My dear Sir

I should have written to you long since, respecting the results of my examination of the specimens you forwarded to me soon after Christmas;¹ had I not been prevented by unforeseen occurrences from subjecting the sections, which were then prepared, to as close an examination as I wished to give them— This, however, I have now done; and I have also made a couple of sections of the last specimen you forwarded to me,—that of the Coquimbo deposit; and the results of this more extended investigation are sufficiently interesting, to make me think it worthwhile to go over the whole subject with you again.

You are of course aware, in the first place, that any calcareous matter, in a state of crystalline aggregation, is transparent under the Microscope; whilst any amorphous deposit is quite opaque, even in a very thin layer. The two conditions are well seen in ordinary Shells of Mollusks, and in the Egg-shell. A tolerably thick section of the former is generally transparent, or at least translucent; the thinnest possible section of the latter is quite opaque. Not unfrequently there is in the Shells of certain Mollusks (e.g. the common Oyster) a layer of amorphous Carbonate of lime, which, altho’ of immeasurable thinness, renders the portion of the section, in which it presents itself, quite opaque.

How far it is possible for any mechanical attrition to reduce the crystalline to the amorphous form, so completely as to change the microscopic character in the manner I have stated,—I can scarcely venture an opinion. That Chemical precipitation will do it,—we well know; but I am inclined to believe that a sufficiently prolonged action of attrition might also effect it.

Now supposing a deposit to have been thus reduced to the amorphous state, the question arises, whether, under any circumstances, it can resume, more or less a crystalline condition. I am strongly inclined to believe that the simple percolation of water charged with a small quantity of carbonic acid (and thereby possessing a solvent power for carbonate of lime) may have this result; and that this is the history of the formation of the Oolites, in which the condition of the calcareous matter,—both that forming the concentric deposits around the nuclei of the egg-like particles,—and that forming the cement which unites them together,—is decidedly crystalline. In this case, the crystallization has taken place, from some unknown cause, around nuclei, which commonly consist of minute Foraminifera, more or less perfectly preserved. In other instances it may occur, I should suppose, in irregular patches or in more distinct strata—

Now to apply these views to the case before us—

In regard to the Chilian deposit, it is, of course sufficiently evident to the unaided eye, that a great part of it consists of comminuted fragments of Shell.² The Microscope shows, that these are mingled with still more minute particles of the same kind; and that there is also a quantity, varying in proportion, of the amorphous substance, which I have alluded to as possibly resulting from the mechanical disintegration of the larger fragments.— I think I can also trace some Spongioid remains in the sections I have made of the specimen you sent me,—about 12 in number.

In the specimen of the Coquimbo deposit which you last sent me,³ I can find no definite appearance of organic structure; it appears to be made up of the amorphous particles of the preceding; in some parts of which a sort of stratification presents itself, some of the layers showing indications of an incipient metamorphosis into the Crystalline condition.

In the various sections I have made of the Pampean deposit, amounting in all to about 25 in number, I trace a considerable variety of appearances. In a large part, I find the same appearances as in the preceding,—namely, a tolerably uniform amorphous character, with traces of incipient crystalline metamorphosis; but I also find the presence of minute rounded concretions,—resembling those of Oolites in size, but not in the arrangement of their particles,—a very frequent character. The substance of these concretions is still amorphous; but that of the connecting cement is often crystalline; as if water, charged with carbonate of lime, had percolated through the deposit, after (from whatever cause) the concretions had been formed. Of organic structures in this deposit, I find a greater variety than in either of the preceding. I have been able distinctly to recognize fragments of Shell,—though rarely. I am fully satisfied, however, that Spongioid bodies were common at the time of its formation; though their remains are by no means distinct. M^r Bowerbank, to whom I have shown the most characteristic specimens, fully agrees with me in this. The remains of the organic fibres, of the Spicula, and I think too of the Gemmules, may be distinguished more clearly than in the Chilian deposit. I am satisfied, also, of the existence of remains of Corals and of Polythalamia, in this deposit. But, as I said in a former communication, the great mass of it is composed of the amorphous matter already referred to;—sometimes uniformly diffused,—sometimes in concretions.

This is, I think, all the light that the Microscope can throw on the history of these deposits; and if it is not entirely satisfactory, nevertheless I hope it may be regarded as a contribution of some value towards the elucidation of their relations.

I shall preserve the series of Sections; and shall have great pleasure in showing them to you, or to any other Geologist who may wish to examine them, at any future time. I shall be in my present domicile for about two months longer, after which I shall return to Ripley until Michaelmas; but it is probable that I shall then remove permanently to the immediate vicinity of London.

I hope that you will not come to Town, without giving me the pleasure of seeing you. I have now a very large collection of preparations of Shell-Structure; and am anxious to make my investigations known to Scientific Men.

Your specimens (which I have brought to Town) shall be forwarded to the Geological Society. I have not been able to make sections of all of them, some being too friable.

Believe me to be, Dear Sir | yours very sincly | William B. Carpenter.