crystals or granules, in the manner of a flux, and thus assist the liquefaction of the mass, and drive it up any fissure that might open for its passage, in a pasty or semi-liquid state, short of complete fusion; so that on reaching the outer air, and parting with the contained steam, together with the heat which the expansion of this element would carry off in a latent form, its surface would consolidate instantly; and the solidification extending rapidly inwards by the formation of crevices giving an outlet to further vapour, the resulting rock would exhibit a cellular or porous structure, and a granular or crystalline texture, in lieu of the compact and glassy one which, if completely fused, it should possess.

These ideas were received at the time with incredulity. But their correctness is now not far from being generally recognised. For example, Professor Phillips, in his recent volume on Vesuvius, admits "the fact that water is present abundantly in the fluid lava," (p. 305), and again (p. 311)," steam pervades some parts-perhaps every part of the fluid mass." He adds, "Lava cooled rapidly might be expected to be glassy in texture. This, however, is rarely, or never, the case in Vesuvius." It is strange that neither the Professor, nor other observers, should have sought for the cause of a fact so different from what was to be expected on the supposition of the complete fusion of the lava-yet so common, that, with the exception of the vitreous lavas of Lipari, Hawaii, Bourbon, and the trachytic obsidians and pearlstones, it may be declared to be universally true of all lavas, whether trachytic or doleritic.

But if we admit the existence of water, or steam, in close and intimate dissemination through every part of a mass of subterraneous lava, and of the rocks from whose liquefaction or semi-fusion by intense heat it proceeds, how are we to believe that this water could have found its way into this position from superficial seas or lakes, through fissures suddenly opened by the earthquakes which accompany an eruption? Such sudden influx of a body of water to a heated mass of mineral matter beneath, might be conceived to give rise to some equally sudden explosion at the point of contact; but the explosion itself, and the rise of lava up the fissure must, it is to be presumed, check any further penetration of water. The effect would be superficial only, and could scarcely so completely saturate the entire mass of heated rock, whether in a liquid or solid state at the time, as to cause its general ebullition, continued too through such lengthened periods as volcanic eruptions are often known to last.

Moreover, if we suppose an earthquake to cause the eruption of a volcano by suddenly admitting a body of water to its heated focus, what, it must be asked, causes the earthquake? Not certainly the influx of this same water through the rents which the earthquake itself only originates. The effect cannot produce the cause. Surely it is more reasonable to suppose that a local increase of heat transmitted from the sides or from beneath a volcanic focus, which had been for a time cooled down by the emission of steam and lava in earlier eruptions, gives occasion to the expansion of a body of subterranean mineral matter (whether in a solid, or fluid, or some intermediate con

dition, but already permeated with water), and by elevatory shocks produces the earthquake-rents, and, if any one of these communicate with the outer air, a volcanic eruption-the lava rising up the fissure to the surface, and the water in it flashing into steam wherever diminished pressure permits, especially in the upper part of the vent, from which the explosions always proceed. Nor is this view of the phenomena of eruption opposed to the general principles of the most eminent geologists, but the contrary. For example, Sir C. Lyell, in several passages of his Principles (see Ed. 1869, p. 233) represents the original motive power of both earthquakes and volcanos to be the lateral shifting of internal heat from one part of the subterranean matter to another; increase of heat occasioning expansion and elevatory movements accompanied by jarring rents causing earthquakes. If, then, we believe the internal heated matter already to contain water, which we know from examination does exist in all granitic and metamorphic rocks, the production of rents in this manner, giving partial freedom to the expansibility of the water, will account at once for earthquakes and volcanic eruptions, without supposing a flood of water from above suddenly to find its way down to and to penetrate every part of the interior of the heated rock in some unintelligible manner. Sir C. Lyell, indeed (Principles, Ed. 1869, p. 233), supposes the rents that have admitted this water may be suddenly closed above, and the water converted into steam below find its way somehow under a mass of fluid lava, which it may drive up some neighbouring volcanic vent. But he has himself shown in an earlier passage (p. 221) that it is only in the upper part of the column of lava in a volcanic vent that the "red-hot or white-hot water entangled in it, under tremendous pressure,' ," is enabled by the reduction of that pressure to flash into steam, and produce the explosive phenomena of a volcanic eruption; and he fails to show-and does not even seem conscious of the necessity for explaining-how "atmospheric or sea-water suddenly descending from above" could become 80 entangled" within and throughout every part of the lava.

I am aware that the theory I am opposing is considered to derive support from the very general occurrence of volcanic vents within or near to seas or large bodies of superficial water. But this same geographical position would result from the generally admitted fact that the continental tracts have been elevated above the sea-level by internal expansions of deeply-seated matter which could not force its way outwards; where, in the words of Mr. Mallet, "uncompleted efforts to establish a volcano" have occurred. Where the effervescent matter beneath has been enabled to find vent, there no elevation of the seabottom (to any great extent) will have taken place, but rather subsidence. And hence the great lines of volcanic eruption on the globe's surface are found within or in the immediate vicinity of areas of subsidence; in other words, of seas or great inland lakes. And further, if we suppose, as may well be conceded, that the elevatory action by which chains of mountains have been (probably through a succession of shocks) raised, is accompanied by the formation of more or less distant downward opening rents parallel to the axis of the elevated

range, we shall find a cause for the general parallelism of the great lines of volcanic eruption to those of the nearest mountain chains or coast-lines of raised land,- —a parallelism which has been often remarked, but unaccounted for that I am aware of upon any other hypothesis (see Volcanos, ed. 1862, p. 309).

One word more. If we are to suppose the water of lavas to have been derived through all past time from the superficial ocean, where are we to seek, it may be asked, the origin of the water of the ocean itself, if not from the interior of the globe, which even now sends out torrents of aqueous vapour from every rent opened through its crust? The believers in the nebular hypothesis will no doubt find it in the original gaseous atmosphere left after the condensation of the neuclus. But even conceding this for the bulk of the ocean, still it may well be supposed that large quantities of water remained "entangled" in the condensed matter.

Without, however, looking back to the beginning of things, as too many geologists are in the habit of doing, in order to explain phenomena of daily occurrence, I think I have shown reason for the belief that the water which evidently permeates the lava beneath a volcanic vent, and by its violent expansion occasions an eruption, existed there before the earthquakes that usually accompany the eruption began, and was not suddenly introduced by the opening of fissures communicating with seas or lakes above. Whether it existed in the material whence the lava is formed from the beginning, or proceeded from any chemical changes in this elementary rock, or magma-or had penetrated there by slow and long-continued filtration from above (which is the opinion of M.M. Daubrée and Fouquè), I do not hazard a conjecture. Our knowledge, at present, of the effects of intense heat and pressure, whether chemical or mechanical, on mineral substances of the influence of terrestrial magnetism-and of the nature and origin of the deeply-seated matter that composes the globe, are too imperfect, I think, in the present state of science, to enable us to solve such problems. But since it has become the fashion, of late, among the leaders of popular geological treatises, to assume as a matter of fact, beyond dispute, that the substance of the globe, immediately beneath its thin superficial crust (and probably to its centre), is in a state of fluid fusion, and that the access of water from the sea above to this molten interior, is the exciting cause of earthquakes and volcanos, I have thought it well to express my reasons for entertaining doubts, to say the least, as to the correctness of either hypothesis.

FAIRLAWN, COBHAM, April 10, 1869.

III.-NOTES ON CONTINENTAL GEOLOGY AND PALEONTOLOGY.
BY THOMAS DAVIDSON, F.R.S., F.G.S.

HAV

(Continued from p. 166).
(PART II.)

AVING in my last communication presented the most recent views entertained by M. Coquand, I now proceed to mention those held by Monsieur Hébert, a most experienced observer, who

CLASSIFICATION OF THE UPPER CRETACEOUS PERIOD, by M. HÉBERT, March, 1869.

Formations or (étages) stages.

STRATA (ASSISES).

Sandstone of the Wanting, except, perhaps in Bel- Maine with Ostrea gium,

biauriculata and

Anorthopygus or-
bicularis.

3. GAULT.4

LOWER CRETACEOUS PERIOD.

Wanting.

Wanting.