The upheaving lava current had therefore not only the weight of the superimposed deposits, but the pressure likewise of an enormous volume of water. It becomes more than probable, therefore, that this aqueous pressure would effectually check the tendency to produce cinders and ashes, and thus as the stream poured upwards through the deposits and came in contact with the waters, the molten matter would extend itself along the bottom of the lake, and thus overlie the secondary strata, as in the present instance.

For farther information on this subject, I would refer the reader to De la Beche's Geological Manual, where will be found some very just and apposite remarks on the point in question.

"It being by no means probable," he says, "that the density of sea water beneath any depth which we can reasonably assign to the ocean, would be such as to render it of greater specific gravity than liquid lava ejected from a volcanic rent, situated beneath the sea, it would follow that so long as the lava continued in a state of fusion, it would arrange itself horizontally beneath the fluid of inferior specific gravity." The question then arises, how long a body of lava in fusion would remain fluid beneath the waters of the sea? The particles of water in contact with the incandescent lava would become greatly heated, and consequently, from their decreased specific gravity, would immediately rise their places being supplied from above by particles of greater density and less temperature. Thus a cooling process would be established on the upper surface of the lava, rendering it solid.

Now as the particles of fluid lava would be prevented from moving upwards by the solid matter above, pressed down by its own gravity and the superincumbent water, they would escape laterally, where not only the cooling process would be less rapid, from the well-known difficulty of heated water moving otherwise than perpendicularly upwards, but where also the power of the fluid lava to escape resistance would be greatest. (See plate)-FIG. 4. Let a be a volcanic rent, through which liquid lava is propelled upwards in the direction d f: the lava being of greater specific gravity than the water b h e c it would tend to arrange itself horizontally in the directions db dc The surface b d c having become solid, the lava would escape from the sides b and c, spreading in a sheet or tabular mass around; and this effect would continue so long as the propelling power at a was sufficient to overcome the resistance opposed to the progress of the lava, or until the termination of the eruption, if that should first happen."*

This clearly stated theoretic problem may now be successfully reduced to practice, and will correctly and exactly apply to the phenomenon under * De la Beche's Geological Manual, p. 125.

consideration. The truth therefore of De la Beche's proposition will be at once established.

(See plate)-FIG. 5. Let us suppose these now inclined strata to be in their original horizontal position, and 2 and 3 forming beds of unconsolidated sandy and muddy deposits beneath the waters of the lake or sea a c e h.

Then a a a a, &c. is a vein of lava or molten trap, which in its endeavours to find vent, upraises and bursts through the solid primary series denoted at 1.

By the heat and pressure thus engendered, the lava indurates the sand at 2, and converting it into sandstone, breaks through it also, and is thus brought in contact with the muddy deposits represented at 3. This deposit being of a specific gravity inferior to the stream of lava, is naturally displaced and forced to contract and furnish room for a stratum of trap at a a a.

The heat and pressure, however, continuing, speedily and almost on the instant, converts the muddy deposit into shale or slate clay. And the lava current bursting through it and the superior stratum of limestone, comes at length to the surface, and in contact with the waters. Here then commences the facts detailed theoretically by De la Beche, as already quoted, and the stratum of trap spread over the surface of the now inclined and consolidated strata of deposits; while the waters of the lake or sea being displaced by the upheavement, effected an escape through the various channels afforded by the disruption of the uprising strata.

It may possibly be objected that the occurrence of a compact stratum of limestone above the shale, and in contact with the trap, will at once invalidate the theory here proposed, from its being a known fact, that when heat is applied to calcareous matter, the carbonic acid is driven off, and the remaining lime rendered infusible.

I shall endeavour therefore to obviate such an objection, by quoting and establishing a theory long since propounded by Dr. Hutton, which at the time of its proposition was looked upon as an ingenious, but perfectly untenable, doctrine.

"He had asserted that calcareous rocks, like every other, had been subjected to the action of heat. But it was well known that when heat was applied to this class of rocks the carbonic acid was driven off in the shape of gas, and the remaining quicklime become infusible. Dr. Hutton indeed had answered this by suggesting, that the pressure of the superincumbent ocean was sufficient to confine the carbonic acid, and to cause it to act as a flux on the quicklime. His theory, however ingenious, was so abundantly gratuitous, that it by no means satisfied even his own disciples. After Dr. Hutton's death, Sir James Hall ascertained by numerous experi

ments that carbonate of lime might readily be fused when exposed to heat, if it were at the same time under a pressure not greater than Dr. Hutton's theory required, or about a mile and a half of sea."*

Now it is easily perceptible, that the result of these experiments is in exact accordance with the effects which the theory here proposed would give rise to.

We have supposed that the present solid strata were once soft and aqueous deposits beneath a vast depth of waters; we thus perceive a beautiful and conclusive illustration of Dr. Hutton's theory in the fact, that when the heat generated by the pressure and condensation from below acted on the superior calcareous stratum at 4, that very stratum was then actually subject to the pressure of the superincumbent waters at A CEH, which by preventing the escape of the carbonic acid gas, and causing it to act as a flux upon the quicklime, converted the stratum, as Dr. Hutton had suggested, into the compact state which it now exhibits.

As theoretic speculations, however just, and however much in accordance with the phenomena observable, they may prove to be, may nevertheless be deemed misplaced in a paper of this kind, I shall leave the subject for a more fitting occasion, and now pass on to a consideration of the remaining facts exhibited in the strata of the Spiti valley.

From Kewrick to the village of Leedung, the strata may be said to be of the same descriptions, namely, talcose schist, quartz rock, greywacke slates, clay slates, sandstone shales and trap, all except the last alternating frequently with each other.

A precise description of each rock belongs rather to the department of the mineralogists than to that of the geologists, and I therefore content myself with pointing out the series rather than individual species, in order that I may hasten on to the theory which the appearances presented suggest.

Passing therefore from Larree via Pokh to the fort of Dunkur, we find the strata to consist of the same alternations of rocks as those already mentioned; but at this latter spot the appearances denote a struggle for the direction of the dip, which merits some attention. The range of hills running along the right bank of the Spiti opposite to Dunkur have a N. W. by W., and S. E. and by E. direction, and at four miles below the fort the strata dip uniformly to the S. W. From that point, however, or near the village of Maness, it would seem that an upheavement had taken place through or along the centre of the range, causing the superior strata to assume a pent or roof-like appearance, throwing them on one side with

* Journal of Science, p. 4.