which they have once evidently covered, is also a phenomena of frequent occurrence; and its explanation must be sought from some of the same causes, that have excavated valleys. This appearance of the strata is called a denudation.

The first of the above theories is that of Werner: he supposed that all the matter of which primary, transition, and secondary rocks are formed, was originally held in solution by water, and that the water, so saturated with mineral matter, covered the whole globe. The primary rocks of granite were formed by chemical precipitation, and their peaked summits and declivities were the result of their original deposition. On the steep sides of these primary mountians were subsequently deposited the different schistose rocks, and all the secondary strata. During the time that these rocks were depositing, the water, though nearly saturated with mineral matter, was capable of supporting animal life, and the shells and remains of zoophytes and fish, were enveloped in the strata, at the period of their deposition. According to this theory, when the water retired from the present continents, the mountains and valleys were already formed.

The theory of Werner requires for its support the admission of conditions, which appear in the present state of our experience impossible, and it is at variance with existing phenomena. The vertical position of beds of puddingstone, sandstone, and the tertiary strata in the Alps, could not have been their original one; nor can the bendings and contortions of the strata, so common in Alpine countries, be explained by original deposition. A further account of part of Werner's theory is given, Chap. IX. p. 148. Werner's theory of valleys is altogether distinct from that theory which ascribes the formation of valleys to inequalities of the surface, subsequently enlarged by the erosion of water.

The second theory, that all valleys have been excavated by the rivers that flow through them, was maintained by Dr. Hutton and Professor Playfair: it formed a part of their general theory of the earth; the leading propositions of which are, that the surface of the present continents is wearing down by the action of the atmosphere and by torrents, and that the materials are carried by rivers into the sea, and there deposited. At a future period, these materials will be melted or consolidated by subterranean heat under pressure of the ocean, and subsequently, by the expansive force of central fire, the bed of the ocean will be elevated, and form new continents. According to this theory, our present continents have been also formed from the ruins of a preceding world, and elevated by a similar cause. is only with that part of the Huttonian system, which relates to the excavation of valleys, that we have at present any concern.

It

It is remarkable, that a theory which maintains that the continents were raised from the ocean by subterranean fire, should limit the formation of valleys to the action of the rivers that run through them; for if the land were raised by an expansive power acting from be

neath, it seems to follow as a necessary corollary, that the surface would be unequally elevated and broken into inequalities by the same cause; unless we suppose, that every part presented an equal degree of resistance to the moving force. There must, therefore, have been originally, inequalities or valleys, which determined the direction of the water-courses in the first instance, though the form of these valleys may have been subsequently modified by the action of water. That all valleys have been excavated by the rivers that flow through them, is opposed by many decisive facts. Before their excavation, the water must have had less force than at present, as the fall would be gentle; and the present effect of rivers in large valleys, is not to excavate them deeper, but to fill them with alluvial depositions.

There are numerous deep valleys in the Alps, that are closed at one end by steep mountains or perpendicular walls of rock, and which were originally closed, and are now nearly closed, at the other end also. Such are the valley of Thones, near Annecy, the valley of Chamouni, and on a larger scale, the valley of Geneva. It is evident that the valley of Thones, and that of Geneva, have once been filled with water, and formed lakes: by an earthquake, or by the erosion of water, a fissure has been made, which has drained the greater part of these valleys; but it is obvious that the valleys could not have been formed by the original lakes, or by the rivers that flowed into them. If valleys were formed by the erosion of rivers, the lakes through which these rivers flow, must have long since been filled up by the materials brought into them. To say that the lakes were once deeper than at present, is giving up the theory; for lakes are only the deeper parts of valleys.

Had the valley of Borrowdale, in Cumberland, been excavated by the water that flows from it, the lake of Keswick, at its entrance, must have received all the materials, and been long since choked up. Or had the valley of the Rhone, ten thousand feet deep and sixty miles in length, been excavated by the Rhone, the quantity of matter brought down by this river, would not only have filled the lake of Geneva, into which it empties itself, but the broad valley in which the lake lies, must also have been filled up, and raised to the height of the Jura. That the Lake of Geneva, and all lakes into which large rivers flow, are gradually filling up, has been before stated; but the valley of the Rhone is not, nor are other valleys becoming deeper. The upper part of this valley, as before stated, has evidently been itself a lake, closed in, or nearly so, by the rocks at Martigny.

The action of torrents in Alpine districts may have been sufficient to widen fissures already made, or to scoop out glens, in the softer beds on the sides of mountains; but they appear inadequate to the original formation of large longitudinal valleys. Water-courses running on the edges of nearly vertical beds, may scoop out a portion

or that the continents emerged, by an expansive force acting beneath them, the effect on the water would be nearly the same. This effect, in scooping out valleys, has been compared to what may be observed in miniature "by the drainage of the retiring tides on muddy shores, especially in confined estuaries, where the fall is considerable and rapid," the water cutting out channels for its passage, as it drains off. The retiring of the ocean suddenly from the present continents, would be a cause sufficient for the excavation of valleys; but I have stated, in the preceding Chapter, the reasons for believing, that continents emerged from the ocean, by the long continued action of an upheaving or expanding force.

The fifth theory, which ascribes the formation of valleys, and the extensive denudations of the strata, to deluges that have suddenly swept over different parts of the globe, has been maintained by Professor Pallas and Sir James Hall. The former conjectured, that the inundations that have covered parts of the Asiatic continent with blocks of stone, beds of gravel, and marine remains, were occasionWithin ed by the formation of volcanic islands in the Indian ocean. the period of authentic history, extensive inundations have been occasioned by volcanoes and earthquakes, which afford probability to the opinion of Pallas. In the year 1650, a new volcanic island rose from the sea in the Grecian Archipelago; and according to the account of Kircher, a contemporary writer, it occasioned the sea to rise forty-five feet in height, at the distance of eighty miles, and destroyed the galleys of the Grand Signior in the port of Candia. The principal damage done by earthquakes to cities adjoining the sea, is often effected by an enormous wave, the sea, retiring from its bed in the first instance, suddenly returns with a prodigious swell, and in a few moments rushes over the adjacent country.

Sir James Hall has given greater extension and consistency to this speculation. He supposes that the upheaving of a large island, like Sumatra, might take place so suddenly as to drive the ocean with great impetuosity over the summits of the highest mountains, and strip off the glaciers, and transport them into distant countries. Ice being specifically lighter than water, the glaciers would carry away with them, the blocks of stone that had fallen from the impending rocks, and had become encased in ice. This theory of Sir James Hall's would, I conceive, offer a better explanation than any other, for the occurrence of groups of fragments of particular rocks, unmixed with fragments of other rocks. Each glacier, loaded with stones from the rocks above it, may be regarded as a ship freighted with specimens of its native mountains, which it deposits, by thawing, in the place where it ultimately rests. Nor would a wave or swell of the sea, that had covered the highest mountains, suddenly subside; it would sweep repeatedly over the whole surface of the globe, at a lower and lower level each time; breaking down opposing obstacles, opening new passages for the water, and scooping out

lakes, and those which empty themselves in the gulf of Mexico, are only separated at their sources, by elevations not exceeding a few feet, and when swelled by rain, the northern and southern rivers sometimes interlock. In this plain there are no mountains. These lakes were probably formed by partial subsidences, at the epoch when the whole country was upheaved from the ocean. The efforts of elevation and depression have been described in the preceding chapter.

Transversal vallies, or those which cut through mountain ranges, nearly at right angles to the direction of the ranges they intersect, may have been originally fissures or openings, made either at the period when the ranges were elevated, or subsequently, by the same causes that have rent and displaced the secondary strata. These fissures may have been afterwards widened by the erosion of water.

Geologists seem now generally agreed, that the action of rivers is not sufficient to explain all the phenomena of valleys, and still less to account for the fragments of rocks scattered over extensive plains, at an immense distance from Alpine districts, where rocks similar to these fragments occur. Another phenomenon, of more importance, is altogether inexplicable by the action of rivers. Immense tracts of the secondary strata, several hundred feet in depth, have in some districts been torn off, and the materials entirely removed, except detached patches, which here and there form isolated caps on distant hills; and incontestably prove, that they were once parts of one continuous stratum or formation. Numerous instances of this might be cited in our own island. It is probable that the beds of chalk that form the north and south downs of Sussex, once extended over the Wealden beds. See p. 192. This local disappearance of a stratum or formation, has properly been called Denudation. The theory advanced by Mr. Farey, to explain these denudations, was, that the surface had been broken and swept away, by the near approach of a comet. But the most rational explanation that can be offered, is that which ascribes the effect to a mighty deluge, sweeping over the surface of the globe, tearing off part of its crust, and transporting the fragments into distant regions, or into the ocean. The case is one which may be truly said to be dignus vindice nodus, and the geologist is compelled to call in the aid of Neptune; for none of the causes in present activity (however we may imagine them to be increased in power or magnitude,) will be found adequate to produce the denudation of an extensive district, and the disappearance of the stony materials, by which it was covered.

The fourth theory, which attributes the formation of valleys to the sudden retreat of the sea from our present continents, is founded on the admitted fact, that the sea has once covered them; and whether we suppose that the bed of the ocean was deepened in one part by a sudden subsidence, which drew off the water from another part;

or that the continents emerged, by an expansive force acting beneath them, the effect on the water would be nearly the same. This effect, in scooping out valleys, has been compared to what may be observed in miniature "by the drainage of the retiring tides on muddy shores, especially in confined estuaries, where the fall is considerable and rapid," the water cutting out channels for its passage, as it drains off. The retiring of the ocean suddenly from the present continents, would be a cause sufficient for the excavation of valleys; but I have stated, in the preceding Chapter, the reasons for believing, that continents emerged from the ocean, by the long continued action of an upheaving or expanding force.

The fifth theory, which ascribes the formation of valleys, and the extensive denudations of the strata, to deluges that have suddenly swept over different parts of the globe, has been maintained by Professor Pallas and Sir James Hall. The former conjectured, that the inundations that have covered parts of the Asiatic continent with blocks of stone, beds of gravel, and marine remains, were occasioned by the formation of volcanic islands in the Indian ocean. Within the period of authentic history, extensive inundations have been occasioned by volcanoes and earthquakes, which afford probability to the opinion of Pallas. In the year 1650, a new volcanic island rose from the sea in the Grecian Archipelago; and according to the account of Kircher, a contemporary writer, it occasioned the sea to rise forty-five feet in height, at the distance of eighty miles, and destroyed the galleys of the Grand Signior in the port of Candia. The principal damage done by earthquakes to cities adjoining the sea, is often effected by an enormous wave, the sea, retiring from its bed in the first instance, suddenly returns with a prodigious swell, and in a few moments rushes over the adjacent country.

Sir James Hall has given greater extension and consistency to this speculation. He supposes that the upheaving of a large island, like Sumatra, might take place so suddenly as to drive the ocean with great impetuosity over the summits of the highest mountains, and strip off the glaciers, and transport them into distant countries. Ice being specifically lighter than water, the glaciers would carry away with them, the blocks of stone that had fallen from the impending rocks, and had become encased in ice. This theory of Sir James Hall's would, I conceive, offer a better explanation than any other, for the occurrence of groups of fragments of particular rocks, unmixed with fragments of other rocks. Each glacier, loaded with stones from the rocks above it, may be regarded as a ship freighted with specimens of its native mountains, which it deposits, by thawing, in the place where it ultimately rests. Nor would a wave or swell of the sea, that had covered the highest mountains, suddenly subside; it would sweep repeatedly over the whole surface of the globe, at a lower and lower level each time; breaking down opposing obstacles, opening new passages for the water, and scooping out