Изображения страниц


earth in the Sloping Chamber, as already stated. From the point where it was first seen, it was everywhere continuous up to the western wall. Its thickness has not been ascertained; for though it was partially broken up in cutting the four-feet section, the bottom of it was not reached. No objects of any kind were found in it. Had Mr. M‘Enery's excavations been carried but a yard further west he must have encountered it, and would have been enabled to solve the problem of the blocks which he so often found in the Caveearth.

The Committee are most anxious to guard against the impression that, in any of the foregoing remarks, they have been unmindful of the service which Mr. M‘Enery rendered to science, or have the most remote wish to depreciate the value of his long-continued labours. Indeed, when they remember that the means at his disposal must have been very limited, and that he was amongst the pioneers in cavern searching, they cannot but feel that the extent and results of his investigations are richly entitled to the warmest praise.

They ventare, however, to take this opportunity of stating that, in order to a thorough and satisfactory investigation, cavern-deposits should be excavated, not by sinking occasional shafts, but continuously in a horizontal direction, to a uniform depth not exceeding 5 or at most 6 feet at first ; that the material should be carefully examined in situ, and then taken to daylight for re-examination. Through not following the first, Mr. M.Enery failed to understand the exact historical order of the Cavern-deposits; and through not being able to accomplish the second, he passed over many specimens calculated to have modified his conclusions, and which he would have been delighted to have found. For example, when speaking of the Sloping Chamber, he says, “ The [Stalagmitic] crust is thickest in the middle . for opening the excavation, the same means were employed as to break up a mass of ancient masonry. Flint blades were detected in it at all depths, even so low as to come in contact with the fossil bones and their earthy matrix, but never below them”*. During the last six months, however, the excavations made in the same Chamber, and in the immediate neighbourhood of his, have brought forth Flint implements from every level of the Cave-earth to which the work has been carried, and they were actually found in greatest numbers in the lowest levels. To this may be added the fact that in his heap of refuse-matter, which he had twice examined, there were, as has been already said, upwards of a dozen flint blades, such as he stated never occurred in the Cave-earth. Had the soil been examined in daylight, they could not have been overlooked; for, instead of being specimens of little value, they are better far than some of those which he figured ; and it is but right to add that many of those found by the Committee were thus detected.

Again, Mr. M‘Enery was keenly watchful for extraneous objects in the Stalagmitic. Floor ; and, from his silence on the question, it may be safely concluded that he never saw fern-impressions in it; nevertheless his refuseheap contained four small slabs of the floor, in each of which was a well-marked impression, requiring not additional manipulation, but simple daylight for their detection. Indeed every specimen of this kind has been recognized outside the Cavern only.

The four slabs just mentioned, as well as the two found by the Committee in the Floor they broke up, have been submitted to Mr. W. Carruthers,

* See Trans. Devon. Assoc. vol. iii. p. 247 (1869).

[ocr errors]

F.R.S., of the British Museum, who has kindly furnished the following note respecting them :

“British Museum, 10 July, 1871. “ The ferns are specimens of Pteris aquilina, Linn., and have belonged to very luxuriant plants; they do not differ from those now growing in England. It is possible that the fragment or may be another species, but it is too imperfect to determine, and it may only be a barren portion of the Pteris, with shorter and broader pinnules than the other specimens.

(Signed) “ WM. CARRUTHERS." Returning for a moment to the Old Crystalline Stalagmitic Floor beneath the Cave-earth, it was observed that, like the modern and granular one, it had here and there on its upper surface conical bosses rising above its general level, and that there were corresponding protuberances vertically above them on the upper floor. The same fact had been noticed in the other branches of the Cavern where the two Floors occurred in the same vertical sections,-a fact apparently warranting the conclusion that the drainage through the Cavern-roof underwent no important change during the entire period represented by the two floors and the intervening Cave-earth. When to this it is added that such bosses are, at least in most cases, vertically beneath Stalactitic pendants on the ceiling, it may be further inferred that the ancient and modern lines of drainage are, in the main, identical.

On the completion of the work in the Sloping Chamber, on July 11, 1871, the excavation of the “Wolf's Den,” which opens out of its northern side, was begun. It was in this Den that Mr. M'Enery found the canines of Machairodus latidens, which have excited so much attention. No such specimens have been met with during the present investigation up to this time.

The Committee, believing it possible that the subject might prove to be connected with their researches, have from time to time mentioned the occasional occurrence of living animals in the Cavern*. Indeed, Kent's Hole is not better known to the palæontologist as a store-house of mammalian remains, than to the Devonshire naturalist as a home of the Great Horseshoe Bat (Rhinolophus ferrum-equinum, Leach); and every visitor, before the present exploration, must have frequently seen them hanging from the walls of the more retired branches. The following facts have presented themselves during the last twelve months :

Whilst the excavation of one of the lateral branches of Smerdon's Passage was in progress, a considerable number of fresh spindle-shaped fæces, about .6 inch long and .2 inch thick, were observed lying on the surface of the Cave-earth, while between it and the roof there was an interspace just sufficient to allow an animal about the size of a Badger to pass.

The workmen having observed that the candles were much nibbled during their absence, that the greasy wooden candlesticks were sometimes carried off and some of them, after a few days, found secreted in small holes, set a suitably baited gin for the suspected offender. Their efforts were rewarded the next morning by finding a rat dead in the trap.

Old newspapers &c. are occasionally sent to the Cavern for the purpose of wrapping up small boxes of specimens, or such delicate objects as need more than ordinary care. On November 28th, 1871, the workmen, using in this way a part of a copy of the Saturday Review,' unintentionally left one complete and sound sheet, i. e. two leaves, near the spot where they had been at work.

* See Reports Brit. Assoc. 1869, p. 204, and 1870, p. 27.

[ocr errors]

The next morning they found the paper precisely where they had left it, but with about one-fifth of one of the leaves gone, and the broken margin of the remainder apparently nibbled. There was nothing to prevent the whole from being taken off, and it was noted that, though left in a precarious position, it had not fallen down. The broken leaf was then torn off and preserved, whilst the unbroken one was allowed to remain as a further experiment. The next morning no trace of it was to be seen. That evening a rat-trap was set at the spot, and very near it another leaf of paper was placed, having on it a small stone, which it was supposed a rat, but not a smaller animal, might be capable of moving. The next morning the paper was found where it had been put, but very much nibbled, whilst the trap and the grease with which it was baited appeared to have not been touched. Before leaving work, the men baited the trap with a tempting end of candle, and placed it on a leaf of paper; whilst another leaf, weighted with a lump of earth, was placed near. On the following morning both pieces of paper were found to be considerably eaten or torn; and it was noted that the injury done to the former was within the margin of the trap placed on it, whilst the trap itself, as well as its bait, remained unaffected, further than that there were on it a few spindle-shaped fæces about a quarter of an inch long. There can be no doubt that some animal, probably smaller than a rat, carried off the missing leaf to a recess in the Cavern, where it may serve to make its nest comfortable, and perhaps hereafter to puzzle a cavern searcher who may discover it.

Fourth Report of the Committee for_the purpose of investigating the

rate of Increase of Underground Temperature downwards in various Localities of Dry Land and under Water. Drawn up by Prof. EVERETT, at the request of the Committee, consisting of Sir Wm. Thomson, F.R.S., Sir CHARLES LYELL, Bart., F.R.S., Prof. J. CLERK MAXWELL, F.R.S., Prof. PHILLIPS, F.R.S., G. J. Symons, F.M.S., Dr. BALFOUR STEWART, F.R.S., Prof. RAMSAY, F.R.S., Prof. A. GEIKIE, F.R.S., JAMES GLAISHER, F.R.S., Rev. Dr. GRAHAM, E. W. BINNEY, F.R.S., GEORGE Maw, F.G.S., W. PENGELLY, F.R.S., S. J. MACKIE, F.G.S., EDWARD HULL, F.R.S., and Prof.

EVERETT, D.C.L. (Secretary). In last year's Report, the intention was expressed of boring down at the bottom of Rosebridge Colliery, if the Association would provide the necessary funds. The circumstances were exceptionally inviting, and the Association very liberally granted the sum asked. The Secretary thereupon paid two visits to Rosebridge, descended and to some extent explored the colliery, in company with Mr. Bryham, and, after a careful study of the plans and sections, agreed upon a particular spot where the bore was to be sunk. Tracings of the plans and sections were kindly sent by Mr. Bryham, who in every way cooperated most cordially, and gave much valuable assistance in arranging the scheme of operations. Several weeks elapsed, which were occupied in making and testing a very large spirit thermometer, suitable for reading in the bad light of a mine, and capable of being read, by estimation, to the hundredth of a degree, from 90° to 110° F.; and on the 7th November the Secretary wrote to Mr. Bryham requesting him to commence operations. Unfortunately, during this brief interval, circumstances had changed. In a neighbouring pit, where the workings were in the same seam of coal as at Rosebridge, though less deep by 200 yards, a considerable quantity of water was found in sinking into the strata underlying this seam. This was a very unexpected circumstance; and as any irruption of water at the bottom of Rosebridge pit, which is now quite dry, would be a most serious affair, Mr. Bryham was afraid to risk the experiment of boring down. Subsequent reflection has only confirmed him in the opinion that such a step would be hazardous, and the Committee have accordingly been most reluctantly compelled to renounce the plan. Mr. Bryham's final refusal was received on the 28th February.

Professor Ansted read a paper last year, in the Geological Section of the Association, upon the Alpine tunnel, commonly called the Mont-Cenis tunnel, and in that paper some interesting statements were made regarding its temperature. Since that time, Professor Ansted has interchanged very numerous letters with the Secretary, and bas furnished much valuable information, gathered from Prof. Sismonda, of Turin, and from M. Borelli, the resident engineer of the tunnel. Observations which appear to be reliable have been made in bore-holes in the sides of the tunnel, and the temperatures thus observed have been compared with the estimated mean temperature at the surface overhead, which in the highest part is a mile above the tunnel, or 2905 metres above sea-level. It is directly under this highest part that the highest temperature is found in the walls of the tunnel, namely 29o.5 C., or 85°:1 F., which is 9° F. lower than the temperature found at the bottom of the Rosebridge shaft at the depth of only 815 yards. But though the tunnel is at more than double this depth from the crest of the mountain over it, we must bear in mind that the surface-temperatures are very different. In a paper published by the engineer of the tunnel, M. F. Giordano, the mean temperature of the air at the crest of the mountain (Mont Frejus) is calculated to be -2°.6 C., or 270.3 F. Assuming this estimate to be correct, we have a difference of 570.8 F. between the deepest part of the tunnel and the air at the surface vertically over it; assuming further, as we did in the case of Rosebridge in last year's Report, that the surface of the hill itself has a mean temperature 1° F. lower than that of the air above it, we have a difference of 560.8 F., and the thickness of rock between is 1610 metres, or 5280 feet (exactly a mile). This gives, by simple division, a rate of increase of 1° F. for 93 feet; but a very large correction must be applied for the convexity of the ground; for it is evident that a point in the ground vertically under a steep crest is more exposed to the cooling influence of the air than a point at the same depth beneath an extensive level surface. No correction for convexity would be needed if the temperature of the air decreased upwards as fast as the temperature of the internal rock ; but this is very far from being the case, the decrease being about 3 times more rapid in the rock than in the air. To form an approximate notion of the amount of this correction, we must determine, as well as we can, the forms of the successive isothermal surfaces in the interior of the mountain. The tendency is for all corners and bends to be eased off as we descend, so that each succeeding isothermal surface is flatter than the one above it. Accordingly, if we have a mountain rising out of a plain, without any change of material, the isothermals will be further apart in a vertical through the crest of the mountain than under the plain on either side ; they will also be further apart



at the highest part of this vertical, that is close under the crest, than at a lower level in the same vertical. It would be absurd to pretend to fix the amount of the correction with accuracy; but it seems pot unreasonable to estimate that, in the present case, the numer of isothermals cut through by a vertical line descending from the crest of the ridge to the tunnel itself is about seven-eighths of the number which would be cut through in sinking through an equal distance in level ground, other circumstances being the same. Instead of 1° in 93 feet, we should thus have lo in / of 93, that is, in 81 feet.

This is a slow rate of increase, and is about the same as Mr. Fairbairn found at Dukenfield. The rocks penetrated by the tunnel consist of highly metamorphosed material, and are described as belonging to the Jurassic series. No fossils have been found in them. For two-thirds of the length of the tunnel, beginning from the Italian end, they are remarkably uniform, and it is in this part that the observations have been taken. The following account of them has been given by Prof. Ansted (Pop. Sci. Review, Oct. 1870, p. 351):—“The rocks on which the observations have been made are absolutely the same, geologically and otherwise, from the entrance to the tunnel, on the Italian side, for a distance of nearly 10,000 yards. They are not faulted to any extent, though highly inclined, contorted, and subjected to slight slips and slides. They contain little water and no mineral veins. They consist, to a very large extent indeed, of silica, either as quartz or in the form of silicates, chiefly of alumina, and the small quantity of lime they contain is a crystalline carbonate.'

This uniformity of material is very favourable to conduction, and the high inclination of the strata (in which respect these rocks resemble those at Dukenfield) also appears to promote either conduction proper or aqueous convection, which resembles conduction in its effects. As regards Mons. Giordano's estimate of the mean air-temperature at the crest, it is obtained in the following way :-The hill of San Theodule is 430 metres higher, and the city of Turin is 2650 metres lower than the crest; the temperature of the former has been determined by one year's observations to be -5°1 C., and that of the latter is 120.5 C. If a decrease of 1° C. for every 174 metres of elevation be assumed (1° F. for 317 feet), we obtain, either by comparison with San Theodule or with Turin, the same determination —20.6 for the air-temperature at the crest of the ridge over the tunnel.

This mode of estimating the temperature appears very fair, though of course subject to much uncertainty; and there is another element of uncertainty in the difference which may exist between the air-temperature and the rock-temperature at the summit.

These two elements of uncertainty would be eliminated if a boring of from 50 to 100 feet were sunk at the summit, and observations of temperature taken in it. The uncertain correction for convexity would still remain to be applied. It would therefore be desirable also to sink a boring, of about the same depth, in the plateau which extends for about a quarter of the length of the tunnel, beginning near the Italian end, its height above the tunnel being about a third of a mile.

In November last, when very little information had reached this country respecting the temperature-observations in the tunnel, an urgent appeal was addressed, jointly by your Committee and by the Geographical Society (of which Prof. Ansted is Foreign Secretary), to M. Sismonda, requesting him to use his influence with the Italian authorities to secure a series of accurate observations of the temperature in the sides of the tunnel, before time had been allowed for this temperature to undergo sensible change from its original

« ПредыдущаяПродолжить »