of the following night. These acting first upon the edges of the air-holes and open spaces between the ice and the shore, caused slight undulations then in the ice itself, and the consequent pulling apart of the feebly cohering prisms, so that, the water surfaces being thereby enlarged, a short time only was necessary for the waves, increasing in altitude and force with the enlarging water surfaces, to send their undulations far before them under the yielding ice. The prisms falling upon their sides, all more or less immersed, affording now large surfaces to the solvent action of water above the melting temperature, and stirred about

by the waves, were quickly dissolved. It is not easy to say

in how short a time, under such circumstances, the great transformation would be wrought, but there ought to be no surprise that all was accomplished in the eight or ten hours of a spring night.

The preliminary process, before alluded to, of the conversion of masses of solid ice into an aggregation of vertical prisms by partial solution must be dependent on the fact that the law of crystallization in that substance yields prisms with vertical axes. That this is the law is indicated by cleavage as well as by solution; for while this is easy and free in planes perpendicular to the upper surface, it is said, truly I believe, not to be attainable in directions oblique or parallel thereto. Beyond this general fact of a vertical arrangement of prisms it is not necessary to go for elucidation of our subject, even if I could give minute specifications as to the crystallization of ice. I am not aware, indeed, that this question in crystallography, interesting as it might prove, has been very thoroughly investigated; but however that may be, we have demonstrated to us by the nat ural process of solution, that ice formed as in the case before us, however solid and homogeneous in appearance, contains a hidden array of crystalline prisms. So much is certain; and this, for the present, is enough on that point. May we not farther assume, that in the process of arrangement about the axes of these prisms, as they are projected downwards into the freezing water, the particles of water, in obeying the law of crystallization, crowd out, radially, the portions of air that would otherwise interfere with their just disposition as ice; and that, at last, this air, by accumulation in spaces between the prisms, suffices to prevent further obedience to the symmetrical principle, causing in these spaces, a confused and porous crystallization peculiarly favorable to the action of a solvent? Whether this be the precise cause or not, a condition favorable to dissolution certainly exists in the irregular spaces between the prisms, as we see by the particulars before given.

The process of Daniel for bringing to view the innate crystallization of apparently amorphous masses--namely, submitting

them to the action of a solution of the same substance, so nearly saturated as to exercise solvent power only when the solidification is imperfect seems to afford a close analogy to that followed by nature, in preparing ice for quick dissolution.

The natural action seems to be this. The early rains of spring throw upon the surface, and by the tributaries, pour under the fields of ice frequent supplies of water, at a temperature melting even at first, and rising with the progress of the year. This warm underlying water, acting chiefly on the porous spaces between the prisms, dissolves them out to the full depth to which the ice is immersed, and perhaps still farther, by capillary action. At the same time, the spongy ice, formed upon the upper surface by melted and refrozen snow, affords warm water, by melting and percolation, to affect similarly the porous spaces between the tops of the prisms.

In this way, during the considerable period intervening between the first spring rains and the final breaking up of the lake, the solid ice is transformed into the condition necessary to a sudden dissolution.

We may assume, indeed, that the solvent action begins on the lower surface, about the time the accretion, by farther freezing, ceases; that it proceeds very slowly, so long as the temperature of the water remains below that of the greatest density, and of course that it goes on more rapidly as the water is lifted above that temperature by the growing warmth of spring.

I regret that I did not take the temperature of the water in the morning after the disappearance of the ice; but on this point I may add to what is said above, that the spring was then well forward, all, or nearly all, the snow had melted from the fields; the early rains and melted snows had for some time been raising the lake, which was then nearly at its greatest height. It was this rise in the lake that had spread a margin of water that did not freeze between the great field of ice and the shore. The inference from all the circumstances, that the temperature of the water at the time of disruption, and for some time previous, was not only above the melting point, but also above that of maximum density, seems to me unavoidable.

I may here be permitted to mention another matter connected with fields of lake-ice that has excited some wonder, namely, the movement towards the shore of boulders, sometimes quite large. The process which must have occurred to intelligent observers, and has probably been heretofore explained, seems to be this: after the rising of the water has supplied an unfrozen margin, a strong wind will sometimes cause the whole field to move until its edge meets adequate resistance upon the shore, all boulders encountered in the way, being pushed before it, into an array upon the shore that accurately marks the extent of the

invasion. These lines of boulders are to be seen in many places, registering accurately, not the work of the preceding year, but the greatest effort of any previous year.

The circumstances of some deep-lying boulders may be such that they are rarely embraced, acted on, or moved, and such may long, by fits, continue to be erratic, though finally to join the general shore parade.

The force of these moving fields is very great, even when the decomposing process is much advanced. I have seen a timber wharf, which was about thirty feet square, ten or fourteen feet high, and filled solidly with earth and stones, shoved along the bottom about thirty feet, by a single continuous push of a great field of ice just ready to be resolved into its prismatic elements. The motion was very slow, only to be seen, indeed, by close observation, while the ice was broken at the edge of contact into innumerable fragments, piling themselves, with a tinkling sound, high upon the wharf and following ice.

A simple and effectual guard against this danger to wharf or pier has been found to be, the giving to the exposed face a certain talus (about one of base to two of height, I think), which turns the ice upwards to the top of the structure, where its fragments accumulate, sometimes to a considerable height. This easy diversion of so great a force is due, of course, to the peculiar crystalline structure of the ice, the degree to which it has been decomposed, and the consequent brittleness against a transverse strain. Should there be an unfrozen margin to permit this motion of large fields of ice, before the solution of continuity in the crystalline arrangement, nothing but the solid earth. could stand before it.

These remarks have extended further than I intended, and I fear much beyond what was required by the state of knowledge on the subject. But I venture, nevertheless, in reference to the first portion of these remarks, one further observation-namely, that nature seems to have especially provided, in the structure of these wintry coverings of water surfaces, for their prompt removal, when their existence would retard the advancing year.

ART. XL.-On some Reactions of the Salts of Lime and Magnesia, and on the Formation of Gypsums and Magnesian Rocks; by T. STERRY HUNT, F.R.S., of the Geol. Survey of Canada. (Continued from this vol., p. 187.)

IV.

Facts in the history of Gypsums, Dolomites, Magnesites and Lime

stones.

43. The gypsums found in nature may be divided into two classes, those directly deposited from water, and those produced by the alteration of beds of limestone. To the latter division belong the gypsums found in the vicinity of solfataras, where, as Dumas has shown, the slow oxydation of moist sulphuretted hydrogen gives rise to sulphuric acid, which transforms beds of carbonate into hydrated sulphate of lime. We must equally refer to the same class those gypsums which are formed among calcareous rocks by the action of waters containing free sulphuric acid. Such a process I have long since described in Western Canada, where numerous springs containing besides sulphates of lime, magnesia, oxyd of iron, alumina, and sulphuretted hydrogen, three or four thousandths of free sulphuric acid, rise through Upper Silurian strata, in the calcareous portions of which they sometimes give rise to masses of gypsum.

Bischof (Chem. Geology, i, 418), who does not appear to have seen my analyses of these acid waters, rejects my view of the epigenic origin of these masses of gypsum, although it will be apparent to every one who examines the facts, that the action of such waters upon calcareous strata must give rise to sulphate of lime. I do not however confound these recently formed masses of sulphate of lime with the older gypsums, which associated with dolomites, sea-salt and sulphur, are abundant in the Saliferous or Onondaga salt group of the same region.-(Am. Jour. Sci., [2], vii, 175; Report Geol. Survey, 1848, 150; Comptes Rendus de l'Acad., 1855, xl, 1348.)

These acid waters which make their appearance in an almost undisturbed region, I conceive to have their origin in deeply buried strata, where gypsum or other sulphates may be undergoing decomposition by the action of water and silica at an elevated temperature, a process analogous to that which gives rise to exhalations of carbonic acid gas.

44. Waters containing free sulphuric acid or ferric or aluminous sulphate, may by flowing into basins where carbonate of lime is present, give rise to solutions of sulphate of lime, and the evaporation of these, of sea-water or other gypseous solutions must give rise to deposits of sulphate of lime, which will

belong to the first division mentioned above. These modes of formation however do not account for an important fact in the history of most stratified gypsums, which is that of their almost constant association with carbonate of magnesia generally in the form of magnesian limestone. Beds of dolomite are often interstratified with or include beds or masses of gypsum, while dolomite and carbonate of magnesia are sometimes found imbedded in gypsum or anhydrite. For a description of the magnesite which is disseminated in the gypsum of Salzburg, see Dufrénoy, Minéralogie, 2d ed., ii, 424. Small masses of compact and crystalline gypsum, occasionally associated with crystals of calcite and quartz, abound in some of the dolomite beds of the so-called Calciferous sandrock in Canada, and crystallized gypsum and anhydrite, together with sulphates of baryta and strontia, and fluor spar, occur in geodes in the magnesian limestone of Niagara. The anhydrous sulphate of lime not only forms beds by itself but is often met with disseminated in masses, grains or crystals through beds of gypsum, and even interstratified with it, as in the south of France, in the Hartz, Switzerland, and in Nova Scotia, as described by Mr. Dawson. (Acadian Geology, 225.) The conversion of beds of anhydrite into gypsum by the absorption of water, and the attendant phenomena, have been described by Charpentier.

45. Both the hydrous and anhydrous sulphate sometimes form the cement of conglomerates or breccias, which enclose flints, fragments of shale and of limestone, as at Pomarance in Tuscany, (Scarabelli, Bull. Soc. Géol. de France, [2], xi, 346,) and also at Bex, where the cement of the conglomerate is a granular anhydrite (Charpentier, Ibid., [2], xii, 546).

Gypsums moreover often include clay and sand, and sometimes contain a considerable admixture of carbonate of lime, which in those of Aix, according to Coquand, amounts to eight per cent. The gypsums of Montmartre also contain, according to Delesse, besides some clay and sand, and several hundredths of carbonate of lime, not less than three per cent of soluble silica intermixed. Silica in the form of flint or chert is sometimes found in concretions with gypsum; thus in the miocene clays near Bologna in Italy, flints are met with associated with sulphates of lime, of baryta and strontia, together with pyrites and sulphur. Masses of sulphate of strontia are likewise found in clays with the gypsums of Montmartre, and the association of sulphate of strontia with the sulphur, gypsum and rock salt of Sicily is well known. The gypsums of Madrid, which occur in tertiary clays, are according to Casiano de Prado, accompanied by beds of chert and of magnesite (Bull. Soc. Géol. de France, [2], xi, 334).