatly in favour of its having been so.

Remarkable sudden changes may be sometimes observed in the quality of the same beds of mountain limestone. At Llanymynah, in Shropshire, (a hill composed of this limestone,) the quality of the limestone, on one side of the hill, is considered by the lime-burners of the very best kind; while, at a little distance, the same strata are so impure, from an intermixture with sand and clay, that they cannot be used with advantage: but what is more remarkable, I have seen, in this hill, a stratum of the best limestone, lying regularly between other strata, suddenly terminate, and a whitish calcareous marle occupy its place, preserving the same degree of thickness, and the same direction. As these strata contain marine organic remains, and were deposited at the bottom of the occan, we may suppose that a submarine current had prevented the limestone from extending further, and supplied its place by a deposition of clay, before the stratum above was deposited. In the former case, where the strata of good limestone become, in some parts, calcareous and impure, we may suppose that submarine currents, carrying away particles of sand, had intermixed them with the calcareous depositions in one part, but not in another. Indeed, this sudden change in the quality of the limestone is so common in that part of Wales, that the workmen have given to it the expressive name of Balkstone. When I was first informed of the balkstone, and saw that it impeded the operations of the quarrymen, I expected to have found a dyke of basalt, and was surprised to observe a mass of stratified linestone, of an impure quality, cutting through the best limestone like a thick wall and left standing, the good limestone being worked away on each side of it. This wall of limestone is of a darker colour than the rest; it contained the remains of the encrinites. It is owing, I conceive, to the irregularities in the deposition of the strata, from causes attending their original formation, that soft and irregular beds or masses of clay occur in mountain limestone, which have subsequently been washed out by subterranean currents of water, and formed excavations and caverns of considerable magnitude. Many instances might be cited of large streams, and even rivers, engulfed in mountain limestone, and rising again at the distance of several miles. In the northern counties these openings are called Swallow Holes. Mr. Farey has enumerated twenty-eight swallow holes in the mountain limestone of Derbyshire.

It is in the lower beds of mountain limestone that enormous natural caverns frequently occur: such are the well-known cavern near Castleton, and Pool's-hole, near Buxton in Derbyshire; and Yordas Cave, under Whernside, in Craven. Gordal Scar and Weathercote Cave, in the same district, cannot properly be called caverns, as they are open to the day; but the latter was probably once a cavern, of which the roof has fallen in. In all these caverns, and others that I have observed in this limestone, there is a stream of running water, which is more or less copious in rainy or dry seasons. I am inclined to believe that the caverns have been formed by the agency of water, percolating through natural fissures, and in the lapse of ages excavating the softer or more broken parts of the rock. The prodigious force with which these subterranean streams rush through the openings of some of these caverns, after continued rains, suggests the probability of this mode of formation. The whole of that enormous mass of limestone in Craven, from Ingleborough and Whernside to Gordal, is intersected by perpendicular fissures, which are narrow at the top, and become wider as they descend, through which the water may be heard to run at a vast depth below. These unseen but everactive streams are slowly but progressively wearing down the internal parts of these calcareous mountains, and depositing them in the sea.

The mountain limestone of Derbyshire demands particular attention from the interesting geological phenomena which it presents; though it has been much visited and frequently described, I believe the accounts hitherto given have been in some respects erroneous. I revisited the country round Matlock soon after my return from the Continent, and was then convinced that the structure of the calcareous mountains had been mistaken, but the state of my health did not permit me to pursue the enquiry. Since the publication of the third edition of this work, I have examined this part of the country carefully, and shall briefly state the result of my observations. Mr. Whitehurst has the merit of being the first observer who discovered some of the leading features of the geology of this district he boldly pronounced that the beds of trap and amygdaloid, provincially called Toadstone, which are interposed in the limestone, were volcanic lava, or at least had an igneous origin. This opinion was much opposed at the time; it is now confirmed by such a weight of evidence, as to leave little doubt respecting its correctness, (See Chap. IX.) though the facts and arguments by which Mr. Whitehurst's views. were then supported were in some respects fallacious.

Mr. Farey, who followed Mr. Whitehurst, adopted the same views of the general structure of the country, though his opinions respecting the formation of the toadstone were entirely different; he considered it to be an aqueous deposition, forming regular strata, like those of sandstone in the coal measures.

Mr. Whitehurst and Mr. Farey describe three beds of toadstone, and four of limestone, in a descending series.

1. The first limestone 150 feet, with much white chert.

2. The first toadstone 48 feet, vesicular and amygdaloidal.

3. The second limestone 150 feet, contains beds of magnesian limestone.

4. The second toadstone 128 feet, more compact than the first toadstone.

5. The third limestone 180 feet, contains black madrepore beds. 6. The third toadstone 66 feet, uncertain.

7. The fourth limestone not pierced through, uncertain.

This may be an approximation to the thickness of the five upper beds near Matlock Bath, but is by no means an accurate statement of the succession and thickness of the beds in other parts of the county. It may be proper to remark also, that the limestone is distinctly stratified, and the strata of limestone are often divided by strata of clay, provincially called way-boards, and also by strata or rather seams of siliceous stone called chert, resembling flint, but less splintery in the fracture, and fusible; which latter property is doubtless owing to an admixture of calcareous earth. These strata of chert occur most frequently in the upper limestones; they contain, like the limestones, remains of shells and encrinites. As loose blocks of chert with encrinites are sometimes ploughed up in the fields, Mr. Farey supposed that these blocks have been converted from limestone into chert by some unknown process,-an opinion for which there is not the slightest foundation. The chert blocks are the remains of hard strata, which have resisted decomposition or destruction, in the same manner as nodules of flint in the upper chalk. Large bivalve shells (Productus) are found both in the limestone and chert. The thick beds of toadstone that divide the upper from the lower limestones, were supposed by Mr. Whitehurst to have been protruded between them in a state of fusion: this opinion will be examined subsequently. Admitting its truth, it would sufficiently account for the great irregularity in the thickness and succession of these beds, which is known to prevail throughout the Peak of Derbyshire. All the miners that I have examined on the subject, agree that the warm springs which abound in the vicinity of Matlock, rise from under the second toadstone, and that when this bed is first pierced through, the water has often a higher temperature than the Matlock Bath water, but its heat is reduced by admixture with cool springs in the upper beds.

I have now to observe that the descending series of limestone and toadstone to No. 5., or the third limestone, may all be found in the vicinity of Matlock, and many other parts of the mining district; but the beds of toadstone are of variable and uncertain thickness. With respect to the third toadstone, its occurrence as a regular bed is extremely doubtful. In some situations there are eruptions of toadstone intervening in the third limestone, which is of vast thickness, but these beds of toadstone are in general extremely irregular :

where they occur, they of course divide the third limestone into two beds. The irregularity of these beds of toadstone, and the disturbance of the regular strata which they have caused, compelled Mr. Farey to call them chance beds, to avoid the admission of their igneous origin. In the same manner he explained the protrusion of the granitic range of rocks in Charnwood Forest; he described them as chance beds in the red marle; it was surely an extraordinary chance, which produced rocks that extend under every other rock formation in the midland counties of England. There are, however, circumstances attending the stratification of the mountain limestone of Derbyshire, that have not been noticed by any of the authors I am acquainted with, who have described this country. There are evident indications of an upheaving force acting on several parts, and bending the strata into arches, the segments of large curves as represented Plate II. fig. 1. and fig. 6. These curves are sometimes complete in the same hill, but frequently their continuity is broken. The strata of Matlock high Tor have been described by former writers as plain, and when seen in face of the rock they appear to be nearly so, but they are in reality curved, as shown Plate I. fig. 6. They enfold the back part of the hill, and are continued into the opposite hill, Masson, which they also enfold. The continuity of the strata is broken by the vale of the Derwent, which makes their true form more difficult to trace: but the arched stratification of the lower part of the same beds is distinctly displayed westward, and may be seen from the road near Matlock toll bar, where a section is made by the Derwent.

A very remarkable instance of the arched stratification, completely formed in one situation, may be seen four miles east of Matlock, in the isolated mountain called Crich Cliff, which rises about nine hundred feet above the Derwent. The strata rise all round, and enfold it, forining nearly spherical segments, as represented in the annexed cut. This section, through the hill, represents the arrangement of the beds of limestone which dip all round the hill ccc, but are somewhat flattened at the top; the shale and gritstone surrounding the lower part of the hill are represented ab. The true structure of the hill has been discovered by recent mining operations; several valuable metallic veins have been explored in it, and a gallery has been driven into it, as represented in the figure f. It is obvious that this arched structure can be formed only by protrusion, whereas the elevation or inclination of plane strata may have been produced by subsidence. Now, when we consider their near proximity to beds of toadstone of igneous formation, we can have little difficulty in assigning a cause for this protrusion; but, fortunately, we are not here obliged to have recourse to conjecture: in driving the gallery towards the centre of the hill, a mass of toadstone was met with (E E,) which was not cut through when I visited the place in 1830. The same toadstone was found by sinking a shaft upon it, as represented in the