2. Crystals containing only stone- or glass-cavities were formed from a state of igneous fusion.

3. Crystals containing both water- and stone- or glass-cavities were formed, under great pressure, by the combined influence of highly heated water and melted rock.

4. That the amount of water present in the cavities may, in some cases, be employed to deduce the temperature at which the crystals were formed.

5. Crystals containing only empty cavities were formed by sublimation, unless the cavities are fluid-cavities that have lost their fluid, or are bubbles due to fusion.

6. Crystals containing few cavities were formed slowly, in comparison with those of the same material that contain many.

7. Crystals that contain no cavities were formed very slowly, or by the cooling from fusion of a pure, homogeneous substance.

Applying these general principles to the study of natural crystalline minerals and rocks, it was shown that the fluid-cavities in rock-salt,—in the calcareous spar of modern tufaceous deposits, of veins, and of ordinary limestone,—and in the gypsum of gypseous marls, indicate that these minerals were formed by deposition from solution in water at a temperature not materially different from the ordinary. The same conclusions apply to a number of other minerals in veins in various rocks, and to many zeolites. The constituent minerals of mica-schist and the associated rocks contain many fluidcavities, indicating that they were metamorphosed by the action of heated water, and not by mere dry heat and partial fusion.

The structure of the minerals in erupted lava proves that they were deposited from a mass in the state of igneous fusion, like the crystals in the slags of furnaces; but, in some of those found in blocks ejected from volcanos (for example, in nepheline and meionite), there are, besides stone- and glass-cavities, many containing water, the relative amount of which indicates that they were formed, under great pressure, at a dull red heat, when both liquid water and melted rock were present. The fluid-cavities in these aqueo-igneous minerals very generally contain minute crystals, as if they had been deposited on cooling from solution in the highly heated water. The minerals in trappean rocks have also such a structure as proves them to be of genuine igneous origin, but they have been much

altered by the subsequent action of water, and many minerals formed in the minute cavities by deposition from solution in water.

The quartz of quartz-veins has a structure proving that it has been rapidly deposited from solution in water: and in some instances the relative amount of water in the fluid-cavities indicates that the heat was considerable. In one good case the temperature thus deduced was 165o C. (329° F.); and apparently, when the heat was still greater, mica and tinstone were deposited, and in some cases probably even felspar. There is then, as has been argued by M. Elie de Beaumont, a gradual passage from quartz-veins to those of granite, and to granite itself; and there is no such distinct line of division between them as might be expected if one was a deposit from water, and the other a rock that had been in such a state of pure igneous fusion as the slags of our furnaces or the erupted lavas. When the constituent minerals of solid granite, far from contact with the stratified rocks, are examined, it is seen that they also contain fluid-cavities. This is especially the case with the quartz of coarsegrained, highly quartzose granites, in which there are so many, that the proportion of a thousand millions in a cubic inch is not at all unusual; and the enclosed water constitutes from one to two per cent. of the volume of the quartz. However, besides these fluid-cavities, the felspar and quartz contain excellent stone-cavities, precisely analogous to those in the crystals of slag, or erupted lavas; and thus the characteristic structure of granite is seen to be the same as that of those minerals formed under aqueo-igneous conditions in the blocks which are ejected from modern volcanos; and the very com. mon occurrence of minute crystals inside the fluid-cavities still further strengthens this analogy.

The conclusion to which these facts appear to lead, is that granite is not a simple igneous rock, like a furnace-slag, or erupted lava, but is rather an aqueo-igneous rock, produced by the combined influence of liquid water and igneous fusion, under similar physical conditions to those existing far below the surface at the base of modern volcanos. These deductions of the author, therefore, strongly confirm the views of Scrope, Scheerer, and Elie de Beaumont; and he agrees with them in considering it probable that the presence of the water during the consolidation of the granite was an instrumental, if not the actual cause of the difference between granite and erupted trachytic rocks.

B. Page 901.-LOGAN- OR ROCKING-STONES.-In that most successful of all the attempts to clothe science in the garb of fancy, -Dr. Paris's delightful volume called "Philosophy in Sport made Science in Earnest,”—there is an interesting account of the rockingstones of Cornwall, which the antiquaries of the last century claimed as Druidical monuments, but which have originated in the natural causes explained in the following description of the celebrated Loganor logging-stone, near the Land's End:

"The foundation of this part of the coast of Cornwall is a stupendous group of granite rocks, which rise in pyramidal clusters to a great altitude, and overhang the sea. The celebrated Logan-stone is an immense block, weighing above sixty tons. The surface in contact with the under rock is of very small extent, and the whole mass is so nicely balanced, that, notwithstanding its magnitude, the strength of a single man applied to its under edge is sufficient to make it oscillate. It is the nature of granite to disintegrate into rhomboidal and tabular masses, which, by the further operation of air and moisture, gradually lose their solid angles, and approach the spheroidal form. The fact of the upper part of the cliff being more exposed to atmospheric agency than the parts beneath will sufficiently explain why these rounded masses so frequently rest on blocks which still preserve the tabular form; and since such spheroidal blocks must obviously rest in that position in which their lesser axes are perpendicular to the horizon, it is equally evident that, whenever an adequate force is applied, they must vibrate on their point of support." Philosophy in Sport, sixth edition, p. 465.

C. Page 918.—The Rev. J. B. Reade, F.R.S. &c., on Fossil InFUSORIA IN MICA.

In a letter to the Author, in reply to an inquiry respecting the possibility of the existence of the organic structure in granite, the Rev. J. B. Reade, after referring to his previous observations on the indestructibility by heat of some of the silicious and calcareous structures of the higher plants (see p. 716), and pointing out that the silicious parts of the Diatomacea and of some of the Protozoa also retain their forms and characters after exposure to fire,marked as follows:-" My original inquiry having thus conducted me to the conclusion, that silicious organization is not destructible by the agency of heat, I thought it not unreasonable to infer that a care

-re

ful and more extended microscopical examination into the condition of silica might lead to the discovery of elementary organic forms even in the primitive strata themselves. It was obviously not necessary to exclude granite from this examination, under the common and apparently natural impression, that the igneous fusion which preceded the present arrangement of its particles would destroy every trace of organization; for I had before me too many manifest proofs that an intense white heat, though capable of fusing glass, was incapable of effecting any change in the minute silicious organization both of plants and diatoms. Moreover there appeared to be a strong suspicion in some minds that every successive surface of our globe had been characterized by its own minute living forms; and you yourself had more than once contended for the existence of life during the granitic period. To give a reality, however, to a first condition, thus pronounced to be probable, we must discover silicious skeletons or shields even in granite itself. But here arises a difficulty which it will baffle our utmost ingenuity to remove; for, though, on the one hand, I met with silicious corpuscules in the primitive rocks, and find, on the other hand, that the indestructible

LIGN. 212.-MINUTE FLAT CIRCULAR BODIES IN MICA.
Corresponding in size and appearance with the rings of GALLIONELLA DISTANS.
Magnified about 500 times linear.

(Discovered and drawn by the Rev. J. B. Reade.)

organic skeletons of recent Infusoria exhibit, even under a power of 900 linear, a striking similarity of form, yet the entire absence of external structure precludes me from assigning a common origin to the ancient and recent organisms. Still, the inquiry, even in its present state, is far from being fruitless; for it cannot be a matter of surprise, that immense mountain-masses should have been found to consist of an aggregation of symmetrical bodies between and ʊʊʊ of an inch in diameter, articulated together in the form of rings

as in chalk (Lagn. 212), or of slender threads, as in limestone and the quartz of granite, and that an exact counterpart of this curious structure in the mineral kingdom should be exhibited in the vegetable by the mouldiness of paste and the Gallionella ferruginea."

LIGN. 213.-The elementary molecules of Chalk, articulated in the form of rings, entire and in fragments. The Rev. J. B. Reade.)

The Rev. J. B. Reade, in a courteous reply to the Editor's inquiry on some points of the interesting researches detailed above, states that

"Some of the discoid-looking spots which I observed in granite might possibly have been the minute air- or water-cavities of Sorby (see p. 883); but those discovered in mica (and figured by Mantell) had too much the appearance of silicious organic structure to allow me to confound them with such cavities."

The Lign. 213 exhibits another interesting result of the Rev. Mr. Reade's microscopical researches in the minute structure of rocks, and was published in the first edition of the "Wonders of Geology." Although at first regarded as being illustrative of the occurrence of infusorial organisms in the chalk, these beaded, discoid, and ring-like bodies are more probably the result of the disintegration of the shells of Foraminifera (as Mr. H. C. Sorby has suggested in connexion with some of his own observations on the minute structure of limestones, &c.). Be this as it may, we cannot but hope that Mr. Reade, Mr. Sorby, Mr. Schafhault, Mr. Bryson, Prof. Ehrenberg, and others will continue their labours in "Microgeology" or "Clinology," as this branch of research has been termed; for it is a wide and promising field, but little cultivated as yet.

D. Page 992.-PARALLEL TERRACES OF GLEN ROY.-I am induced to notice, in this place, a remarkable phenomenon observable in some