Part 15 (1/2)

[Ill.u.s.tration: Fig. 96. Cross section.]

This appearance of a range of marble seats circling round the head of a valley, or of great flights of steps descending from the top to the bottom, on the opposite sides of a gorge, may be accounted for, as already hinted, by supposing the sea to have stood successively at many different levels, as at _a a_, _b b_, _c c_, in the accompanying fig. 96. But the causes of the gradual contraction of the valley from above downwards may still be matter of speculation. Such contraction may be due to the greater force exerted by the waves when the land at its first emergence was smaller in quant.i.ty, and more exposed to denudation in an open sea; whereas the wear and tear of the rocks might diminish in proportion as this action became confined within bays or channels closed in on two or three sides. Or, secondly, the separate movements of elevation may have followed each other more rapidly as the land continued to rise, so that the times of those pauses, during which the greatest denudation was accomplished at certain levels, were always growing shorter. It should be remarked, that the cliffs and small terraces are rarely found on the opposite sides of the Sicilian valleys at heights so precisely answering to each other as those given in fig. 96., and this might have been expected, to whichever of the two hypotheses above explained we incline; for, according to the direction of the prevailing winds and currents, the waves may beat with unequal force on different parts of the sh.o.r.e, so that while no impression is made on one side of a bay, the sea may encroach so far on the other as to unite several smaller cliffs into one.

Before quitting the subject of ancient sea-cliffs, carved out of limestone, I shall mention the range of precipitous rocks, composed of a white marble of the Oolitic period, which I have seen near the northern gate of St. Mihiel in France. They are situated on the right bank of the Meuse, at a distance of 200 miles from the nearest sea, and they present on the precipice facing the river three or four horizontal grooves, one above the other, precisely resembling those which are scooped out by the undermining waves. The summits of several of these ma.s.ses are detached from the adjoining hill, in which case the grooves pa.s.s all round them, facing towards all points of the compa.s.s, as if they had once formed rocky islets near the sh.o.r.e.[78-A]

Captain Bayfield, in his survey of the Gulf of St. Lawrence, discovered in several places, especially in the Mingan islands, a counterpart of the inland cliffs of St. Mihiel, and traced a succession of s.h.i.+ngle beaches, one above the other, which agreed in their level with some of the princ.i.p.al grooves scooped out of the limestone pillars. These beaches consisted of calcareous s.h.i.+ngle, with sh.e.l.ls of recent species, the farthest from the sh.o.r.e being 60 feet above the level of the highest tides. In addition to the drawings of the pillars called the flower-pots, which he has published[78-B], I have been favoured with other views of rocks on the same coast, drawn by Lieut. A. Bowen, R. N. (See fig. 97.)

[Ill.u.s.tration: Fig. 97. Limestone columns in Niapisca Island, in the Gulf of St. Lawrence. Height of the second column on the left, 60 feet.]

In the North-American beaches above mentioned rounded fragments of limestone have been found perforated by _lithodomi_; and holes drilled by the same mollusks have been detected in the columnar rocks or ”flower-pots,” showing that there has been no great amount of atmospheric decomposition on the surface, or the cavities alluded to would have disappeared.

[Ill.u.s.tration: Fig. 98. The North Rocks, Bermuda, lying outside the great coral reef. A. 16 feet high, and B. 12 feet. _c._ _c._ Hollows worn by the sea.]

We have an opportunity of seeing in the Bermuda islands the manner in which the waves of the Atlantic have worn, and are now wearing out, deep smooth hollows on every side of projecting ma.s.ses of hard limestone. In the annexed drawing, communicated to me by Lieut. Nelson, the excavations _c_, _c_, _c_, have been scooped out by the waves in a stone of very modern date, which, although extremely hard, is full of recent corals and sh.e.l.ls, some of which retain their colour.

When the forms of these horizontal grooves, of which the surface is sometimes smooth and almost polished, and the roofs of which often overhang to the extent of 5 feet or more, have been carefully studied by geologists, they will serve to testify the former action of the waves at innumerable points far in the interior of the continents. But we must learn to distinguish the indentations due to the original action of the sea, and those caused by subsequent chemical decomposition of calcareous rocks, to which they are liable in the atmosphere.

Notwithstanding the enduring nature of the marks left by littoral action on calcareous rocks, we can by no means detect sea-beaches and inland cliffs everywhere, even in Sicily and the Morea. On the contrary, they are, upon the whole, extremely partial, and are often entirely wanting in districts composed of argillaceous and sandy formations, which must, nevertheless, have been upheaved at the same time, and by the same intermittent movements, as the adjoining calcareous rocks.

FOOTNOTES:

[67-A] Western Islands, vol. ii. p. 93. pl. 31. fig. 4.

[69-A] See Mammat's Geological Facts, &c. p. 90. and plate.

[69-B] Conybeare's Report to Brit. a.s.soc. 1842, p. 381.

[70-A] Prestwich, Geol. Trans. second series, vol. v. pp. 452. 473.

[75-A] Section given by Dr. Christie, Edin. New Phil. Journ. No.

xxiii., called by mistake the Cave of Mardolce, by the late M. Hoffmann.

See account by Mr. S. P. Pratt, F. G. S. Proceedings of Geol. Soc.

No. 32. 1833.

[78-A] I was directed by M. Deshayes to this spot, which I visited in June, 1833.

[78-B] See Trans. of Geol. Soc., second series, vol. v. plate v.

CHAPTER VII.

ALLUVIUM.

Alluvium described--Due to complicated causes--Of various ages, as shown in Auvergne--How distinguished from rocks in situ--River-terraces--Parallel roads of Glen Roy--Various theories respecting their origin.

Between the superficial covering of vegetable mould and the subjacent rock there usually intervenes in every district a deposit of loose gravel, sand, and mud, to which the name of alluvium has been applied. The term is derived from _alluvio_, an inundation, or _alluo_, to wash, because the pebbles and sand commonly resemble those of a river's bed or the mud and gravel spread over low lands by a flood.

A partial covering of such alluvium is found alike in all climates, from the equatorial to the polar regions; but in the higher lat.i.tudes of Europe and North America it a.s.sumes a distinct character, being very frequently devoid of stratification, and containing huge fragments of rock, some angular and others rounded, which have been transported to great distances from their parent mountains. When it presents itself in this form, it has been called ”diluvium,” ”drift,” or the ”boulder formation;” and its probable connexion with the agency of floating ice and glaciers will be treated of more particularly in the eleventh and twelfth chapters.