Page images
PDF
EPUB

of being hewn into the pedestal of some colossal statue ; but instead of being thus appropriated to form part of a monument, it has lately been converted of itself into a whole monument. When I last passed the way, I found it dedicated, in an inscription of nine-inch letters, to the memory of the immortal Scott.' Nature had dedicated it to the memory of one of her great revolutions ages before ; but since the dedicator had determined on adding, in Highland fashion, a stone to the cairn of Sir Walter, it would certainly have been no easy matter to have added to it a nobler one.

GRANITIC GNEISS AND SANDSTONE, WITH THE CONDITIONS OF

THEIR UPHEAVAL.

On entering on the granitic rock, we find the strata, strangely disturbed and contorted, lying, in the course of a few yards, in almost every angle, and dipping in almost every direction. And not only must there have been a complexity of character in the disturbing forces, but the rock on which they acted must have been singularly susceptible of being disturbed. The strata of the sandstone were, at the period of their upheaval, the same brittle, rigid plates of solid stone that they are now. The strata of the granitic gneiss were characterized, on the contrary, during their earlier periods of disturbance, by a yielding flexibility: they were capable of being bent into sharp angles without breaking. We see them running in zig-zag lines along the precipices, now striking downwards, now ascending upwards, now curved like a relaxed Indian bow in one direction, curved in a contrary one, like the same bow when fully bent. The strata of the sandstone, like a pile of glass-panes laid parallel, existed in a state in which they could be either raised in any given angle, or, if the acting forces were vio

now

lent and partial, broken up and shivered; whereas the granitic strata existed in the state of the same glass-panes brought to a bright red heat, and capable, from their extreme flexibility, of being bent and twisted in any direction. We find, too, that there occur occasional patches in which the lines of the stratification have been altogether obliterated. We can trace the strata with much distinctness on every side of these ; but there is a gradual obscuration of the lines, and we see what was a granitic gneiss in one square yard of rock existing as a compact homogeneous mass in the next. The effect is exactly that which would be produced in the heated panes of my illustration, were the heat kept up until portions of them began to run; and the circumstance serves to throw light on some of the other phenomena of the gneiss. The stone, in its average specimens, is a ternary, consisting of red feldspar, white quartz, and a dingy-coloured mica; but no one, notwithstanding, could mistake it for a true granite. It has its granite veins, however; and these veins, truly such in some cases, are, in not a few others, mere strata of the gneiss, which have evidently been formed into granite where they lie. There are no marks of injection,-no accompanying disturbance : all their conditions, with the exception of their being true granites, are exactly those of the layers which repose over and under them. Now the homogeneous patches serve, as I have said, to throw light on the secret of the formation of these. In one important respect the granitic rocks differ widely among themselves. Some of them contain potass and soda in such large proportions, and have such a tendency to disintegrate, in consequence, that they furnish much less durable materials for building than the better sandstones; while others, of an almost indestructible quality, are devoid of these salts altogether. Potass and soda form powerful fluxes; and it seems at least natural to infer that, should wide tracts of granitic rock be exposed to an intense but equable heat, the portions of the mass in which the fluxes exist in large proportions must pass into a much higher state of fluidity than the portions in which they are less abundant, or which are altogether devoid of them. Single strata and detached masses might thus come to be in the state of extremest fusion of which their substance was capable, and all their particles, disengaged, might be entering freely into the combinations peculiar to the plutonic rocks, when all around them continued to bear the semi-chemical, semi-mechanical characteristics of the metamorphic ones. Hence it is possibly the origin of some of those granite veins, open above, and terminating below in wedge-like points, which have so puzzled the Huttonians of a former age, and which have been so triumphantly referred to by their opponents as evidences that the granite had been precipitated by some aqueous solution.

SEPTARIA OR CEMENT-STONES OF THE LIAS.

OBSERVE these nodular masses of pale blue limestone, that seem as if they had cracked in some drying process, and had afterwards the cracks carefully filled up with a lightcoloured cement. The flaws are occupied by a rich calcareous spar; and in the centre of each mass we find, in most instances, a large ill-preserved Ammonite, which has also its spar-filled cracks and fissures, as if it, too, had been burst asunder by the process which had rent the surrounding matrix. These nodular masses are the characteristic septaria or cement-stones of the Lias, so much used in England for making a hard, enduring mortar, that has the quality of setting under water. Their bluish-coloured portions are so largely charged with the argillaceous matter of the bed in which they occur, and contain, besides, so considerable a mixture of iron, that, refusing to slake like common lime, they have to be crushed, after calcination, by mechanical means; while the fossil in the centre, and the semi-transparent spar of the cracks, are composed of matter purely cal

And from this peculiar mixture this cement seems to derive those setting qualities which render it of such value.

careous.

AMMONITES OF THE NORTHERN LIAS.

THE Ammonites of the upper beds of the Lias approach more to the type of the Ammonite communis, being comparatively flat when viewed sectionally, and having the whorls broadly visible, as in the Ionic volute; while the Ammonites of the lower beds approach in type to the Ammonite heterophyllus,-each succeeding whorl covering so largely the whorl immediately under it, that the spiral line seems restricted to a minute hollow in the centre, scarce equal in extent, in some specimens, to the twentieth part of the entire area. In other words, the Ammonites of the Upper Lias in this deposit represent, as a group, the true ammonite type ; while in the Lower Lias they approach more nearly as a group to the type of the nautilus. And not only are they massier in form, but also absolutely larger in size. I have found Ammonites in the more ponderous septaria, that fully doubled in bulk any I ever saw in the upper shales. We occasionally find nodules that, having formed in the outer rings of these larger shells, somewhat resemble the rims of wheels,-in some cases, wheels of not very diminutive size.

BELEMNITES OF THE NORTHERN LIAS.

We find the Belemnites of the lower deposit, like its Ammonites, of a bulkier form than those of the upper beds.

The Belemnites abbreviatus and elongatus, both large, massy species, especially the former, are of common occurrence ; while those most abundant in the upper beds are the Belemnites longissimus and penicillatus, both exceedingly slim species. It is worthy of remark, that Sir R. Murchison, in his list of fossils peculiar to the Lias as developed in the midland counties of England, specifies the Belemnites penicillatus as characteristic of its upper, and the Belemnites abbreviatus and elongatus of its lower division.

CUTTLE-FISH,

Is the reader acquainted with at once the largest and most curious of British Mollusca,—the cuttle-fish,-a creature which stands confessedly at the head of the great natural division to which it belongs ? Independently of its intrinsic interest to the naturalist, it bears for the commentator and the man of letters an interest of an extrinsic and reflected kind. No other mollusc occupies so prominent a place in our literature. It is furnished with an ink-bag, from which, when pursued by an enemy, it ejects a dingy carbonaceous fluid, that darkens the water for yards around, and then escapes in the cloud,- like some Homeric hero worsted by his antagonist, but favoured by the gods, or some body of military retreating unseen from a lost field, under the cover of a smoking shot. And there has scarce arisen a controversy since the days of Cicero, in which the cuttle-fish, with its ink-bag, has not furnished some one of the controversialists with an illustration. It has attained to some celebrity, too, on another and altogether different account. That enormous monster, the kraken of Norway, of which our earlier geographers tell such surprising stories, was held to belong to this curious family. And though the monster has disappeared from the treatises of our naturalists

« ՆախորդըՇարունակել »