MR. DARWIN'S VIEW OF THESE PHENOMENA. 



77 



tion of even those who feel no interest regarding 

 the causes which have modelled the surface of the 

 land. The origin," adds Mr. Darwin, " of the 

 terraces of Coquimbo is precisely the same, accord- 

 ing to my view, with that of the plains of Patagonia; 

 the only difference is, that the plains are rather 

 broader than the terraces, and that they front the 

 Atlantic Ocean instead of a valley, — which valley, 

 however, was formerly occupied by an arm of the 

 sea, but now by a fresh-water river. In every 

 case it must be remembered that the successive 

 cliffs do not mark so many distinct elevations, but, 

 on the contrary, periods of comparative repose, 

 during the gradual, and perhaps scarcely sensible, 

 rise of the land. In the valley of Guasco we have 

 the record of seven such nights of rest, in the 

 action of the subterranean powers*." 



As the whole history of these singular phenomena 

 has been lately fully investigated by Mr. Darwin, 

 and as I feel that my description given above is 

 not only meagre, but probably inaccurate, from 

 the inevitable haste in which the observations 

 were made, I felt it due to the subject, as well 

 as to all the parties who have treated of it, to 

 request Mr. Darwin to give me his frank opinion 

 upon my statement, to state the analogy exist- 

 ing between the phenomena at Coquimbo, and 

 those of Patagonia, and also to point out the 

 parts of his recent paper on the Parallel Roads of 

 Glen Roy in Scotland, which bear most directly 

 on this curious question. The following is the 

 answer with which Mr. Darwin favoured me : — 



" 12, Upper Gower Street, 15th March, 1840. 

 " My Dear Sir, 



" I much regret that, from the state of my health, 

 I am incapable of answering your question at the 

 length which I should much wish to do. I forget 

 what I said to Mr. Lyell, but I remember that, 

 from your description, I had expected a much 

 larger valley. If the valley be considered as 

 bounded by the mountains of granitic rock, its 

 width is between three and four miles. But the width 

 of the valley, in which the river flows, is only about 

 a mile. I think, too, you have considerably over- 

 stated the distance up the valley to which the 

 terraces extend, at least as far as I could discover. 

 There are five terraces, of which three, as you 

 observe, are best characterised. The height of 

 the edge of the upper plain, close behind the town 

 of Coquimbo, is 364 feet. This upper plain slopes 

 down, but insensibly to the eye, towards Herradura 

 Bay, where it is chiefly formed of calcareous 

 rock, in the place of gravel, and its height is only 

 252 feet. This calcareous rock, contains recent 

 marine shells. On the lower terraces, I also found 

 existing shells. The upper plain, (whose edge is 

 364 feet close behind Coquimbo,) rises (but insen- 

 sibly to the eye), in its course up the true valley 

 of Coquimbo, and at two miles up the valley is 

 420 feet above the sea, — that is, 55 feet higher 

 behind the town of Coquimbo. 



"The sketch I have given in my Journal of 

 Researches, of the theory of their origin, is I 



* Darwin's Journal, in the Voyage of the Beagle, p. 423. 



believe accurate. You will understand it better, 

 if you will be so good as to read what I have 

 written about the plains of Patagonia, at pp. 200 to 

 208. When I wrote p. 423 of my Journal, I had 

 not visited Glen Roy. I now consider the cases 

 as somewhat different. The appearances at Glen 

 Roy are almost entirely due to the cumulative 

 power of the sea, on steep slopes during a period 

 of rest. The terraces of Coquimbo and Patagonia, 

 are due to the abrading action of the sea, on 

 gently inclined surfaces, during such periods. 

 The parallelism of the terraces are, consequently, 

 far less exact than those of the " Roads" of Glen 

 Roy. If you think it worth the trouble to read 

 my Glen Roy paper, in the Philosophical Trans- 

 actions, you will perceive that the formation of 

 terraces, by the abrasion of the matter accu- 

 mulated in a gentle slope in the valleys during 

 the rising, is a somewhat complex action. The 

 upper terrace, or plain of Coquimbo, is, I believe, 

 strictly analogous to the fringe of stratified allu- 

 vium in Glen Roy, described at p. 50 in my paper ; 

 its origin is explained in the hypothesis given at 

 p. 59. The successive terraces at Coquimbo, I 

 believe, are analogous to some appearances in the 

 mouth of the Spean, which I have just alluded to 

 at p. o7. 



" Glen Roy and Coquimbo, or Guasco, offer two 

 grand instances of slight modifications of the 

 action of the sea on land, during periods of rest 

 in its gradual elevation. 



" I much fear this note will be scarcely intel- 

 ligible ; I should have much enjoyed conversing 

 with you on this subject, but I am not at present 

 capable of such exertion. If the subject is worth 

 your attention, I am sure you will fully compre- 

 hend all I know, by comparing what I have 

 written on Glen Roy and Patagonia at pp. 200 

 to 208. I should feel extreme interest in hearing 

 your judgment on the theory I have proposed to 

 account for the whole class of appearances under 

 question. I think you will be pleased to hear, 

 that traces of parallel roads have been discovered 

 in other parts of Scotland, since I published 

 my paper in the Philosophical Transactions for 

 1839. — Believe me, my dear sir, 



" Yours very truly, 



" Charles Darwin." 



I have only to add that, after having examined 

 the parallel roads of Glen Roy, and carefully 

 perused Mr. Darwin's paper on that wonderful 

 series of shelves, as well as what he says of the 

 gigantic terraces of Patagonia, and having wit- 

 nessed in various other parts of the world many 

 analogous phenomena, I feel compelled (cheer- 

 fully, I grant,) to surrender my judgment on this 

 point into his hands, and to abandon many of my 

 former notions on the subject. I consider Mr. 

 Darwin's generalisations on this point as not more 

 distinguished by boldness of speculation, than by 

 the most careful, minute, and progressive induc- 

 tion — qualities by which geological theories are 

 not always characterised. 



