We are alfo of opinion, that the more temperate, as well as more minute, comparison of the British conftitution with that which has been adopted by the National Affembly, is much better calculated to render us fatisfied with our prefent form of government, than the paffionate rhapsodies with which we, on this fide of the water, have been lately amufed. *** A tranflation of M. NECKER's work has appeared in our own language, fince the preceding article was written;—of which farther notice will be taken in our Review. Cog. ART. XIII. FOURTH LETTER to Dr. JAMES HUTTON, F. R. S. Edinburgh, On the THEORY OF THE EARTH. Windfor, Aug. 29, 1791. SIR, IN N my last letter*, I explained an error which has been productive of various imaginary Theories of the Earth. It had been thought impoffible to difcover, from the phenomena obferved on the furface of our continents, the time when they first exifted; which idea has left full fcope to imagination, for creating fyftems refpecting the caufes of their existence. But the fact is the very reverfe: many phenomena clearly lead us to determine the age of our continents, within fuch limits as exclude every hypothefis whatsoever which afcribes to them an unbounded antiquity. The principal aim of my last letter was to give you, in a very common clafs of phenomena, a precife idea of those natural chronometers; and in this I propofe to fhew you the confequence to which they irrefiftibly lead. 1. The following are the principles of investigation which may enable us to difcover thene when our continents began to exift.-ft, To find out fuch effects, as could only begin at that time, but muft neceflarily have begun then.-28, To determine the mode of action of the caufes which produce thofe effects.-3d, To examine, whether the whole of the operations thus determined, may be known; by endeavouring to discover, what was the ftate of things in thofe parts of our continents at their origin.-4th, To compare the parts of thofe effects which have been performed within the memory of men, with the whole of what has been produced.-5th, To direct the fame inquiry to various claffes of effects having thofe determined characters, in order to find out whether the feparate deductions * See the Appendix to the Monthly Review, New Series, vol. iii. drawn drawn from them, coincide in refpect of the time when their various caufes began to act on our new-born continents. 2. Whatsoever be the clafs of geological phenomena which we examine with that view, the principles of inquiry are the fame: but in my last letter, in giving you an example of the manner in which thofe neceffary data may be obtained, I chofe a class of phenomena, which, at the fame time, difpel the vague notions of fome geologifts, who attribute to the action of running waters the great and characteristic features of the furface of our continents. Poetical defcriptions of dreadful effects fometimes produced by fwollen torrents and rivers, have here been fubftituted for inquiries relative to the following neceffary points: -Whence proceed those waters which, at times, are so turbulent? What is the courfe they must have taken when they began to flow?-Can they have effentially altered their courfe from that time?-In what circumftances may they be hurtful to the ground?-Laftly, What are the alterations which they may produce? By thefe inquiries, the opinion above mentioned is entirely contradicted; fince the original state of every great part of our continents, and all the fubfequent alterations, can be afcertained. It is evident, for inftance, that all the waters which unite in one main fiream arriving laftly at the fea, proceed from a fet of grounds, whofe declivities tend towards that common receptacle: and that no material alteration can have happened to that fet from the beginning; fince, for the production of any change in that refpect, the original high boundaries of the various fyftems of grounds, thus diftinguished, ought first to have been deftroyed; while thofe high lands are the very parts of our continents where the action of running waters is at its minimum, and confequently incapable of producing any effential alteration in thote boundaries. That incontrovertible propofition puts a ftop to all wandering conjectures: each of the various fyftems of grounds whence a main fream proceeds, is the fame that it was at the origin of our continents: and by letting out from that firft refult of theory and obfervation, I have clearly determined two diftinct causes of alteration in the original fate of thofe fyftems; one proceeding from the decay of their fleep parts; the other from the action of running waters. The effects of those two only causes of deftruction, are as diftinct as the caufes themselves; their accumulation, from the time when they began, is very confpicuous; and their progress in a given time, is traced by monuments. From thofe very intelligible phenomena, I demonftrated in my last letter, that our continents have undergone but very fmall and well determined alterations; and in my prefent inquiry into their age, I fhall begin by the fame phenomena, in which we fhall find various kinds of chronometers. 3. Let us first return to the highest boundaries between separate fyftems of running waters on our hemifphere; namely, the tops of the Alps. There we find immenfe ruins of ftrata, the moft part of which, being turned up, form the towering fummits of thofe mountains. The chaotic ftate of those high grounds must have existed at the time when all the known caufes acting on our continents began their action: for, not only the power of fireams is there at its minimum, but those high valleys are filled up with ice, which prevents the propulfion of any large materials by waters: and as for the dust and fmall gravel, which fometimes come out from under that ice, they have been ftopped in hollow places, not yet filled with them; and their very diftinct mafs cannot account for any fenfible part of the cavities obferved above. However, the broken ftrata which furround thofe cavities are conftantly mouldering down: but the falling materials accumulate under them in the form of flopes, which fhew fenfibly the quantity of that effect from its beginning, coeval with our continents. Confequently, the degree of rapidity in the decay of those summits, may give us a first idea of the time when all the continental causes began to act. On that particular, I fhall only use the expresfions of a celebrated obferver of the Alps, M. DE SAUSSURE. Among other material obfervations which he has published from a fortnight's ftay, in July 1788, on the Col-du-géant, a fummit of thofe mountains elevated 1880 fathoms above the level of the fea, is the following: "The water proceeding from the thawing of fnow, filtrates conftantly in the crevices of the inclined strata of those fummits, and being expanded by froft, it splits them affunder. It is well known by all who frequent thofe mountains, that their rocks are conftantly crumbling down; but on the Col-du-géant, that phenomenon is imprefled on the mind in a very forcible manner, by the frequency of the reports of crushing maffes of rocks: I fhall not exaggerate in faying, that we did not, pafs one hour, without hearing fuch reports as equalled the roaring of thunder, produced by the falls of rocks from the fides of the Mont-blanc, of the Aiguille-marbrée, and of the very eminence on which we had fixed our ftation." This is not a flow progrefs; and when compared with the whole that has been performed, it cannot carry us far back in tracing its beginning. 4. That fact is the greatest of its kind exhibited on our hemifphere; but the circumftances and caufes on which it depends, being common to a multitude of grounds, the fame effects may be obferved over the whole furface of the land. The vague operations attributed to running waters, veiled that important field of inquiry into the age of our continents: but attention only, with the most common knowlege of the laws of hydraulics, hydraulics, will now direct every obferver, in finding out, within his rambles, or in his travels, many fleep grounds which must have been fuch from the origin of our continents. For that purpose it is fufficient to examine, whether any cause, belonging to the present state of things, can have either raised the fteep eminences obferved, or funk the ground before them. Thofe objects, of great importance to geology, will be found, not in a few spots, but in millions; not in mountains only, but in hills and hillocks: they will be diftinguished along Streams; as the fancied effects of these are now exploded; by which means two particular progresses will be there obfervable, the smoothing of originally abrupt chafms, and the fettling of the channel of the ftream. In that way of investigation, founded on real principles, vegetation in particular, as afcending over the flopes of rubbish under the decaying grounds, will afford, in almost every country, one of thofe chronometers. Vegetation tends inceffantly to take poffeffion of thofe flopes, and it is only protracted by the falling rubbish. Many opes are already fixed; the feep grounds from which they have proceeded, were, either less high, or more difpofed to crumble down, than others: but in numberlefs fpots, that operation is ftill going on; and by comparing the whole that has been produced, as fhewn by the mafs of the flope, with determined points in the progress, and examining in that refpect, the steps upwards of vegetation, this general conclufion will be obtained every where, that our continents are not very old. 5. If the laws affigned to fome great operation of nature, be real, they generally are confirmed by various claffes of phenomena, which have the fame remote caufe; and here we have many confirmations of that fort. The fand and fmall gravel carried off from the great mountains, has been (as I have explained,) ftopped in various receptacles, where, if it has not yet filled them up, it lies ftill without any fenfible lofs. Those fpaces, as they now are obferved, have a general level, confisting, partly of water, and partly of gathered fediments of the river which laft are as diftinct as the water itself, from the furrounding original boundaries of their common receptacle. The known lakes are the most confpicuous inftances of that kind; we find, in all of them, new grounds formed round the mouth of every stream discharged there. The fediments depofited by rivers at their meeting that almoft ftagnant water, being conftantly accumulated against the fhore by the waves, are thereby raised fome feet above the common level of the lake, and the most ancient parts are feldom overflowed. Thefe new grounds being very fertile, are inhabited as foon as, by the help of fome embankments, there is fafety in fettling upon Q94 them. them. Thus, traditions and records indicate the progress of thofe acquifitions of lands; and if, in confulting thofe hourglafs-like chronometers, we did not confider, that it has required more or less time for the first fediments to rife to the level of the water, the calculated time fince thofe rivers began to defcend from the mountains, would fall fhort of what we know, from clear documents of hiftory, on the age of our continents. 6. The whole of the operations performed by rivers along their courfe, affords the fame refult in various nodes; but here I must confine myfelf to fuch general outlines, as may lead to particular obfervations. It is evidently against the laws of hydraulics to fuppofe, that a fiream which, at its beginning, found a fucceffion of natural troughs in the interfccted declivity of the grounds, has fince changed its main courfe: but its inflections may have been altered; and as this is a process which required time, it becomes alfo a chronometer. When a river, in following fome original troughs, met with high grounds which stepped its paffage, it was forced to change its course; but at the fame time it attacked the obitacle which repulsed it, and there began an operation which could end only when the stream, having acquired a proper fweep, fhould ceafe to act against a fteep bank. I fhall not fpeak of rocky channels, as no material alterations have been produced in their original forms: loofe grounds only can fhew us the work of fireams; and in these it is very diftinguishable, because they have really cut fome parts of their beds through them. When the rivers were firft ftopped against fuch grounds, by undermining them, they formed a peculiar fort of cliffs, very diftinct from the flap grounds which had been produced by the convulfions of the flrata before the birth of our continents; for, in the first, there is always a horizontal foil oppofite to them on the other fide of the river, proceeding from the heavy materials fallen from the cliffs which have been accumulated on the parts abandoned by that river. Thefe cliffs then, being produced by demolition, gradually retreated; and the materials which continued to fall from them, accumulating at their foot, began to embank the fream. Such of the cliffs as thereby ceafed to be attacked, continuing however to moulder down, were reduced into fiopes, foon covered by vegetation; and fo ended the destructive power of the river in fuch parts, by its bed being there fettled. 7. Thefe are the evident operations performed by rivers in their originally fhort turnings; which effects have been either favoured or retarded by particular circumstances, eafily dif covered on almost every spot. If that operation is completed, the cliffs first formed, are seen at some distance, either cultivated, or covered with verdure; and horizontal grounds are found on the |