of that of iodine, a still more remarkable image emerged on mercurializing. This image, like the former, was circular and black, but all round it for a certain space there was an annulus of narrow dimensions of pure unmercurialized silver, the deep black of which contrasted strikingly with the blue black of the spectre, and its outer circumference was marked by a faint whitening of the plate,-faint, but as sharp as it is possible to conceive.

In a third trial things were conducted as before, except that now chlorine, diluted with atmospheric air, was used; the spectre again came out, and did not differ in any observable manner from that produced by iodine.

In a fourth trial the vapour of nitrous acid was used as a detithonizer. In this case the edges of the spectre commonly had a gradually shading outline, and only in one instance did I find that sharpness of termination exhibited in the other cases.

We therefore perceive that iodine, bromine, chlorine, and nitrous acid can detithonize a surface on which light has fallen: they can undo what the tithonic rays have done.

In repeating these experiments, as for example the only one by iodine, if the common iodine-box be used to effect the detithonization, two or three seconds of time is all that is required. A longer period is demanded when the vapour is very weak, but when strong the effect is almost instantaneous.

This detithonization and production of spectral images can therefore be accomplished in an incredibly short space of time.

I made trials with other substances, such as hydrogen gas and the vapour of liquid muriatic acid. The former to a certain extent though not near so powerfully as the electro-negative bodies mentioned, could procure the change in question; the latter seemed to be without any perceptible action.

To the list, with the other electro-negative substances, I believe oxygen ought to be added; for, on repeating the same experiment and raising the temperature of the plate in atmospheric air so as to maintain the tithonized surface of about 200° Fahr. for several minutes, a certain effect which in an imperfect way resembled those already described was exhibited. Oxygen, therefore, diluted as in atmospheric air, at 200° Fahr., may be regarded as possessing to a small extent the property in question.

Without multiplying the description of these experiments further, -for the ingenuity of any one who repeats them will suggest many modifications which may give rise to striking results,-I will in conclusion give the reasons which have led me to suppose that in all these phenomena two different principles are engaged,-vapour action, and radiation.

I have stated that the Electro-negative bodies possess this detithonizing quality in a very marked manner. I do not wish it to be understood, however, that there is any relation of antagonization between that particular class of substances and the tithonic rays. It appears to me that their peculiar quality, in the circumstances

described, may be traced to the fact, that silver, an electro-positive substance, happens to afford the sensitive surface. I have however prepared a paper which takes up the consideration of the conditions and theory involved, and will not at present anticipate what has to be offered when that paper shall be published.

The action, then, which these different gases and vapours exhibit, is so intense as to mask the feebler effect of radiation. Thus it takes several hours' exposure in the dark, and after a long subsequent process of mercurialization, to prove the true radiant effect,→ a slow detithonization, which could be brought about by vapour action in an instant. But whoever has seen the symmetrical or rather geometrical lines that are left, when the slower process is followed, must be struck with the persuation that the phenomenon he witnesses is obeying geometrical laws, and is not due to the irregular action of a dilute and varying current of vapour.

Thus, on repeating carefully the experiment cited at the close of my last paper, in which a lens is laid on a tithonized surface and left in the dark, I found that after the mercurial process was completed, the plate exhibited a dark central spot surrounded by a white annulus. On drawing upon paper a section of the lens and the sensitive surface, I found that a line drawn from the extreme edge of the white annulus to the edge of the lens was a tangent to the lens at that point; that a line drawn from the extreme edge of the central dark spot would, after reflection by the convex surface of the lens, be found precisely on the edge of the white annulus; the edge of the annulus and the edge of the spot thus having a true catoptrical relation to the curvature of the lens.

Now, although in laboratories such has that in which my experiments are conducted, the vapours of these differrent electro-negative bodies are unquestionably present, and may produce a part of the phenomenon witnessed, yet inasmuch as that phenomenon follows laws that are apparently of a strict geometrical kind, and to those floating vapours we could hardly assign any thing like symmetrical results,-guided, also, by the analogy of cooling bodies, which lose part of their heat through radiation, and part through the current action of the air, and part through the conducting power of their supports, I have been led to take the view of the phenomena in question which I have set forth.

University, New York, Dec. 5, 1842,

(Phil. Mag.)

On the Geology of Egypt. By Lieut. NEWBOLD, F.R.S., of the

Madras Army*.

MR. NEWBOLD first describes the physical features of Egypt, and 2ndly, the formations of which the country is composed.

I. Physical Features.-After alluding to the natural boundaries of Egypt, namely, the Mediterranean on the north, the Libyan desert on the west, the mountains of Nubia on the south, and the Red Sea, with the Isthmus of Suez, on the east, and stating that the area Proceedings of the Geological Society.

thus circumscribed comprises about 100,000 square miles, the author shows that Egypt has three great physical divisions: 1. the mountainous region extending between the Red Sea and the Nile; 2. the deserts east and west of the Nile; and 3, the fertile valley of that river, with its delta.

The mountainous region is naked and deary in aspect, on account of the deficiency of springs, rain, and dew: and it presents bare or sand-covered rocks, intersected by deep ravines. The peculiarly tabular features of Central and part of Upper Egypt are due to the horizontal stratification of the prevailing limestone, which supports the desert districts, and terminate near the banks of the Nile, from Esneh to Cairo, in mural escarpments. Between Kossier and Ghen. nah the aspect is rendered more varied and irregular by pinnacles and dome-shaped masses of plutonic or hypogene rocks. The deserts present a series of undulating plains sometimes studded with clusters of low hills, and are covered chiefly with unproducted saline, often calcareous and gypseous sand, marl, and gravel. The Oases of the deserts and the mountainous region, Mr. Newbold regards simply as valleys, supplied with moisture either by springs or by the drainage-water of the deserts, held up by the impervious clay constituting the subsoil. In a few cases the moisture, he thinks, may be due to percolation from the Nile. The greatest altitude of the desert between Suez and Cairo is about 700 feet above the "ocean;" and its general character between the Red Sea and the Nile is that of a flattish irregular plateau rising towards the centre and terminating in each direction in abrupt escarpments. The flat marshy districts between Suez and Pelusium are stated, on the authority of Laborde, to be 24 feet below the sea-level.

The aspect of the valley and delta of the Nile varies with the seasons, presenting while the country is inundated a vast freshwater lake, studded with palm-shaded hamlets; and after the subsidence of the waters, exhibiting along the course of the river a line of brilliant verture winding through higher sterile tracts. When the grain has been gathered, the prospect consists of one monotonous brown, dusty plain, traversed by the sluggish Nile. The dip of the country from the first cataract to the Mediterranean is, according to Mr. Wallace, only two inches in a mile; but the descent a little north of Assuan is seven inches, lessening however on approaching the delta, and the canal between the Nile and Alexandria, a distance of sixty miles, has not a single lock.

From the horizontal stratification of the rocks composing the greater part of Egypt, it is difficult, Mr. Newbold says, to trace any particular line of elevation. The mural cliffs which flank the valley of the Nile to the vicinity of Cairo, there deviate towards the east and west, and similar but less abrupt cliffs flank both shores of the Red Sea. This horizontal formation is traversed 'by valleys and ravines or wadis, having a north and south, and east and west direction, or which intersect each other at right angles, the most considerable being that of the Nile.

In the eastern desert of Upper Egypt, Mr. Newbold traced these valley to a north and south anticlinal line, caused by plutonic rocks which attain an altitude of 1000 feet above the sea-level; and their upheaval, he says, accounts for the intersecting systems of valleys, and illustrates forcibly the truth of Mr. Hopkins's observations on the laws of fracture. In the vicinity of the erupted rocks the sedimentary strata exhibit considerable proofs of disturbance, but as the distance increases the inclination diminishes, proving, Mr. Nowbold states, that the strata were elevated to their present position with no more force than was necessary to produce the fissures or valleys; and he adds, that in proportion as the horizontality is recovered, the frequency, depth, and extent of the fissures decrease.

*

Some of the valleys, as that of Kossier, are considered to have been widened by aqueous causes no longer in operation, and that of the Nile by the still continued erosion of the river; while others, as the Bahr-bila Maieh, or waterless river, and that which separates the petrified wood formation from the Red Mountain, to have been formed entirely by them. The surface of these valleys is covered, for the greater part, with the detritus of the neighbouring rocks and of distantly transported rolled pebbles, which often rest on ledges and hills much above the general drainage-level. In the valley of Kossier, near the Red Sea, the gravel consists principally of pebbles of plutonic and hypogene rocks derived from the interior; but near the hills or to the westward of the parent rocks few of these pebbles are found, proving, the author says, the eastwardly direction of the transporting currents.

The natural drainage of the country is remarkably simple. The greater portion of the small quantity of rain which falls in Central and Upper Egypt is absorbed by the deserts and collected in natural basins like the Oases; the remainder is partly carried off by the great evaporation, and partly conducted to the Red Sea by transverse cracks on the eastern side of the anticlinal axis, or to the valley of the Nile by similar cracks on the western flank of that axis. The drainage of the Libyan dessert is also effected through the valley of the Nile. The amount of water which escapes by these means is however so small, that the Nile throughout the last 1350 miles, or about one-half of its course, does not receive what may be termed a single tributary.

II. Formations.-The deposits of which Egypt consists, are arranged by Mr. Newbold under the heads of, 1. hypogene rocks with argillaceous schist, 2. breccia di verde, 3. lower sandstone, 4. marine limestone, 5. upper sandstone, 6. post-pliocene deposits, 7. drift, 8. volcanic rocks, 9. alluvial accumulations, 10. sand-drifts.

1. Hypogene Rocks.-These constitute a small portion of Egypt. Between the Red Sea and the Nile, Mr. Newbold observed them only

• Mr. Newbold objects to the opinion entertained by some travellers that this valley was anciently the channel of the Nile, as it contains no rich dark-coloured alluvium.

in the latitude of Kossier (26° 8') resting against granite in highly inclined or curved strata, and forming an east and west zone 30 miles in breadth. He is of opinion that the same beds may probably range south by east, hypogene rocks appearing at Gebel Zerbára (lat. 24° 30′). In a northerly direction they have been traced to the cataracts, resting on "granite."

Gneiss, with thin "veins" of marble, usually constitutes the lowest strata, which are overlaid conformably by micaceous talcose, hornblende, and argillaceous schists and quartzite. Dykes or masses of basalt, greenstone, porphyry, and serpentine are associated with the whole series. All the hypogene rocks assume a crystalline character near the granite or trap, the gneiss and hornblende schist becoming garnetiferous and abounding in actynolite, both crystallized and compact; the talcose schist also passes into potstone and nephrite with iron pyrites, as at Mount Baram; the micaceous schists at Gebel Zerbára yields emeralds, avanturine, hæmatitic, and specular iron ore; and the clay-slate changes into basanite or flinty slate.

2. Breccia di Verde.-The argillaceous slate is overlaid conformably, in lat. 26° 8', by the celebrated breccia di verde. This rock presents thick-bedded strata, which become more horizontal on receding from the granite, and is composed principally of angular and rounded fragments of greenstone, gneiss, porphyry, argillaceous and flinty slates, serpentine and marble, also sometimes of light green compact felspar and hypogene rocks, cemented by a slightly calcareous paste of various shades of green and purplish red. No fossils have yet been noticed in the rock. The cliffs composed of this breccia rarely exceed 200 feet in height above the level of the desert.

3. Lower Sandstone.-Above the breccia di verde occurs a sandstone of apparently limited extent, and confined to the southern part of Egypt, passing thence into Nubia. It is displayed on both flanks of the anticlinal axis between Kossier and Ghennah, reposing near Bir Anglaise conformably on greenstone; it is exposed also on the banks of the Nile, and, according to Lefevre, it ranges from a little south-west of Esneh (lat. about 26° 10′) nearly to Syene or Assuan, 70 miles, where it is dislocated by the syenite, and near its junction with that rock passes into a conglomerate and becomes agatiferous; it also, from the smallness of the fragments composing the breccia strata, and the altered cyrstalline structure of the mass near the plutonic rocks, often resemble certain porphyries, but the true nature of the rock is easily recognizable in the beds at a greater distance.

The sandstone varies from a loose siliceous aggregate with a felspathic, calcareous or ferruginous cement, to a compact quartz rock; and the pebbles in the interstratified breccia consist usually of chert, flinty slate, agate or jasper. Associated with the sandstone are occasionally thin beds of green and purple clay, containing gypsum and chloride of soda. Veins of white, brown, and amethystine quartz also traverse it, and copper as well as specular iron ore are stated to have been found in it near Hummamet. This sandstone was exten

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