I believe that the true explanation of the curious phenomena presented by these caves in general, is to be found in Deluc's theory, fortified by such facts as those which I have now stated. The mean temperature of the rock at Besançon, where the elevation above the sea is comparatively so small, renders the temptation to suggest some chemical cause very strong.
The question of ice in summer where thaw prevails in winter, may fairly be considered to have been eliminated from the discussion of such caves as I have seen, in spite of the persistent assertions of some of the peasantry. The observations, however, in caverns in volcanic formations, and in basaltic débris, are so circumstantial that it is impossible to reject them; and in such cases a theory similar to that enunciated by Mr. Scrope[[193]] seems to be the only one in any way satisfactory, though I have not heard of such marvellous results being produced elsewhere by evaporation. One observer, for instance, of the cavern near the village of Both, in the Eiffel, found a thickness of 3 feet of ice; and in that case it was melting in summer, instead of forming. In some cases it has been suggested that the length of time required for external heat or cold to penetrate through the earth and rock which lie above the caves is sufficient to account for the phenomenon of summer frost and winter thaw. Thus, it is said, the thickness of the superincumbent bed may be such that the heat of summer only gets through to the cave at Christmas, and then produces thaw, while in like manner the greatest cold will reach the cave in mid-summer. But there is a fatal objection to this idea in the fact that the invariable stratum--i.e., the stratum beyond which the annual changes of external temperature are not felt--is reached about 60 feet below the surface in temperate latitudes,[[194]] while at the tropics such changes are not felt more than a foot below the surface. Humboldt calculated that in the latitude of central France the whole annual variation in temperature at a depth of 30 feet would not amount to more than one degree.[[195]]
CHAPTER XVIII.
ON THE PRISMATIC STRUCTURE OF THE ICE IN GLACIÈRES.
It was natural to suppose that the prismatic structure which I found so very general in the glacières was the result of some cause or causes coming into operation after the first formation of the ice. On this point M. Thury's visit to the Glacière of S. Georges in the spring of 1852 affords valuable information, for at that time the coating of ice on the wall, evidently newly formed, did not present the structure aréolaire which he had observed in his summer visit to the cave. He suggests that, since ice is less coherent at a temperature of 32° F.--which is approximately the temperature of the ice-caves during several months of the year--than when exposed to a greater degree of cold, its molecules will then become free to assume a fresh system of arrangement.[[196]] On the other hand, Professor Faraday has found that ice formed under a temperature some degrees below the ordinary freezing point has a well-marked crystalline structure.[[197]] M. Thury suggests also, as a possibility, what I have found to be the case, by frequent observations, that the prismatic ice has greater power of resisting heat than ordinary ice; and on this supposition he accounts for the fact of hollow stalactites being found in the Cavern of S. Georges.[[198]] At the commencement of the hot season, the atmospheric temperature of the glacières rises gradually; and when it has almost reached 32° F., the prismatic change takes place in the ice, extending to a limited depth below the surface. The central parts of the stalactites retain their ordinary structure, and are after a time exposed to a general temperature rather above than below the freezing point; and thus they come to melt, the water escaping either by accidental fissures between some of the prisms, or by the extremity of the stalactite, or by some part of the surface which has chanced to escape the prismatic arrangement, and has itself melted under increased temperature.[[199]]
M. Héricart de Thury describes the peculiar structure of the ice which he found in the Glacière of the Foire de Fondeurle.[[200]] He found that the crystallised portions were very distinctly marked, displaying for the most part a six-sided arrangement; and in the interior of a hollow stalactite he found numerous needles of ice perfectly crystallised, the crystals being some triangular and some six-sided. He was unable to detect any perfect pyramid.[[201]] I have already quoted Olafsen's observations on the polygonal lining which he saw on the surface of the ice in the Surtshellir. The French Encyclopædia[[202]] relates that M. Hassenfratz saw ice served up at table at Chambéry which broke into hexagonal prisms; and when he was shown the ice-houses where it was stored, he found considerable blocks of ice containing hexahedral prisms terminated by corresponding pyramids.
In vol. xv. (New Series) of the American Journal of Science,[[203]] an extract is given from a letter describing the 'Ice Spring' in the Rocky Mountains, which the mountaineers consider to be one of the curiosities of the great trail from the States to Oregon and California. It is situated in a low marshy 'swale' to the right of the Sweetwater river, and about forty miles from the South Pass. The ground is filled with springs; and about 18 inches below the turf lies a smooth and horizontal sheet of ice, which remains the year round, protected by the soil and grass above it. On July 12th, 1849, it was from 2 to 4 inches thick; but one of the guides stated that he had seen it a foot deep. It was perfectly clear, and disposed in hexagonal prisms, separating readily at the natural joints. The ice had a slightly saline taste,[[204]] the ground above it being impregnated with salt, and the water near tasting of sulphur. The upper surface of the stratum of ice was perfectly smooth.
In Poggendorff's Annalen (1841, Erganzsband, 517-19,--Boué, an old offender in that way, says 1842) there is an account of ice being found in the Westerwald, near the village of Frickhofen at the foot of the Dornburg, among basaltic débris about 500 feet above the sea.[[205]] Commencing at a depth of 2 feet below the surface, the ice reaches from 20 to 22 feet farther down, where the loose stones give place to dry sand. The ice is in thin layers on the stones, and is deposited in the form of clear and regular hexagonal crystals. The lateral extent through which this phenomenon obtains is from 40 to 50 feet each way, and is greater in winter than in summer. As in other cases that have been noticed in basaltic débris, the snow which falls upon the surface here is speedily melted. The Allgemeine Zeitung (1840, No. 309), from which the account in Poggendorff is taken, suggested that the melted snow-water which would thus run down among the interstices would readily freeze below the surface, while the heavy cold air of winter would be stored up at the lower levels, and the poor conducting powers of basaltic rock[[206]] would favour its permanence through the summer. The temperature of the cold current which was perceptible in the parts of the mass of débris where the ice existed was 1° R. (34°·25 F.). Nothing but a few lichens grow on the surface of the débris.
These are, I think, all the references I have met with to the prismatic structure of subterranean ice. But there is an interesting account in Poggendorff 's Annalen,[[207]] by a private teacher in Jena, of the crystalline appearance of ice under slow thaw near that town. In the winter of 1840, the Saale was frozen, and the ice remained unbroken till the middle of January, when the thermometer rose suddenly, and the river in consequence overflowed the lower grounds, and carried large masses of ice on to the fields, where it was left when the water subsided. On the 20th of January the thermometer fell again, and remained below the freezing point till the 12th of February: some of the ice did not disappear till the following month.