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.—¹ I forget what I said to M^r Lyell,² 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 rocks its width is between 3 & 4 miles. But the width of the valley in which the river flows is only about a mile.— I have not a copy of your work, & I am unable to leave the house, but I think you have considerably overstated 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 characterized.—³ 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 & 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 insensibly to the eye), in its course up the true valley of Coquimbo, & at two miles up the valley is 420 feet above the sea,—that is 55 feet higher. than behind town of Coquimbo. The sketch I have given in my Journal of Researches of the theory of their origin, is, I 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 p. 200 to 208⁴

When I wrote p. 423 of my Journal, I had not visited Glen Roy.⁵ 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 parallelisms 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 Philosoph Transact.⁶ you will perceive that the formation of terraces by the abrasion of the matter, accumulated in a gentle slope in the valleys during the rising, is a somewhat complex action.— The upper terrace or plain of Coquimbo I believe is strictly analogous to the fringe of stratified alluviums in Glen Roy, described at p. 50 in my paper.—its origin 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,⁷ which I have just alluded to at p. 67.—

Glen Roy & 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.—⁸

I much fear this note will be scarcely intelligible: 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 comprehend all I know, by comparing what I have written on Glen Roy & Patagonia at 200 to 208.— I should feel extreme interest in hearing your judgment on this theory I have proposed to account for the whole class of appearances under question.—

Believe me My dear Sir | Yours very truly | Chas. Darwin.

P. S. | 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.— And I have received several private communications on the subject.—⁹