Imagens das páginas
PDF

such close approximation that the microscope does not detect an interstice, though the least force separates them.

"From the appearance of the fossil, its coniferous structure is almost self-evident. But to prove that it was a pine wood, as nearly as our present knowledge of fossil botany will admit of, it is necessary to examine so thin a slice that the nature of the woody fibre may be microscopically observed by transmitted light: such slices have hitherto only been prepared by the most skilful lapidary, and at a great cost. In this instance the wood is already separated into lamellae admirably adapted for this purpose, and far more beautifully than could possibly be effected by hand. Under these circumstances, with a good microscope, each of these fibres is seen to bear the distinctive character of pine-wood, being marked with a series of discs, considered as glands, and which constitute the glandular woody tissue.

"The nature of these discs is still, perhaps, disputable, and is not immediately connected with the present subject. Such a structure is nearly confined to the conifera, and is essential to them, so far as we at present know.

"Hence it is almost certain that the present fossil belonged to trees of an order whose different species never grow separately, but cover immense tracts of land with, often, a gigantic vegetation.

"How the silification was effected without there existing a bond of union between the separate fibres, is a most interesting question; and further, the nature of the cleavage of the fossil, some other circumstances connected with it, and the ease with which it can be examined, may be expected to add much to what is already known of the physiology of trees, their growth and developement."*

• "Tasmanian Journal of Natural Science," &c. No. I. p. 25.

The tree described by Dr. Hooker is found at Rose Garland, the property of Mr. Barker, to whom great credit is due, not only for having discovered this relic, but for the pains he has taken to preserve it from the injury of Vandal collectors. His unremitting exertions have led to the discovery of some other most interesting fossils, particularly of casts, in basalt, of consumed trees, which throw light upon the state of the forest at the time of the irruption.

Not less wonderful than these fossil trees, and equally interesting, are the erratic blocks or boulders found in the same valley of the Derwent. The masses are composed of cylindrical, somewhat flattened, columns of basalt, confusedly heaped together, with a detritus of pebbles mixed with spheroid boulders of greenstone rocks, all lodged against an escarpment situated at the bottom of the valley, and on the right bank of the Derwent.

This escarpment belongs to the carboniferous strata, and was once connected with another escarpment running across the bed of the river, so as to dam up the present outlet of the waters, and thus to form, in conjunction with the other lines yet existing, the perfect and continuous margin of a basin. The violence with which this embankment was burst asunder is obvious, as is also the action of the water upon it. The position of the detritus, and the direction of the axes of the columns, which he in position corresponding to the present fall of the country, that is at the lowest level of the valley, prove that the disturbing forces acted from within the basin.

This is corroborated further by the evidences of the basaltic and trachytic irruption which occurred after the deposition of the variegated sandstones in Van Diemen's Land. That irruption seems to have appeared first about Rose Garland, which is the centre of the valley. The trees there, which had been fossilised, withstood the intensity of the incandescent matter: other trees, placed in circumstances less favourable to their previous fossilisation, were consumed; but being either saturated with water, or still green, they resisted in some measure the process of combustion, and have left behind longitudinal moulds in the basaltic scoria?, with parietal cavities and impressions, similar to the rugged appearance which the carbonisation of a tree assumes externally. Into some of these moulds, a second irruptive force appears to have injected fresh lava, thus forming casts of the consumed trees, and records of the succession of volcanic agencies.

This irruption was followed by that of greenstone in the upper part of the valley; which, accompanied as it was by a sudden upward movement of the bottom, must have precipitated the waters from one side of the basin to the other, by which, the barrier being ruptured at the place where the present escarpment is seen, the drainage of the valley was effected.

In this movement an area of 1200 square miles seem to have been raised to the height of 4000 feet, and the valley to have bfeen overflowed by streams of greenstone and basalt issuing from five mouths — the present lakes of the so-called upper country of the Derwent. (PI. V. fig. 2.)

»

CONCLUSION.

In our attempt to sketch the general physical aspect of New South Wales and Van Diemen's Land, we have followed a continuous chain of mountains for upwards of 1500 miles—first along the eastern coast of New Holland to Wilson's Promontory; thence to Bass's Straits; thence again, in zigzag direction, through Van Diemen's Land; beholding it every

L s

where towering above the country through which it winds its course.

The lithological character of this chain, and that of the spurs which belong to it, has been found to be chiefly due to the presence of crystalline rocks; the irruption of some, being confined to particular epochs, while that of others has extended itself to all the geological eras — into the newest, with which our inquiry has closed.

Thus the irruption of granite, sienite, hyalomicte, and protogene was stated to have taken place only at the beginning of the first epoch; that of quartz rock and porphyries, during the first two epochs; that of basalt and its varieties, during the last two; while that of the greenstone operated continually throughout all the four.

These facts tend to the following conclusions:—

1. That the continuity of the chain in the mean

direction of N. E. to S. W., connected as it is by the islands of the Straits, shows that the action of the force which up-heaved it was uniform in direction.

2. But although uniform, .the movement was not

synchronous, on the whole line, but was exerted during four different and distinct epochs.

3. That the difference in the height of the peaks,

which range between 6500 and 1000 feet above the sea-level, proves that the uplifting movement was exerted with different degrees of intensity.

4. That the position and the character of the foci

of the maximum and minimum of that intensity is such as to lead to the connection of the origin of the chain of mountains with a series of volcanoes of "elevation," operating along a longitudinal fissure of the earth ranging from N. E. to S. W.

[ocr errors]

5. Lastly, that from the lithological character, and from the geological phenomena which have been found grouped along its course, the above mountain chain may be looked upon as the Australian Eastern Axis of Perturbation.

The sedimentary rocks which we have seen divided by that axis, and at present incumbent upon it, have been traced to four different epochs.

The First is characterised by the presence of micaslate, argillaceous and siliceous slate, and the absence of gneiss.

The Second, by the arenaceous, calcareous, and argillaceous stratified deposits, and by the following organic remains which pervade them: —

Polyparia.

Stenopora informis (Lonsdale).

tasmaniensis (Lonsdale).

ovata (Lonsdale).

crinita (Lonsdale).

Fenestella ampla (Lonsdale).

internata (Lonsdale).

fossula (Lonsdale).

Amplexus arundinaceus (Lonsdale).
Hemitrypa sexangula (Lonsdale).
Favosites gothlandica (Lamarck).

Crinoidea.
Crinoidal columns or stems.

« AnteriorContinuar »