and some much larger. The beds, being inclined at a high angle, are easily removed in large masses.

All geologists familiar with the appearance of rocks in mines and tunnels are aware of the variety of dip that frequently shows itself in the interior of the earth, and of the fact that the dip of the rocks at the surface is by no means always coincident with that of the interior. The difference may be small, but it is generally clear and well marked. It may often be observed even in quarries and deep railway cuttings. In the Mont Cénis tunnel perhaps the most striking fact seems to be that of a constant variety of dip, but always within certain limits. This is no doubt due in a very great degree, if not entirely, to the contortion consequent on squeezing; and this again is the result of the elevatory forces that have brought up these materials, originally deposited horizontally on a sea bottom, and now the crests of great mountain chains many thousand feet above the sea. With the variety of dip is observed at the same time a singular complication of small slips and troubles. None of these seems to have been important or due to any other cause than the same contortions so prominently illustrated. Still the phenomena of glazed surfaces, slickensides, and smoothed, polished faces in close contact are to be observed almost incessantly, and they are accompanied by a few, but very few, open cracks and fissures, generally consisting of small pockets partly filled or lined with crystals of quartz, calcite, or dolomite, and crystals of iron and copper pyrites. Galena and blende are also found but are very rarely distributed, and in exceedingly small quantities, in the same localities.

So far, then, as the general appearance and construction of the rocks can be determined by aid of the extensive and remarkable excavation now in progress and nearly completed, we may say that they confirm what has been already observed in mines and tunnels of smaller extent in similar rock, and show the phenomena of metamorphosis of secondary rocks (originally, no doubt, fossiliferous and regularly bedded) to present the same general character in the interior of the earth that they do nearer the surface. No intimation whatever of the existence of older or harder or more altered rock is seen. The rocks are not, perhaps, quite so hard or tough in the middle of the tunnel as they were found to be near the entrance, but they retain all their characteristics throughout, and we may fairly presume that the mass of the metamorphosed jurassic rock not yet converted into granite or protogine retains now, both in the interior and near the surface, the same condition that was induced when the metamorphism was first effected, and the elevation of the mountain mass of the Alps was completed. All the phenomena of slides and small faults conform accurately

with what has been already observed in other places, and with other rocks that have been exposed to similar action; and thus the geologist is strengthened in his own conclusions, and satisfied with the correctness of the conclusions drawn by others in this department of geology, by the additional light thrown on his pursuits by the study of the works carried on in this remarkable tunnel.

While alluding to this part of the subject, it would not be fair to exclude mention of a supposed discovery of a rounded pebble, said to have been found in a narrow dry cleft or fissure, from four to six inches in width, near the middle of the tunnel. The cleft was very irregular, and was partly filled with crystalline quartz, and was said to be open above. The pebble was oval, and somewhat resembled the undular concretions of limestone or ironstone often met with in slags and shales. The enclosing rock was a talcose schist, with quartz of the ordinary kind. As there was a possibility that the specimen might have belonged to the rock, it was carefully broken, in the presence and at the request of the writer; but the structure showed it to be gritstone rounded by water. The specimen had been brought to the resident engineer by the foreman of the works, who had not before or since found any curiosities, and who claimed to have taken it himself from underground. The story is given as it was related.

The condition of the interior of the earth with regard to water is a subject on which it may be expected that great light will be thrown by the perforation of the Alps. A clean cut through seven miles of rock under a mass of 5,000 feet of overlying material nearly of the same kind may be expected to afford information of great value in this respect. The anticipations that might have been made, judging from experience in other tunnels, are, however, not altogether borne out by the reality, and the result is almost negative. The total quantity of water entering the tunnel has at no time been large, and, compared with its great length, must be regarded as wonderfully small. The average quantity is stated not to exceed one litre per second from each end, or less than 40,000 gallons per day from the whole excavation as at present completed. This quantity has been increased from time to time, when certain open but very narrow fissures and small cavities have been reached. In these cases there has generally been a rush of water, evidently under pressure, but the total content of the water cavities, including all their communications, has at no time been very large, and they have each in succession been exhausted, a very few days being generally sufficient for this purpose. In the case of the largest of them a quarter of a million gallons of water would seem to represent the full con

tent. As this corresponds to about 40,000 cubic feet of space, some idea may be formed of the nature of the largest of the fissures; and as the water was very quickly drained off, it is evident that no open communication could exist with any other fissure, or with the surface. There has been no seasonal change in the quantity of water entering at either end. The pressure of the water in the cavities reached was sufficient to throw a jet forwards into the tunnel to a distance of as much as 20 feet, this lasting in one or two cases for some hours. No exact record seems to have been made of these fissures. When examined, the largest of them was not more than from four to six inches, and they do not appear to have extended with any regularity even across the tunnel. The largest of these watercontaining fissures was reached in 1861, not very long after the commencement of active perforation. Another, of some importance, was cut in 1867, when the middle had nearly been reached. Both of these were on the Italian side. Others have been cut on the French side, differing little in the circumstances, but generally smaller. The contact of rocks of very different nature does not seem to have been marked by the presence of springs, nor did the limestones on the French side, though of considerable extent, yield more water than the compact schists on the Italian side. The temperature of the water varied considerably. That of the largest spring, reached in 1861, was 64°; that of the next largest, reached in 1867, was 79°. The water in the clefts on the French side was between 60° and 62°. The waters, on being roughly analysed, were found to contain oxide of iron, sulphate of lime, and sulphate of magnesia. As crystals of dolomite have been found, and there are large deposits of gypsum, no surprise can be felt at this result. It is important, however, to notice, that even at the distance of midway in the tunnel, and with so large a depth of rock above, the waters are not in any sense thermal, and do not proceed from deep sources. The result is that which would take place of necessity, if water percolating from above should find an outlet in the interior after a long and probably very slow

course.

On the whole, then, the conclusion arrived at from the observations made in the tunnel tend to show that geological speculations as to the existence of free currents of water in the earth's interior, and especially of heated water as connected with metamorphic action, are not altogether correct. At a depth of 5,000 feet the temperature of water in the earth's interior should, according to these views, be very high, and the quantity of water should be large. Considering the large size of the excavation, and its great length, much more water than 40,000 gallons per day might be expected to come in, and the

quantity entering might fairly have been expected rather to increase than diminish. Such has not been the case. The water supply that has been reached was very soon exhausted, and the water itself that issued from the fissures intersected was neither very hot, nor was it loaded with mineral matter, nor has it increased in quantity. It can hardly be said to have been in circulation. Thus, so far as metamorphic rocks are concerned, the metamorphosis seems to have been carried on away from water action of any kind.

The temperature of rock in the earth's interior is another enquiry of very great interest and importance. It is to be regretted that, owing to want of proper arrangement from the first, the method of taking the temperature of the rock has not been fully and properly carried out on both sides. On the Italian side the observations are better than on the French side; but no complete record of either has yet been prepared. The system adopted was to bore holes to about ten feet in the solid rock, at intervals of about 500 mètres, and put in thermometers. But the thermometers themselves were not, in all cases, maximum and minimum instruments, and occasionally accidents have happened. It is understood, however, that further observations may at any time be made.

The general result, as represented by Signor Borelli, the resident engineer on the Italian side, shows a remarkable uniformity in the temperature of the rock throughout. At the distance of 6,200 mètres (6,506 yards), or nearly midway, and at a depth of about 5,000 feet, the temperature of the rock was found to be only 80°. The mean annual temperature of the surface at the mouth of the tunnel and above is not very closely determined; but, under any circumstances, the increment, compared with that recorded in observations made in mines, is exceedingly small. The greatest depth reached in mines has been 2,150 feet, and the average increment is generally taken at 1° Fah. for every 60 feet. Allowing 80 feet to reach the stratum of invariable temperature, the permanent temperature at 5,000 feet should be at least 80° above the mean annual temperature. It is certainly not more than 50°, and probably less. There have, however, been other cases observed, at which the increment amounted to 1° for upwards of 100 feet; and as there is a general absence of mineral veins and metalliferous deposits of every kind in the rocks through which the tunnel is bored, it may be possible that this smaller increment belongs to the earth generally, the higher being due to chemical action, induced by the presence of certain metals, metalliferous minerals, and water.

At the same time it must be remembered that the very large proportion of all the observations of subterranean temperature

whether made in mines or borings for water, but in no case to half the depth of the central part of the tunnel, shows the rise to be one degree in about 54 feet as an average and the stratum of invariable temperature to be about 60 feet. This is the rate not only when the boring commences near the sea but also near Geneva in a deep sinking commenced 1,600 feet above the sea. The calculated temperature, therefore, in the tunnel under the crest, would certainly be enormously higher than the observed rate. It may perhaps be suggested that something should be allowed for the steep slope of the mountain, but this slope is not sufficient to make the distance from any point of the surface much less than the distance from the crest.

It may be considered, then, that in this matter the result of the observation of temperature of the rock within the tunnel tends to shake confidence in the conclusions hitherto received with regard to the average rate of increase of heat in the interior of the earth. And this is the case in more ways than one. It is hardly possible to imagine a case more strictly average in its general nature than the one before us. The rocks in 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 ten thousand 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. Such rocks appear to possess no cause that could affect a fair estimate of the rate of increase of temperature. The result, if accepted, would reduce. the rate of increment to a degree in about a hundred feet.

It is very much to be regretted that we have not access to all the observations made, and that the character of the observations is not so good as the subject demands. At the same time there is no doubt that the general conclusion is correct, and that the average just stated is a near approximation to a correct one. It is certain that in this tunnel, which affords the best opportunities for close and accurate observation ever obtained, the result is such as to demand a reconsideration of all geological arguments based on the rapid and regular increase of temperature towards the centre of the earth. The temperature of the water already quoted at two points would seem to correspond sufficiently with the observed temperatures of the rock. Thus, in the somewhat powerful spring tapped in 1861 when the work had only proceeded a comparatively short distance, the temperature was 64°, and the superincumbent rock was about 2,500 feet. The rate is also about one