My dear Lyell

I have received your letter from Down, & I have been studying my S. American Book.

I ought to have stated more clearly, but undoubtedly in W. Tierra del Fuego, where Clay-slate passes by alternation into a grand district of Mica-schist, & in the Chonos isl^ds. & La Plata, where glossy slates occur within the metamorphic schists, the foliation is parallel to the cleavage, ie parallel in strike & dip; but here comes, I am sorry & ashamed to confess a great hiatus in my reasoning, I have assumed that the cleavage in these neighbouring or intercalated beds was (as in more distant parts) distinct from stratification;¹ if you choose to say that here the cleavage was or might be parallel to true bedding, I cannot gainsay it, but can only appeal to apparent similarity,—to the great areas of uniformity of strike & high angle,—all certainly unlike as far as my experience goes, to true stratification.— I have long known how easily I overlook flaws in my own reasoning, & this a flagrant case.—

I have been amused to find (for I had quite forgotten) how distinctly I give suspicion (p. 155) (top of page) the idea, before Sharpe, of cleavage, (not foliation) being due to the laminæ forming parts of great curves;² I well remember the fine section at the end of a region where the cleavage (certainly cleavage) had been most uniform in strike & most variable in dip.—

I made with really great care (& in M.S in detail)³ observations on a case which I believe is new, & bears on your view of metamorphosis (p. 149 at bottom)⁴ In a clay-slate porphyry regions, where certain thin sedimentary layers of tuff, had by self-attraction, shortened themselves into little curling pieces, & then again into crystals of feldspar of large size, & which consequently were all strictly parallel. The series was perfect & beautiful. Apparently also the rounded grains of quartz had in other parts aggregated themselves into crystalline nodules of quartz.

I have not been able to get Sorby⁵ yet, but shall not probably have anything to write on it.—

I am delighted you have taken up subject, even if I am utterly floored.

Your’s most truly | C. Darwin

P.S (Do read this P.S.) | I have a presentiment it will turn out that when clay-slate has been metamorphosed the foliation in the resultant schists has been due generally (if not as I think always) to the cleavage, & this to a certain degree will “save my bacon,” (please look at my saving clause p. 167.)⁶ but when other rocks, then that stratification has been the ruling agent,—the strike, but not the dip, being in such cases parallel to any adjoining clay-slate.— If this be so, preexisting planes of division, we must suppose, on my view of cause determine the lines of crystallization & segregation, & not planes of division produced for the first time, during the act of crystallisation, as in volcanic rocks. If this sh^d ever be proved, I shall not look back with utter shame at my work.—⁷