CHAPTER X. CHAP. X. PART III. THE forms of valleys exhibit phænomena, which, in the view of the mineral geology, suppose physical operations that cannot be limited to the periods of time and the revolutions represented in the Mosaical geology and chronology. It is especially in the formation of valleys, that this science observes; Time, which has such narrow limits for us, has none for Nature; for her, it is as indefinite as space: both surpass 66 even the conception of our imagination1!" In assigning therefore a cause for those formations, it makes the indefiniteness of time answerable for the soundness of its conclusions. To reduce that indefiniteness of time into finite parts and smaller measures, it has indeed suggested, for our convenience, what it terms ecliptic days, borrowed from the archives of the old Chaldaic philosophy: "If (it says) the dif "ferent epochas or revolutions in which our planet was reduced from a chaotic state to its present habitable form, be measured by those See above, p. 24. 66 66 CHAP. X. great ecliptic days, (each consisting of above PART III. 20,000 years,) sufficient time will be allowed "for the various changes'." This is very accommodating; but, one material thing is here forgotten, which defeats the whole intent; and that is, to inform us who it is that has authority, either to use that measure, or to make that allowance: for certainly, it is not left ad libitum of the mineral geology. There is no article in geology, in which the mineral system betrays more manifestly its need of a guide to conduct it, with relation both to fact and time, than in speculating upon the causes which produced valleys. Let us hear it pronounce its own speculations upon this subject. "The disposition, direction, and "structure of valleys, their form, the stratifica"tion of the mountains which border them, are indications of their origin. Every body has remarked the manner in which rain-water, especially after a violent thunder-shower, "furrows the surface of hillocks, or any ground presenting a sloping surface. In the disposi"tion of these minute ravines relatively to the "declivity on which they occur, in the "sinuosities, and deviations of their directions, "in their ramifications, in the form of the 1 BAKEWELL, Elem. of Geol. p. 429. portions of soil enclosed between them, &c. we possess the most faithful representation of "the facts of the same kind, which are presented "by valleys and mountains. In examining in "our cabinets the models in relief of mountain ous chains, we seem to behold some of those very hillocks furrowed in the manner which "has been described. At the sight of this singular disposition and ramification of valleys, we cannot but acknowledge, with "Mr. Playfair, that it is the blows of the same "instrument often repeated, which has engraved these characters so deeply on the surface of the globe; and this is, above all things, the " erosive and decomposing force of water. With "respect to myself, it is impossible for me to " doubt it, when, stationed in a chain of moun"tains, I consider all these characters, that is "to say, the valleys of different orders and "their gorges; and when I see them all, even "to their last ramifications, directed according "to the line of the greatest declivity of the "sides of the mountains. For, in fact, the "waters are capable of producing the effect; they tend to produce the effect; and every "thing is as if they had actually produced it1." D'AUBUISSON, tom. i. p. 245, 6. CHAP. X. There is nothing more fallacious, or against PART III. which reason ought habitually to guard itself with more vigilant circumspection, than general and superficial resemblances which tend "na 66 turally to lead us to infer identity." We have exposed this fallacious principle, in the first part of this disquisition; and, perhaps, few more striking examples to confirm the importance of the remark could be found, than this which this able mineralogist here inadvertently affords us. It is evident, that in this scheme of geology, the origin of valleys and of the beds of rivers must be ascribed to one and the same cause; namely, the action of water, eroding and excavating" avec le tems1-with the aid of time," both the valleys and the river-beds. For, the excavation of river-beds, must have been a continuation of the effect of the excavation of the valleys from which their waters proceeded; and the erosion of the valleys must therefore, according to this hypothesis, have been accompanied by the erosion of the channels which carried off the waters that are supposed to have formed the valleys; so that the river-beds, equally with the valleys, will answer to the minute ravines furrowed by the erosive and decomposing power of 'D'AUBUISSON, tom. i. p. 238. CHAP. X. PART III. the rain-water. The question therefore proceeds, from the power of water to erode and excavate, generally; but it terminates, in its power to erode and excavate the channels of rivers, in particular. Let us then consider this supposed operation in the latter case, with some attention; because, if physics and sound reasoning should find that it is an operation impracticable in the latter case, it will go to determine the probability of the operation in the former case. Now, it will be plain to reflection: 1. That there is no known power, or law, in what we denominate nature, by the operation of which the waters of a river could form for themselves a bed, such as are the actual beds of rivers, in a surface originally compact, extended, and nearly horizontal. 2. That the waters of rivers would never have reached the distant points at which they now discharge themselves into the sea, if beds leading to those points had not previously been opened to conduct them thither. 3. That there would, consequently, have been no rivers on the globe, if river beds had not been provided, anterior to the procession of their waters. 1. It is granted, that a mass of waters descending from a mountainous ridge, through a passage determining the first diameter of its bulk, would form a column of water whose |