CHAPTER XXIII.

ON THE FORMATION OF VALLEYS, AND THE GEOLOGICAL THEORIES RELATING TO VALLEYS AND DENUDATIONS.

On the Causes that have broken the Surface of the Globe.-Erosive Action of running Water, illustrated by the Process called Hushing-Bursting of Lakes. -Some valleys originally formed by Elevation or Subsidence, and subsequently enlarged by the Action of Water.-Different Theories respecting the Formation of Valleys.-Theory of Werner-of Hutton.-Of Elevation. Of the retiring Waters of the Ocean.-Theory of Excavation and Denudation by Deluges. Modification of this Theory by Sir James Hall; its Application to Explain Denudations, and Transportation of Blocks of Granite from the Alps.-Particular Phenomena, presented by the scattered Blocks in the Vicinity of Geneva.-Denudation of Stratified Rocks, effected by the same Causes which have broken the Primary Rocks, and scattered their Fragments into distant Districts.

FROM what has been stated in the preceding chapter, respecting the elevation and submersion of the earth's surface, the geological student might infer, that such elevations and submergences offer a satisfactory explanation of the formation of valleys, but the inference would be erroneous. There are two distinct causes which have modified the surface of the globe; the one internal, dependent on the atmosphere which surrounds it; beside these, there is the ceaseless flux and reflux of the ocean, dependent on the attractive forces of the sun and moon, and on the earth's diurnal revolution on its axis. The two former causes have been principally concerned in the formation of valleys; and there are few valleys in which the combined effects of both these causes may not be traced. The inequalities of surface produced by the upheaving of mountain ranges, or the emergence of continents from the ocean, must have originally determined the course of the retiring water, or of atmospheric water precipitated in rain. Of the power of atmospheric water, to act upon the surface of the globe, we can form but a very feeble idea, froin what we observe in our own country. In warm climates, as much rain will sometimes fall in one hour, as falls at different times, during three months, in northern latitudes: added to this, when the rain descends in mountainous regions, the water is suddenly collected into powerful rivers, rushing with incredible violence to the lower valleys. At remote epochs, it is highly probable that many elevated depressions, which are now mountain valleys in alpine regions, upheld the waters and formed lakes, that have subsequently burst their barriers, and have ploughed a passage for the succeeding rivers, when the drainage of the country became more regular.

To enable the reader to form some notion of the force of falling water, carrying with it loose stones that occur in its passage, it may be useful to describe a process called hushing, in Westmoreland.

The quarrymen, when in search for good beds of slate, where the side of a mountain is covered with stones and vegetation, form a lake or pool near the top of the mountain, by damming up a rivulet where it passes through a depression or small valley. When the water has accumulated in sufficient quantity, they dig a trench near the dam, to direct the current where they wish it to flow, and then break down part of the dam. The water flows first through the trench, and rushing with accelerated velocity down the mountain, carries with it the stones near the surface, and in a very short time ploughs a deep channel in the rocks, exposing every bed to view. Thus, in a few hours is effected, what the labour of many men, continued for months, could not have accomplished. I have been informed, that in the upper part of the valley of Long Sleddale, when the process of hushing takes place, the river Ken, (as it flows by Kendal, twelve miles distant,) is made turbid for some days, by the quantity of débris carried into it. If such an effect can be produced by the small quantity of water thus pent up, it will not be difficult to believe, that the bursting of extensive mountain lakes, may have scooped out passages for mighty rivers. Even the bursting of a small mountain lake, in the valley of Bagnes, in the year 1818, produced the most terrific effects. The lake had been formed by a barrier of ice damming up the river at a great elevation: this barrier suddenly gave way, and precipitated the water into the great valley of the Rhone, near Martigny, tearing down and overturning every obstacle it met in its passage. From the quantity of mud and stones which it bore along, it resembled a moving mass of stones and earth. An English gentleman who was descending the valley at the time, observed his horse exhibit by its motions, great trepidation, of which he could not discover the cause, until a loud rushing noise occasioned him to look back, when he beheld what appeared like a wall filling up the bottom of the valley, and advancing rapidly towards him. He instantly alighted, and scrambled up the adjacent rocks, leaving his horse to its fate. Two years afterwards, when I was at Martigny, the desolating ravages of this catastrophe were apparent.

Many of the valleys in the Alps have, evidently, once been lakes. The upper valley of the Rhone, from its source to Martigny, formed one lake the whole valley of Geneva, between the Alps and the Jura, formed a lower and more extensive lake, before a passage was opened for the water at Porte l'Ecluse. When a fissure was once made by earthquakes or by subsidence, the rushing of water charged with stones, would enlarge and deepen the passage, and thus lay dry and reduce the ancient lakes in a comparatively short period. In the year 1819, part of a mountain immediately above the river Isère, and opposite to the city of Moutiers, in the Tarentaise, suddenly fell down into the river, and formed a dam across it, over which persons might pass from one side to the other. When I was there in the year 1821, all this mass of stone, had been carried away by the riv

er. The action of rivers in extensive and level valleys, tends rather to fill them with débris, brought from the more elevated countries in which the rivers had their origin, than to excavate them deeper.

The formation of the greater number of valleys cannot be explained by the action of water alone. There are valleys of elevation formed by the raising of the strata on each side-valleys of subsidence, formed by the sinking of the ground, leaving the adjacent rocks unmoved-valleys formed on the line of faults, in which the rocks on one side have been thrown up or depressed-valleys of disruption, where a range of mountains, or an extent of country has been rent by earthquakes or by subsidence. Most of the valleys formed originally by these causes, have been subsequently enlarged or modified by the action of water. There are, indeed, instances of valleys and ravines formed entirely by the continued erosion of water; such is the valley of Niagara, between Queenstown and the Falls. (See the frontispiece to the present volume.) Other instances might be cited, in which the action of water is equally evident. In many cases, however, where water appears to have been the sole agent in excavating rocks, I am inclined to believe, that an original break or fissure has greatly accelerated the process. In many broader valleys, the excavation must often have been effected by more powerful agents than any which we perceive in present operation; and when a broad outlet is once made, the subsequent drainage of a country may work its way to the sea in a very sinuous course; but this sinuous course, does not prove that the valley had been originally formed by the river that flows through it.

Besides the action of nountain torrents, the bursting of lakes, and the regular flowing of rivers, many geologists believe that the excavation of valleys, and the transportation of loose rocks, have been effected by the more powerful agency of the ocean, thrown over the surface of the land by the great convulsions that have upheaved mountain ranges and continents. For the benefit of the geological student, I shall endeavour to give a brief outline of the principal theories that have been maintained respecting the formation of valleys; but the first of these theories is now admitted to be untenable.

The formation of valleys has been ascribed to the following caus

es:

1st, To the original unequal deposition of the earth's surface. 2d, To excavation, by the rivers that flow through them. 3d, To the elevation or subsidence of part of the earth's surface. 4th, To excavations, caused by the sudden retreat of the sea from our present continents.

5th, To excavations by inundations or deluges, that have suddenly swept over the surface of different parts of the globe. I shall notice the leading facts that favour or oppose each of these theories. The disappearance of large portions of strata from districts

The quarrymen, when in search for good beds of slate, where the side of a mountain is covered with stones and vegetation, form a lake or pool near the top of the mountain, by damming up a rivulet where it passes through a depression or small valley. When the water has accumulated in sufficient quantity, they dig a trench near the dam, to direct the current where they wish it to flow, and then break down part of the dam. The water flows first through the trench, and rushing with accelerated velocity down the mountain, carries with it the stones near the surface, and in a very short time ploughs a deep channel in the rocks, exposing every bed to view. Thus, in a few hours is effected, what the labour of many men, continued for months, could not have accomplished. I have been informed, that in the upper part of the valley of Long Sleddale, when the process of hushing takes place, the river Ken, (as it flows by Kendal, twelve miles distant,) is made turbid for some days, by the quantity of débris carried into it. If such an effect can be produced by the small quantity of water thus pent up, it will not be difficult to believe, that the bursting of extensive mountain lakes, may have scooped out passages for mighty rivers. Even the bursting of a small mountain lake, in the valley of Bagnes, in the year 1818, produced the most terrific effects. The lake had been formed by a barrier of ice damming up the river at a great elevation: this barrier suddenly gave way, and precipitated the water into the great valley of the Rhone, near Martigny, tearing down and overturning every obstacle it met in its passage. From the quantity of mud and stones which it bore along, it resembled a moving mass of stones and earth. An English gentleman who was descending the valley at the time, observed his horse exhibit by its motions, great trepidation, of which he could not discover the cause, until a loud rushing noise occasioned him to look back, when he beheld what appeared like a wall filling up the bottom of the valley, and advancing rapidly towards him. He instantly alighted, and scrambled up the adjacent rocks, leaving his horse to its fate. Two years afterwards, when I was at Martigny, the desolating ravages of this catastrophe were apparent.

Many of the valleys in the Alps have, evidently, once been lakes. The upper valley of the Rhone, from its source to Martigny, formed one lake the whole valley of Geneva, between the Alps and the Jura, formed a lower and more extensive lake, before a passage was opened for the water at Porte l'Ecluse. When a fissure was once made by earthquakes or by subsidence, the rushing of water charged with stones, would enlarge and deepen the passage, and thus lay dry and reduce the ancient lakes in a comparatively short period. In the year 1819, part of a mountain immediately above the river Isère, and opposite to the city of Moutiers, in the Tarentaise, suddenly fell down into the river, and formed a dam across it, over which persons might pass from one side to the other. When I was there in the year 1821, all this mass of stone, had been carried away by the riv

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