that process, of which cleavage is the first effect" (Ibid., page 167).),
but [with] other rocks than 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, pre-existing planes of division, we must
suppose on my view of the cause, determining the lines of crystallisation
and segregation, and not planes of division produced for the first time
during the act of crystallisation, as in volcanic rocks. If this should
ever be proved, I shall not look back with utter shame at my work.


LETTER 543. TO J.D. HOOKER.
Down, September 8th [1856].

I got your letter of the 1st this morning, and a real good man you have
been to write. Of all the things I ever heard, Mrs. Hooker's pedestrian
feats beat them. My brother is quite right in his comparison of "as strong
as a woman," as a type of strength. Your letter, after what you have seen
in the Himalayas, etc., gives me a wonderful idea of the beauty of the
Alps. How I wish I was one-half or one-quarter as strong as Mrs. Hooker:
but that is a vain hope. You must have had some very interesting work with
glaciers, etc. When will the glacier structure and motion ever be settled!
When reading Tyndall's paper it seemed to me that movement in the particles
must come into play in his own doctrine of pressure; for he expressly
states that if there be pressure on all sides, there is no lamination. I
suppose I cannot have understood him, for I should have inferred from this
that there must have been movement parallel to planes of pressure. (543/1.
Prof. Tyndall had published papers "On Glaciers," and "On some Physical
Properties of Ice" ("Proc. R. Inst." 1854-58) before the date of this
letter. In 1856 he wrote a paper entitled "Observations on 'The Theory of
the Origin of Slaty Cleavage,' by H.C. Sorby." "Phil. Mag." XII., 1856,