o the amount of nitrogen in the soil, though not of course if the
whole country is considered. I saw in your paper something about
the difference in the amount of nitrogen at different depths in the
superficial mould, and here worms may have played a part. I wish that
the problem had been before me when observing, as possibly I might have
thrown some little light on it, which would have pleased me greatly.

2.IX.VIII. MISCELLANEOUS, 1846-1878.

(552/1. The following four letters refer to questions connected with the
origin of coal.)

LETTER 552. TO J.D. HOOKER. Down, May [1846].

I am delighted that you are in the field, geologising or
palaeontologising. I beg you to read the two Rogers' account of the
Coal-fields of N. America; in my opinion they are eminently instructive
and suggestive. (552/1. "On the Physical Structure of the Appalachian
Chain," by W.B. and H.D. Rogers. Boston, 1843. See also "Geology of
Pennsylvania," by H.D. Rogers. 4 volumes. London and Philadelphia,
1843.) I can lend you their resume of their own labours, and, indeed, I
do not know that their work is yet published in full. L. Horner gives
a capital balance of difficulties on the Coal-theory in his last
Anniversary Address, which, if you have not read, will, I think,
interest you. (552/2. "Quart. Journ. Geol. Soc." Volume II., 1846, page
170.) In a paper just read an author (552/3. "On the Remarkable Fossil
Trees lately discovered near St. Helen's." By E.W. Binney. "Phil.
Mag." Volume XXIV., page 165, 1844. On page 173 the author writes: "The
Stigmaria or Sigillaria, whichever name is to be retained... was a
tree that undoubtedly grew in water.") throws out the idea that the
Sigillaria was an aquatic plant (552/4. See "Life and Letters," I.,
pages 356 et seq.)--I suppose a Cycad-Conifer with the habits of the
mangrove. From simple geological reasoning I have for some time been led
to suspect that the great (and great and difficult it is) problem of
the Coal would be solved on the theory of the upright plants having been
aquatic. But even on such, I presume improbable notion, there are, as it
strikes me, immense difficulties, and none greater than the width of the
coal-fields. On what kind of coast or land could the plants have lived?
It is a grand problem, and I trust you will grapple with it. I shall
like much to have some discussion with you. When will you come here
again? I am very sorry to infer from your letter that your sister has
b