a tendency in the old points of eruption to become
the points of subsequent injection and disturbance, and consequently of
denudation, it ceases to be surprising that the streams of lava in the
porphyritic claystone conglomerate formation, and in other analogous
cases, should most rarely be traceable to their actual sources." The
latter part of this letter is published in "Life and Letters," I.,
pages 377, 378.) I remember being struck with your discussion on the
Mississippi beds in relation to Pampas, but I should wish to read them
over again; I have, however, re-lent your work to Mrs. Rich, who, like
all whom I have met, has been much interested by it. I will stop about
my own Geology. But I see I must mention that Scrope did suggest (and I
have alluded to him, page 118 (483/5. "Geological Observations," Edition
II., 1876. Chapter VI. opens with a discussion "On the Separation of the
Constituent Minerals of Lava, according to their Specific Gravities."
Mr. Darwin calls attention to the fact that Mr. P. Scrope had speculated
on the subject of the separation of the trachytic and basaltic series
of lavas (page 113).), but without distinct reference and I fear not
sufficiently, though I utterly forgot what he wrote) the separation of
basalt and trachyte; but he does not appear to have thought about the
crystals, which I believe to be the keystone of the phenomenon. I cannot
but think this separation of the molten elements has played a great part
in the metamorphic rocks: how else could the basaltic dykes have come in
the great granitic districts such as those of Brazil? What a wonderful
book for labour is d'Archiac!...(483/6. Possibly this refers to
d'Archiac's "Histoire des Progres de la Geologie," 1848.)

LETTER 484. TO LADY LYELL. Down, Wednesday night [1849?].

I am going to beg a very very great favour of you: it is to translate
one page (and the title) of either Danish or Swedish or some such
language. I know not to whom else to apply, and I am quite dreadfully
interested about the barnacles therein described. Does Lyell know Loven,
or his address and title? for I must write to him. If Lyell knows him I
would use his name as introduction; Loven I know by name as a first-rate
naturalist.

Accidentally I forgot to give you the "Footsteps," which I now return,
having ordered a copy for myself.

I sincerely hope the "Craters of Denudation" prosper; I pin my faith to
this view. (484/1. "On Craters of Denudation, wit