A Manual of Elementary Geology Part 14

[62-B] Geol. Trans. second series, vol. v. p. 452.

[64-A] Conybeare and Phillips, Outlines, &c. p. 376.

[64-B] Phillips, Geology, Lardner's Cyclop. p. 41.

[65-A] See the results of the "Geological Survey of Great Britain;"

Memoirs, vols. i. and ii., by Sir H. De la Beche, Mr. A. C. Ramsay, and Mr. John Phillips.

CHAPTER VI.

DENUDATION.

Denudation defined--Its amount equal to the entire ma.s.s of stratified deposits in the earth's crust--Horizontal sandstone denuded in Ross-s.h.i.+re--Levelled surface of countries in which great faults occur--Coalbrook Dale--Denuding power of the ocean during the emergence of land--Origin of Valleys--Obliteration of sea-cliffs--Inland sea-cliffs and terraces in the Morea and Sicily--Limestone pillars at St. Mihiel, in France--in Canada--in the Bermudas.

Denudation, which has been occasionally spoken of in the preceding chapters, is the removal of solid matter by water in motion, whether of rivers or of the waves and currents of the sea, and the consequent laying bare of some inferior rock. Geologists have perhaps been seldom in the habit of reflecting that this operation has exerted an influence on the structure of the earth's crust as universal and important as sedimentary deposition itself; for denudation is the inseparable accompaniment of the production of all new strata of mechanical origin. The formation of every new deposit by the transport of sediment and pebbles necessarily implies that there has been, somewhere else, a grinding down of rock into rounded fragments, sand, or mud, equal in quant.i.ty to the new strata. All deposition, therefore, except in the case of a shower of volcanic ashes, is the sign of superficial waste going on contemporaneously, and to an equal amount elsewhere. The gain at one point is no more than sufficient to balance the loss at some other. Here a lake has grown shallower, there a ravine has been deepened. The bed of the sea has in one region been raised by the acc.u.mulation of new matter, in another its depth has been augmented by the abstraction of an equal quant.i.ty.

When we see a stone building, we know that somewhere, far or near, a quarry has been opened. The courses of stone in the building may be compared to successive strata, the quarry to a ravine or valley which has suffered denudation. As the strata, like the courses of hewn stone, have been laid one upon another gradually, so the excavation both of the valley and quarry have been gradual. To pursue the comparison still farther, the superficial heaps of mud, sand, and gravel, usually called alluvium, may be likened to the rubbish of a quarry which has been rejected as useless by the workmen, or has fallen upon the road between the quarry and the building, so as to lie scattered at random over the ground.

If, then, the entire ma.s.s of stratified deposits in the earth's crust is at once the monument and measure of the denudation which has taken place, on how stupendous a scale ought we to find the signs of this removal of transported materials in past ages! Accordingly, there are different cla.s.ses of phenomena, which attest in a most striking manner the vast s.p.a.ces left vacant by the erosive power of water. I may allude, first, to those valleys on both sides of which the same strata are seen following each other in the same order, and having the same mineral composition and fossil contents. We may observe, for example, several formations, as Nos.

1, 2, 3, 4, in the accompanying diagram (fig. 89.); No. 1. conglomerate, No. 2. clay, No. 3. grit, and No. 4. limestone, each repeated in a series of hills separated by valleys varying in depth. When we examine the subordinate parts of these four formations, we find, in like manner, distinct beds in each, corresponding, on the opposite sides of the valleys, both in composition and order of position. No one can doubt that the strata were originally continuous, and that some cause has swept away the portions which once connected the whole series. A torrent on the side of a mountain produces similar interruptions; and when we make artificial cuts in lowering roads, we expose, in like manner, corresponding beds on either side. But in nature, these appearances occur in mountains several thousand feet high, and separated by intervals of many miles or leagues in extent, of which a grand exemplification is described by Dr. MacCulloch, on the north-western coast of Ross-s.h.i.+re, in Scotland.[67-A] The fundamental rock of that country is gneiss, in disturbed strata, on which beds of nearly horizontal red sandstone rest unconformably. The latter are often very thin, forming mere flags, with their surfaces, distinctly ripple-marked.

They end abruptly on the declivities of many insulated mountains, which rise up at once to the height of about 2000 feet above the gneiss of the surrounding plain or table land, and to an average elevation of about 3000 feet above the sea, which all their summits generally attain. The base of gneiss varies in height, so that the lower portions of the sandstone occupy different levels, and the thickness of the ma.s.s is various, sometimes exceeding 3000 feet. It is impossible to compare these scattered and detached portions without imagining that the whole country has once been covered with a great body of sandstone, and that ma.s.ses from 1000 to more than 3000 feet in thickness have been removed.

[Ill.u.s.tration: Fig. 89. Valleys of denudation. _a._ alluvium.]

[Ill.u.s.tration: Fig. 90. Denudation of red sandstone on north-west coast of Ross-s.h.i.+re. (MacCulloch.)]

In the "Survey of Great Britain" (vol. i.), Professor Ramsay has shown that the missing beds, removed from the summit of the Mendips, must have been nearly a mile in thickness; and he has pointed out considerable areas in South Wales and some of the adjacent counties of England, where a series of palaeozoic strata, not less than 11,000 feet in thickness, have been stripped off. All these materials have of course been transported to new regions, and have entered into the composition of more modern formations.

On the other hand, it is shown by observations in the same "Survey," that the palaeozoic strata are from 20,000 to 30,000 feet thick. It is clear that such rocks, formed of mud and sand, now for the most part consolidated, are the monuments of denuding operations, which took place on a grand scale at a very remote period in the earth's history. For, whatever has been given to one area must always have been borrowed from another; a truth which, obvious as it may seem when thus stated, must be repeatedly impressed on the student's mind, because in many geological speculations it is taken for granted that the external crust of the earth has been always growing thicker, in consequence of the acc.u.mulation, period after period, of sedimentary matter, as if the new strata were not always produced at the expense of pre-existing rocks, stratified or unstratified. By duly reflecting on the fact, that all deposits of mechanical origin imply the transportation from some other region, whether contiguous or remote, of an equal amount of solid matter, we perceive that the stony exterior of the planet must always have grown thinner in one place whenever, by accessions of new strata, it was acquiring density in another. No doubt the vacant s.p.a.ce left by the missing rocks, after extensive denudation, is less imposing to the imagination than a vast thickness of conglomerate or sandstone, or the bodily presence as it were of a mountain-chain, with all its inclined and curved strata. But the denuded tracts speak a clear and emphatic language to our reason, and, like repeated layers of fossil nummulites, corals or sh.e.l.ls, or like numerous seams of coal, each based on its under clay full of the roots of trees, still remaining in their natural position, demand an indefinite lapse of time for their elaboration.

No one will maintain that the fossils entombed in these rocks did not belong to many successive generations of plants and animals. In like manner, each sedimentary deposit attests a slow and gradual action, and the strata not only serve as a measure of the amount of denudation simultaneously effected elsewhere, but are also a correct indication of the rate at which the denuding operation was carried on.

Perhaps the most convincing evidence of denudation on a magnificent scale is derived from the levelled surfaces of districts where large faults occur. I have shown, in fig. 87. p. 63., and in fig. 91., how angular and protruding ma.s.ses of rock might naturally have been looked for on the surface immediately above great faults, although in fact they rarely exist.

This phenomenon may be well studied in those districts where coal has been extensively worked, for there the former relation of the beds which have s.h.i.+fted their position may be determined with great accuracy. Thus in the coal field of Ashby de la Zouch, in Leicesters.h.i.+re (see fig. 91.), a fault occurs, on one side of which the coal beds _a b c d_ rise to the height of 500 feet above the corresponding beds on the other side. But the uplifted strata do not stand up 500 feet above the general surface; on the contrary, the outline of the country, as expressed by the line _z z_, is uniform and unbroken, and the ma.s.s indicated by the dotted outline must have been washed away.[69-A] There are proofs of this kind in some level countries, where dense ma.s.ses of strata have been cleared away from areas several hundred square miles in extent.

[Ill.u.s.tration: Fig. 91. Faults and denuded coal strata, Ashby de la Zouch. (Mammat.)]

In the Newcastle coal district it is ascertained that faults occur in which the upward or downward movement could not have been less than 140 fathoms, which, had they affected equally the configuration of the surface to that amount, would produce mountains with precipitous escarpments nearly 1000 feet high, or chasms of the like depth; yet is the actual level of the country absolutely uniform--affording no trace whatever of subterranean movements.[69-B]

The ground from which these materials have been removed is usually overspread with heaps of sand and gravel, formed out of the ruins of the very rocks which have disappeared. Thus, in the districts above referred to, they consist of rounded and angular fragments of hard sandstone, limestone, and ironstone, with a small quant.i.ty of the more destructible shale, and even rounded pieces of coal.

Allusion has been already made to the shattered state and discordant position of the carboniferous strata in Coalbrook Dale (p. 62.). The collier cannot proceed three or four yards without meeting with small slips, and from time to time he encounters faults of considerable magnitude, which have thrown the rocks up or down several hundred feet. Yet the superficial inequalities to which these dislocated ma.s.ses originally gave rise are no longer discernible, and the comparative flatness of the existing surface can only be explained, as Mr. Prestwich has observed, by supposing the fractured portions to have been removed by water. It is also clear that strata of red sandstone, more than 1000 feet thick, which once covered the coal, in the same region, have been carried away from large areas. That water has, in this case, been the denuding agent, we may infer from the fact that the rocks have yielded according to their different degrees of hardness; the hard trap of the Wrekin, for example, and other hills, having resisted more than the softer shale and sandstone, so as now to stand out in bold relief.[70-A]

_Origin of valleys._--Many of the earlier geologists, and Dr. Hutton among them, taught that "rivers have in general hollowed out their valleys." This is true only of rivulets and torrents which are the feeders of the larger streams, and which, descending over rapid slopes, are most subject to temporary increase and diminution in the volume of their waters. The quant.i.ty of mud, sand, and pebbles const.i.tuting many a modern delta proves indisputably that no small part of the inequalities now existing on the earth's surface are due to fluviatile action; but the princ.i.p.al valleys in almost every great hydrographical basin in the world, are of a shape and magnitude which imply that they have been due to other causes besides the mere excavating power of rivers.

Some geologists have imagined that a deluge, or succession of deluges, may have been the chief denuding agency, and they have speculated on a series of enormous waves raised by the instantaneous upthrow of continents or mountain chains out of the sea. But even were we disposed to grant such sudden upheavals of the floor of the ocean, and to a.s.sume that great waves would be the consequence of each convulsion, it is not easy to explain the observed phenomena by the aid of so gratuitous an hypothesis.

On the other hand, a machinery of a totally different kind seems capable of giving rise to effects of the required magnitude. It has now been ascertained that the rising and sinking of extensive portions of the earth's crust, whether insensibly or by a repet.i.tion of sudden shocks, is part of the actual course of nature, and we may easily comprehend how the land may have been exposed during these movements to abrasion by the waves of the sea. In the same manner as a mountain ma.s.s may, in the course of ages, be formed by sedimentary deposition, layer after layer, so ma.s.ses equally voluminous may in time waste away by inches; as, for example, if beds of incoherent materials are raised slowly in an open sea where a strong current prevails. It is well known that some of these oceanic currents have a breadth of 200 miles, and that they sometimes run for a thousand miles or more in one direction, retaining a considerable velocity even at the depth of several hundred feet. Under these circ.u.mstances, the flowing waters may have power to clear away each stratum of incoherent materials as it rises and approaches the surface, where the waves exert the greatest force; and in this manner a voluminous deposit may be entirely swept away, so that, in the absence of faults, no evidence may remain of the denuding operation. It may indeed be affirmed that the signs of waste will usually be least obvious where the destruction has been most complete; for the annihilation may have proceeded so far, that no ruins are left of the dilapidated rocks.

Although denudation has had a levelling influence on some countries of shattered and disturbed strata (see fig. 87. p. 63. and fig. 91. p. 69.), it has more commonly been the cause of superficial inequalities, especially in regions of horizontal stratification. The general outline of these regions is that of flat and level platforms, interrupted by valleys often of considerable depth, and ramifying in various directions. These hollows may once have formed bays and channels between islands, and the steepest slope on the sides of each valley may have been a sea-cliff, which was undermined for ages, as the land emerged gradually from the deep. We may suppose the position and course of each valley to have been originally determined by differences in the hardness of the rocks, and by rents and joints which usually occur even in horizontal strata. In mountain chains, such as the Jura before described (see fig. 71. p. 55.), we perceive at once that the princ.i.p.al valleys have not been due to aqueous excavation, but to those mechanical movements which have bent the rocks into their present form. Yet even in the Jura there are many valleys, such as C (fig.

71.), which have been hollowed out by water; and it may be stated that in every part of the globe the unevenness of the surface of the land has been due to the combined influence of subterranean movements and denudation.

I may now recapitulate a few of the conclusions to which we have arrived: first, all the mechanical strata have been acc.u.mulated gradually, and the concomitant denudation has been no less gradual: secondly, the dry land consists in great part of strata formed originally at the bottom of the sea, and has been made to emerge and attain its present height by a force acting from beneath: thirdly, no combination of causes has yet been conceived so capable of producing extensive and gradual denudation, as the action of the waves and currents of the ocean upon land slowly rising out of the deep.

Now, if we adopt these conclusions, we shall naturally be led to look everywhere for marks of the former residence of the sea upon the land, especially near the coasts from which the last retreat of the waters took place, and it will be found that such signs are not wanting.

I shall have occasion to speak of ancient sea-cliffs, now far inland, in the south-east of England, when treating in Chapter XIX. of the denudation of the chalk in Surrey, Kent, and Suss.e.x. Lines of upraised sea-beaches of more modern date are traced, at various levels from 20 to 100 feet and upwards above the present sea-level, for great distances on the east and west coasts of Scotland, as well as in Devons.h.i.+re, and other counties in England. These ancient beach-lines often form terraces of sand and gravel, including littoral sh.e.l.ls, some broken, others entire, and corresponding with species now living on the adjoining coast. But it would be unreasonable to expect to meet everywhere with the signs of ancient sh.o.r.es, since no geologist can have failed to observe how soon all recent marks of the kind above alluded to are obscured or entirely effaced, wherever, in consequence of the altered state of the tides and currents, the sea has receded for a few centuries. We see the cliffs crumble down in a few years if composed of sand or clay, and soon reduced to a gentle slope. If there were sh.e.l.ls on the beach they decompose, and their materials are washed away, after which the sand and s.h.i.+ngle may resemble any other alluviums scattered over the interior.

[Ill.u.s.tration: Fig. 92. Section of inland cliff at Abesse, near Dax.

_a._ Sand of the Landes.

_b._ Limestone.

_c._ Clay.]

The features of an ancient sh.o.r.e may sometimes be concealed by the growth of trees and shrubs, or by a covering of blown sand, a good example of which occurs a few miles west from Dax, near Bordeaux, in the south of France. About twelve miles inland, a steep bank may be traced running in a direction nearly north-east and south-west, or parallel to the contiguous coast. This sudden fall of about 50 feet conducts us from the higher platform of the Landes to a lower plain which extends to the sea. The outline of the ground suggested to me, as it would do to every geologist, the opinion that the bank in question was once a sea-cliff, when the whole country stood at a lower level. But this is no longer matter of conjecture, for, in making excavations in 1830 for the foundation of a building at Abesse, a quant.i.ty of loose sand, which formed the slope _d e_, was removed; and a perpendicular cliff, about 50 feet in height, which had hitherto been protected from the agency of the elements, was exposed. At the bottom appeared the limestone _b_, containing tertiary sh.e.l.ls and corals, immediately below it the clay _c_, and above it the usual tertiary sand _a_, of the department of the Landes. At the base of the precipice were seen large partially rounded ma.s.ses of rock, evidently detached from the stratum _b_. The face of the limestone was hollowed out and weathered into such forms as are seen in the calcareous cliffs of the adjoining coast, especially at Biaritz, near Bayonne. It is evident that, when the country was at a somewhat lower level, the sea advanced along the surface of the argillaceous stratum _c_, which, from its yielding nature, favoured the waste by allowing the more solid superinc.u.mbent stone _b_ to be readily undermined. Afterwards, when the country had been elevated, part of the sand, _a_, fell down, or was drifted by the winds, so as to form the talus, _d e_, which masked the inland cliff until it was artificially laid open to view.

When we are considering the various causes which, in the course of ages, may efface the characters of an ancient sea-coast, earthquakes must not be forgotten. During violent shocks, steep and overhanging cliffs are often thrown down and become a heap of ruins. Sometimes unequal movements of upheaval or depression entirely destroy that horizontality of the base-line which const.i.tutes the chief peculiarity of an ancient sea-cliff.

It is, however, in countries where hard limestone rocks abound, that inland cliffs retain faithfully the characters which they acquired when they const.i.tuted the boundary of land and sea. Thus, in the Morea, no less than three, or even four, ranges of what were once sea-cliffs are well preserved. These have been described, by MM. Boblaye and Virlet, as rising one above the other at different distances from the actual sh.o.r.e, the summit of the highest and oldest occasionally exceeding 1000 feet in elevation. At the base of each there is usually a terrace, which is in some places a few yards, in others above 300 yards wide, so that we are conducted from the high land of the interior to the sea by a succession of great steps. These inland cliffs are most perfect, and most exactly resemble those now washed by the waves of the Mediterranean, where they are formed of calcareous rock, especially if the rock be a hard crystalline marble. The following are the points of correspondence observed between the ancient coast lines and the borders of the present sea:--1. A range of vertical precipices, with a terrace at their base. 2. A weathered state of the surface of the naked rock, such as the spray of the sea produces. 3. A line of littoral caverns at the foot of the cliffs. 4. A consolidated beach or breccia with occasional marine sh.e.l.ls, found at the base of the cliffs, or in the caves. 5. Lithodomous perforations.

In regard to the first of these, it would be superfluous to dwell on the evidence afforded of the undermining power of waves and currents by perpendicular precipices. The littoral caves, also, will be familiar to those who have had opportunities of observing the manner in which the waves of the sea, when they beat against rocks, have power to scoop out caverns.

As to the breccia, it is composed of pieces of limestone and rolled fragments of thick solid sh.e.l.l, such as _Strombus_ and _Spondylus_, all bound together by a crystalline calcareous cement. Similar aggregations are now forming on the modern beaches of Greece, and in caverns on the sea-side; and they are only distinguishable in character from those of more ancient date, by including many pieces of pottery. In regard to the _lithodomi_ above alluded to, these bivalve mollusks are well known to have the power of excavating holes in the hardest limestones, the size of the cavity keeping pace with the growth of the sh.e.l.l. When living they require to be always covered by salt water, but similar pear-shaped hollows, containing the dead sh.e.l.ls of these creatures, are found at different heights on the face of the inland cliffs above mentioned. Thus, for example, they have been observed near Modon and Navarino on cliffs in the interior 125 feet high above the Mediterranean. As to the weathered surface of the calcareous rocks, all limestones are known to suffer chemical decomposition when moistened by the spray of the salt water, and are corroded still more deeply at points lower down where they are just reached by the breakers. By this action the stone acquires a wrinkled and furrowed outline, and very near the sea it becomes rough and branching, as if covered with corals. Such effects are traced not only on the present sh.o.r.e, but at the base of the ancient cliffs far in the interior. Lastly, it remains only to speak of the terraces, which extend with a gentle slope from the base of almost all the inland cliffs, and are for the most part narrow where the rock is hard, but sometimes half a mile or more in breadth where it is soft. They are the effects of the encroachment of the ancient sea upon the sh.o.r.e at those levels at which the land remained for a long time stationary. The justness of this view is apparent on examining the shape of the modern sh.o.r.e wherever the sea is advancing upon the land, and removing annually small portions of undermined rock. By this agency a submarine platform is produced on which we may walk for some distance from the beach in shallow water, the increase of depth being very gradual, until we reach a point where the bottom plunges down suddenly. This platform is widened with more or less rapidity according to the hardness of the rocks, and when upraised it const.i.tutes an inland terrace.

But the four princ.i.p.al lines of cliff observed in the Morea do not imply, as some have imagined, four great eras of sudden upheaval; they simply indicate the intermittence of the upheaving force. Had the rise of the land been continuous and uninterrupted, there would have been no one prominent line of cliff; for every portion of the surface having been, in its turn, and for an equal period of time, a sea-sh.o.r.e, would have presented a nearly similar aspect. But if pauses occur in the process of upheaval, the waves and currents have time to sap, throw down, and clear away considerable ma.s.ses of rock, and to shape out at certain levels lofty ranges of cliffs with broad terraces at their base.

There are some levelled s.p.a.ces, however, both ancient and modern, in the Morea, which are not due to denudation, although resembling in outline the terraces above described. They may be called Terraces of Deposition, since they have resulted from the gain of land upon the sea where rivers and torrents have produced deltas. If the sedimentary matter has filled up a bay or gulf surrounded by steep mountains, a flat plain is formed skirting the inland precipices; and if these deposits are upraised, they form a feature in the landscape very similar to the areas of denudation before described.

In the island of Sicily I have examined many inland cliffs like those of the Morea; as, for example, near Palermo, where a precipice is seen consisting of limestone at the base of which are numerous caves. One of these called San Ciro, about 2 miles distant from Palermo, is about 20 feet high, 10 wide, and 180 above the sea. Within it is found an ancient beach (_b_, fig. 93.), formed of pebbles of various rocks, many of which must have come from places far remote. Broken pieces of coral and sh.e.l.l, especially of oysters and pectens, are seen intermingled with the pebbles.

Immediately above the level of this beach, _serpulae_ are still found adhering to the face of the rock, and the limestone is perforated by _lithodomi_. Within the grotto, also, at the same level, similar perforations occur; and so numerous are the holes, that the rock is compared by Hoffmann to a target pierced by musket b.a.l.l.s. But in order to expose to view these marks of boring-sh.e.l.ls in the interior of the cave, it was necessary first to remove a ma.s.s of breccia, which consisted of numerous fragments of rock and an immense quant.i.ty of bones of the mammoth, hippopotamus, and other quadrupeds, imbedded in a dark brown calcareous marl. Many of the bones were rolled as if partially subjected to the action of the waves. Below this breccia, which is about 20 feet thick, was found a bed of sand filled with sea-sh.e.l.ls of recent species; and underneath the sand, again, is the secondary limestone of Monte Grifone. The state of the surface of the limestone in the cave above the level of the marine sand is very different from that below it. _Above_, the rock is jagged and uneven, as is usual in the roofs and sides of limestone caverns; _below_, the surface is smooth and polished, as if by the attrition of the waves.

[Ill.u.s.tration: Fig. 93. Cross section.

_a._ Monte Grifone.

_b._ Cave of San Ciro.[75-A]

_c._ Plain of Palermo, in which are Newer Pliocene strata of limestone and sand.

_d._ Bay of Palermo.]

The platform indicated at _c_, fig. 93., is formed by a tertiary deposit containing marine sh.e.l.ls almost all of living species, and it affords an ill.u.s.tration of the terrace of deposition, or the last of the two kinds before mentioned (p. 74.).

There are also numerous instances in Sicily of terraces of denudation. One of these occurs on the east coast to the north of Syracuse, and the same is resumed to the south beyond the town of Noto, where it may be traced forming a continuous and lofty precipice, _a b_, fig. 94., facing towards the sea, and const.i.tuting the abrupt termination of a calcareous formation, which extends in horizontal strata far inland. This precipice varies in height from 500 to 700 feet, and between its base and the sea is an inferior platform, _c b_, consisting of similar white limestone. All the beds dip towards the sea, but are usually inclined at a very slight angle: they are seen to extend uninterruptedly from the base of the escarpment into the platform, showing distinctly that the lofty cliff was not produced by a fault or vertical s.h.i.+ft of the beds, but by the removal of a considerable ma.s.s of rock. Hence we may conclude that the sea, which is now undermining the cliffs of the Sicilian coast, reached at some former period the base of the precipice _a b_, at which time the surface of the terrace _c b_ must have been covered by the Mediterranean. There was a pause, therefore, in the upward movement, when the waves of the sea had time to carve out the platform _c b_; but there may have been many other stationary periods of minor duration. Suppose, for example, that a series of escarpments _e_, _f_, _g_, _h_, once existed, and that the sea, during a long interval free from subterranean movements, advances along the line _c b_, all preceding cliffs must have been swept away one after the other, and reduced to the single precipice _a b_.

[Ill.u.s.tration: Fig. 94. Cross section.]

[Ill.u.s.tration: Fig. 95. Valley called Gozzo degli Martiri, below Melilli, Val di Noto.]

That such a series of smaller cliffs, as those represented at _e_, _f_, _g_, _h_, fig. 94., did really once exist at intermediate heights in place of the single precipice _a b_, is rendered highly probable by the fact, that in certain bays and inland valleys opening towards the east coast of Sicily, and not far from the section given in fig. 94., the solid limestone is shaped out into a great succession of ledges, separated from each other by small vertical cliffs. These are sometimes so numerous, one above the other, that where there is a bend at the head of a valley, they produce an effect singularly resembling the seats of a Roman amphitheatre. A good example of this configuration occurs near the town of Melilli, as seen in the annexed view (fig. 95.). In the south of the island, near s.p.a.ccaforno, Scicli, and Modica, precipitous rocks of white limestone, ascending to the height of 500 feet, have been carved out into similar forms.