cleared up.


LETTER 542. TO C. LYELL.
27, York Place, Baker Street [1855].

I have received your letter from Down, and I have been studying my S.
American book.

I ought to have stated [it] more clearly, but undoubtedly in W. Tierra del
Fuego, where clay-slate passes by alternation into a grand district of
mica-schist, and in the Chonos Islands and La Plata, where glossy slates
occur within the metamorphic schists, the foliation is parallel to the
cleavage--i.e. parallel in strike and dip; but here comes, I am sorry and
ashamed to say, 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. 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 and 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, and this is a flagrant case. I have been amused to find,
for I had quite forgotten, how distinctly I give a suspicion (top of page
155) to the idea, before Sharpe, of cleavage (not foliation) being due to
the laminae forming parts of great curves. (542/1. "I suspect that the
varying and opposite dips (of the cleavage-planes) may possibly be
accounted for by the cleavage-laminae...being parts of large abrupt curves,
with their summits cut off and worn down" ("Geological Observations on S.
America," page 155). I well remember the fine section at the end of a
region where the cleavage (certainly cleavage) had been most uniform in