I should be extremely obliged if any of your chemical readers would inform me whether salt and carbonate of lime (under the form of sea-shells) would, if slightly moistened and left in great masses long together, act in any degree on each other?¹ It is, I believe, known that masses of the same substances will act on each other, of which smaller quantities will not. I do not ask this question for agricultural purposes (though possibly the answer might be of some interest in that point of view), but from having found in Peru a great bed of upraised recent shells, mixed with salt, which are decayed and corroded in a singular manner, so that the surfaces of the shells are scaling off and falling into powder.² I may mention, as explaining one element in the value of sea-shells as manure, that they are dissolved by water with greater facility than apparently any other form of carbonate of lime: one proof of this I observed in a curious rock, from Chili, chiefly composed of small fragments of recent shells, which are all enveloped and cemented together by a pellucid calcareous deposit; but in some parts of this rock the little included fragments are in every stage of decay and disappearance; in other parts they are entirely dissolved, the little calcareous envelopes being left quite empty. Here we see that water, capable of dissolving shelly matter, has penetrated through their thin films or envelopes of carbonate of lime, without having acted on them; these films, moreover, being a deposition from water within quite recent times.—³ C. Darwin.