More Letters of Charles Darwin Volume II

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I fear I must have wearied you with the length of this letter, which I have
not had time to arrange properly. I could argue at great length against
Mr. Milne's theory of barriers of detritus, though I could help him in one
way--viz., by the soundings which occur at the entrances of the deepest
fiords in T. del Fuego. I do not think he gives the smallest satisfaction
with respect to the successive and comparatively sudden breakage of his
many lakes.

Well, I enjoyed my trip to Glen Roy very much, but it was time thrown away.
I heartily wish you would go there; it should be some one who knows glacier
and iceberg action, and sea action well. I wish the Queen would command
you. I had intended being in London to-morrow, but one of my principal
plagues will, I believe, stop me; if I do I will assuredly call on you. I
have not yet read Mr. Milne on Elevation (522/11. "On a Remarkable
Oscillation of the Sea, observed at Various Places on the Coasts of Great
Britain in the First Week of July, 1843." "Trans. R. Soc. Edinb." Volume
XV., page 609, 1844.), so will keep his paper for a day or two.

P.S.--As you cannot want this letter, I wish you would return it to me, as
it will serve as a memorandum for me. Possibly I shall write to Mr.
Chambers, though I do not know whether he will care about what I think on
the subject. This letter is too long and ill-written for Sir J. Clark.

LETTER 523. TO LADY LYELL.
[October 4th, 1847.]

I enclose a letter from Chambers, which has pleased me very much (which
please return), but I cannot feel quite so sure as he does. If the
Lochaber and Tweed roads really turn out exactly on a level, the sea theory
is proved. What a magnificent proof of equality of elevation, which does
not surprise me much; but I fear I see cause of doubt, for as far as I
remember there are numerous terraces, near Galashiels, with small intervals
of height, so that the coincidence of height might be cooked. Chambers
does not seem aware of one very striking coincidence, viz., that I made by
careful measurement my Kilfinnin terrace 1202 feet above sea, and now Glen
Gluoy is 1203 feet, according to the recent more careful measurements.
Even Agassiz (523/1. "On the Glacial Theory," by Louis Agassiz, "Edinb.
New Phil. Journ." Volume XXXIII., page 217, 1842. The parallel terraces
are dealt with by Agassiz, pages 236 et seq.) would be puzzled to block up
Glen Gluoy and Kilfinnin by the same glacier, and then, moreover, the lake
would have two outlets. With respect to the middle terrace of Glen Roy--
seen by Chambers in the Spean (figured by Agassiz, and seen by myself but
not noticed, as I thought it might have been a sheep track)--it might yet
have been formed on the ice-lake theory by two independent glaciers going
across the Spean, but it is very improbable that two such immense ones
should not have been united into one. Chambers, unfortunately, does not
seem to have visited the head of the Spey, and I have written to propose
joining funds and sending some young surveyor there. If my letter is
published in the "Scotsman," how Buckland (523/2. Professor Buckland may
be described as joint author, with Agassiz, of the Glacier theory.), as I
have foreseen, will crow over me: he will tell me he always knew that I
was wrong, but now I shall have rather ridiculously to say, "but I am all
right again."

I have been a good deal interested in Miller (523/3. Hugh Miller's "First
Impressions of England and its People," London, 1847.), but I find it not
quick reading, and Emma has hardly begun it yet. I rather wish the scenic
descriptions were shorter, and that there was a little less geologic
eloquence.

Lyell's picture now hangs over my chimneypiece, and uncommonly glad I am to
have it, and thank you for it.

LETTER 524. TO C. LYELL.
Down, September 6th [1861].

I think the enclosed is worth your reading. I am smashed to atoms about
Glen Roy. My paper was one long gigantic blunder from beginning to end.
Eheu! Eheu! (524/1. See "Life and Letters," I., pages 68, 69, also pages
290, 291.)

LETTER 525. TO C. LYELL.
Down, September 22nd [1861].

I have read Mr. Jamieson's last letter, like the former ones, with very
great interest. (525/1. Mr. Jamieson visited Glen Roy in August 1861 and
in July 1862. His paper "On the Parallel Roads of Glen Roy, and their
Place in the History of the Glacial Period," was published in the
"Quarterly Journal of the Geological Society" in 1863, Volume XIX., page
235. His latest contribution to this subject was published in the
"Quarterly Journal," Volume XLVIII., page 5, 1892.) What a problem you
have in hand! It beats manufacturing new species all to bits. It would be
a great personal consolation to me if Mr. J. can admit the sloping Spean
terrace to be marine, and would remove one of my greatest difficulties--
viz. the vast contrast of Welsh and Lochaber valleys. But then, as far as
I dare trust my observations, the sloping terraces ran far up the Roy
valley, so as to reach not far below the lower shelf. If the sloping
fringes are marine and the shelves lacustrine, all I can say is that nature
has laid a shameful trap to catch an unwary wretch. I suppose that I have
underrated the power of lakes in producing pebbles; this, I think, ought to
be well looked to. I was much struck in Wales on carefully comparing the
glacial scratches under a lake (formed by a moraine and which must have
existed since the Glacial epoch) and above water, and I could perceive NO
difference. I believe I saw many such beds of good pebbles on level of
lower shelf, which at the time I could not believe could have been found on
shores of lake. The land-straits and little cliffs above them, to which I
referred, were quite above the highest shelf; they may be of much more
ancient date than the shelves. Some terrace-like fringes at head of the
Spey strike me as very suspicious. Mr. J. refers to absence of pebbles at
considerable heights: he must remember that every storm, every deer, every
hare which runs tends to roll pebbles down hill, and not one ever goes up
again. I may mention that I particularly alluded to this on S. Ventanao
(525/2. "Geolog. Obs. on South America," page 79. "On the flanks of the
mountains, at a height of 300 or 400 feet above the plain, there were a few
small patches of conglomerate and breccia, firmly cemented by ferruginous
matter to the abrupt and battered face of the quartz--traces being thus
exhibited of ancient sea-action.") in N. Patagonia, a great isolated rugged
quartz-mountain 3,000 feet high, and I could find not one pebble except on
one very small spot, where a ferruginous spring had firmly cemented a few
to the face of mountain. If the Lochaber lakes had been formed by an ice-
period posterior to the (marine?) sloping terraces in the Spean, would not
Mr. J. have noticed gigantic moraines across the valley opposite the
opening of Lake Treig? I go so far as not to like making the elevation of
the land in Wales and Scotland considerably different with respect to the
ice-period, and still more do I dislike it with respect to E. and W.
Scotland. But I may be prejudiced by having been so long accustomed to the
plains of Patagonia. But the equality of level (barring denudation) of
even the Secondary formations in Britain, after so many ups and downs,
always impresses my mind, that, except when the crust-cracks and mountains
are formed, movements of elevation and subsidence are generally very
equable.

But it is folly my scribbling thus. You have a grand problem, and heaven
help you and Mr. Jamieson through it. It is out of my line nowadays, and
above and beyond me.

LETTER 526. TO J.D. HOOKER.
Down, September 28th [1861].

It is, I believe, true that Glen Roy shelves (I remember your Indian
letter) were formed by glacial lakes. I persuaded Mr. Jamieson, an
excellent observer, to go and observe them; and this is his result. There
are some great difficulties to be explained, but I presume this will
ultimately be proved the truth...

LETTER 527. TO C. LYELL.
Down, October 1st [1861].

Thank you for the most interesting correspondence. What a wonderful case
that of Bedford. (527/1. No doubt this refers to the discovery of flint
implements in the Valley of the Ouse, near Bedford, in 1861 (see Lyell's
"Antiquity of Man," pages 163 et seq., 1863.) I thought the problem
sufficiently perplexing before, but now it beats anything I ever heard of.
Far from being able to give any hypothesis for any part, I cannot get the
facts into my mind. What a capital observer and reasoner Mr. Jamieson is.
The only way that I can reconcile my memory of Lochaber with the state of
the Welsh valleys is by imagining a great barrier, formed by a terminal
moraine, at the mouth of the Spean, which the river had to cut slowly
through, as it drained the lowest lake after the Glacial period. This
would, I can suppose, account for the sloping terraces along the Spean. I
further presume that sharp transverse moraines would not be formed under
the waters of the lake, where the glacier came out of L. Treig and abutted
against the opposite side of the valley. A nice mess I made of Glen Roy!
I have no spare copy of my Welsh paper (527/2. "Notes on the Effects
produced by the Ancient Glaciers of Caernarvonshire, and on the Boulders
transported by Floating Ice," "Edinb. New Phil. Journ." Volume XXXIII.,
page 352, 1842.); it would do you no good to lend it. I suppose I thought
that there must have been floating ice on Moel Tryfan. I think it cannot
be disputed that the last event in N. Wales was land-glaciers. I could not
decide where the action of land-glaciers ceased and marine glacial action
commenced at the mouths of the valleys.

What a wonderful case the Bedford case. Does not the N. American view of
warmer or more equable period, after great Glacial period, become much more
probable in Europe?

But I am very poorly to-day, and very stupid, and hate everybody and
everything. One lives only to make blunders. I am going to write a little
book for Murray on Orchids (527/3. "On the Various Contrivances by which
Orchids are Fertilised by Insects," London, 1862.), and to-day I hate them
worse than everything. So farewell, in a sweet frame of mind.

LETTER 528. TO C. LYELL.
Down, October 14th [1861].

I return Jamieson's capital letter. I have no comments, except to say that
he has removed all my difficulties, and that now and for evermore I give up
and abominate Glen Roy and all its belongings. It certainly is a splendid
case, and wonderful monument of the old Ice-period. You ought to give a
woodcut. How many have blundered over those horrid shelves!

That was a capital paper by Jamieson in the last "Geol. Journal." (528/1.
"On the Drift and Rolled Gravel of the North of Scotland," "Quart. Journ.
Geol. Soc." Volume XVI., page 347, 1860.) I was never before fully
convinced of the land glacialisation of Scotland before, though Chambers
tried hard to convince me.

I must say I differ rather about Ramsay's paper; perhaps he pushes it too
far. (528/2. "On the Glacial Origin of Certain Lakes, etc." "Quart.
Journ. Geol. Soc." Volume XVIII., page 185. See Letter 503.) It struck me
the more from remembering some years ago marvelling what could be the
meaning of such a multitude of lakes in Friesland and other northern
districts. Ramsay wrote to me, and I suggested that he ought to compare
mountainous tropical regions with northern regions. I could not remember
many lakes in any mountainous tropical country. When Tyndall talks of
every valley in Switzerland being formed by glaciers, he seems to forget
there are valleys in the tropics; and it is monstrous, in my opinion, the
accounting for the Glacial period in the Alps by greater height of
mountains, and their lessened height, if I understand, by glacial erosion.
"Ne sutor ultra crepidam," I think, applies in this case to him. I am hard
at work on "Variation under Domestication." (528/3. Published 1868.)

P.S.--I am rather overwhelmed with letters at present, and it has just
occurred to me that perhaps you will forward my note to Mr. Jamieson; as it
will show that I entirely yield. I do believe every word in my Glen Roy
paper is false.

LETTER 529. TO C. LYELL.
Down, October 20th [1861].

Notwithstanding the orchids, I have been very glad to see Jamieson's
letter; no doubt, as he says, certainty will soon be reached.

With respect to the minor points of Glen Roy, I cannot feel easy with a
mere barrier of ice; there is so much sloping, stratified detritus in the
valleys. I remember that you somewhere have stated that a running stream
soon cuts deeply into a glacier. I have been hunting up all old references
and pamphlets, etc., on shelves in Scotland, and will send them off to Mr.
J., as they possibly may be of use to him if he continues the subject. The
Eildon Hills ought to be specially examined. Amongst MS. I came across a
very old letter from me to you, in which I say: "If a glacialist admitted
that the sea, before the formation of the shelves, covered the country
(which would account for the land-straits above the level of the shelves),
and if he admitted that the land gradually emerged, and if he supposed that
his lakes were banked up by ice alone, he would make out, in my opinion,
the best case against the marine origin of the shelves." (529/1. See
Letter 522.) This seems very much what you and Mr. J. have come to.

The whole glacial theory is really a magnificent subject.

LETTER 530. TO C. LYELL.
Down, April 1st [1862].

I am not quite sure that I understand your difficulty, so I must give what
seems to me the explanation of the glacial lake theory at some little
length. You know that there is a rocky outlet at the level of all the
shelves. Please look at my map. (530/1. The map accompanying Mr.
Darwin's paper in the "Phil. Trans. R. Soc." 1839.) I suppose whole valley
of Glen Spean filled with ice; then water would escape from an outlet at
Loch Spey, and the highest shelf would be first formed. Secondly, ice
began to retreat, and water will flow for short time over its surface; but
as soon as it retreated from behind the hill marked Craig Dhu, where the
outlet on level of second shelf was discovered by Milne (530/2. See note,
Letter 521.), the water would flow from it and the second shelf would be
formed. This supposes that a vast barrier of ice still remains under Ben
Nevis, along all the lower part of the Spean. Lastly, I suppose the ice
disappeared everywhere along L. Loggan, L. Treig, and Glen Spean, except
close under Ben Nevis, where it still formed a barrier, the water flowing
out at level of lowest shelf by the Pass of Mukkul at head of L. Loggan.
This seems to me to account for everything. It presupposes that the
shelves were formed towards the close of the Glacial period. I come up to
London to read on Thursday a short paper at the Linnean Society. Shall I
call on Friday morning at 9.30 and sit half an hour with you? Pray have no
scruple to send a line to Queen Anne Street to say "No" if it will take
anything out of you. If I do not hear, I will come.

LETTER 531. TO J. PRESTWICH.
Down, January 3rd, 1880.

You are perfectly right. (531/1. Prof. Prestwich's paper on Glen Roy was
published in the "Phil. Trans. R. Soc." for 1879, page 663.) As soon as I
read Mr. Jamieson's article on the parallel roads, I gave up the ghost with
more sighs and groans than on almost any other occasion in my life.

2.IX.IV. CORAL REEFS, FOSSIL AND RECENT, 1841-1881.

LETTER 532. TO C. LYELL.
Shrewsbury, Tuesday, 6th [July, 1841].

Your letter was forwarded me here. I was the more glad to receive it, as I
never dreamed of your being able to find time to write, now that you must
be so very busy; and I had nothing to tell you about myself, else I should
have written. I am pleased to hear how extensive and successful a trip you
appear to have made. You must have worked hard, and got your Silurian
subject well in your head, to have profited by so short an excursion. How
I should have enjoyed to have followed you about the coral-limestone. I
once was close to Wenlock (532/1. The Wenlock limestone (Silurian)
contains an abundance of corals. "The rock seems indeed to have been
formed in part by massive sheets and bunches of coral" (Geikie, "Text-book
of Geology," 1882, page 678.), something such as you describe, and made a
rough drawing, I remember, of the masses of coral. But the degree in which
the whole mass was regularly stratified, and the quantity of mud, made me
think that the reefs could never have been like those in the Pacific, but
that they most resembled those on the east coast of Africa, which seem
(from charts and descriptions) to confine extensive flats and mangrove
swamps with mud, or like some imperfect ones about the West India Islands,
within the reefs of which there are large swamps. All the reefs I have
myself seen could be associated only with nearly pure calcareous rocks. I
have received a description of a reef lying some way off the coast near
Belize (terra firma), where a thick bed of mud seems to have invaded and
covered a coral reef, leaving but very few islets yet free from it. But I
can give you no precise information without my notes (even if then) on
these heads...

Bermuda differs much from any other island I am acquainted with. At first
sight of a chart it resembles an atoll; but it differs from this structure
essentially in the gently shelving bottom of the sea all round to some
distance; in the absence of the defined circular reefs, and, as a
consequence, of the defined central pool or lagoon; and lastly, in the
height of the land. Bermuda seems to be an irregular, circular, flat bank,
encrusted with knolls and reefs of coral, with land formed on one side.
This land seems once to have been more extensive, as on some parts of the
bank farthest removed from the island there are little pinnacles of rock of
the same nature as that of the high larger islands. I cannot pretend to
form any precise notion how the foundation of so anomalous an island has
been produced, but its whole history must be very different from that of
the atolls of the Indian and Pacific oceans--though, as I have said, at
first glance of the charts there is a considerable resemblance.

LETTER 533. TO C. LYELL.
[1842.]

Considering the probability of subsidence in the middle of the great oceans
being very slow; considering in how many spaces, both large ones and small
ones (within areas favourable to the growth of corals), reefs are absent,
which shows that their presence is determined by peculiar conditions;
considering the possible chance of subsidence being more rapid than the
upward growth of the reefs; considering that reefs not very rarely perish
(as I cannot doubt) on part, or round the whole, of some encircled islands
and atolls: considering these things, I admit as very improbable that the
polypifers should continue living on and above the same reef during a
subsidence of very many thousand feet; and therefore that they should form
masses of enormous thickness, say at most above 5,000 feet. (533/1.
"...As we know that some inorganic causes are highly injurious to the
growth of coral, it cannot be expected that during the round of change to
which earth, air, and water are exposed, the reef-building polypifers
should keep alive for perpetuity in any one place; and still less can this
be expected during the progressive subsidences...to which by our theory
these reefs and islands have been subjected, and are liable" ("The
Structure and Distribution of Coral Reefs," page 107: London, 1842).)
This admission, I believe, is in no way fatal to the theory, though it is
so to certain few passages in my book.

In the areas where the large groups of atolls stand, and where likewise a
few scattered atolls stand between such groups, I always imagined that
there must have been great tracts of land, and that on such large tracts
there must have been mountains of immense altitudes. But not, it appears
to me, that one is only justified in supposing that groups of islands stood
there. There are (as I believe) many considerable islands and groups of
islands (Galapagos Islands, Great Britain, Falkland Islands, Marianas, and,
I believe, Viti groups), and likewise the majority of single scattered
islands, all of which a subsidence between 4,000 and 5,000 feet would
entirely submerge or would leave only one or two summits above water, and
hence they would produce either groups of nothing but atolls, or of atolls
with one or two encircled islands. I am far from wishing to say that the
islands of the great oceans have not subsided, or may not continue to
subside, any number of feet, but if the average duration (from all causes
of destruction) of reefs on the same spot is limited, then after this limit
has elapsed the reefs would perish, and if the subsidence continued they
would be carried down; and if the group consisted only of atolls, only open
ocean would be left; if it consisted partly or wholly of encircled islands,
these would be left naked and reefless, but should the area again become
favourable for growth of reefs, new barrier-reefs might be formed round
them. As an illustration of this notion of a certain average duration of
reefs on the same spot, compared with the average rate of subsidence, we
may take the case of Tahiti, an island of 7,000 feet high. Now here the
present barrier-reefs would never be continued upwards into an atoll,
although, should the subsidence continue at a period long after the death
of the present reefs, new ones might be formed high up round its sides and
ultimately over it. The case resolves itself into: what is the ordinary
height of groups of islands, of the size of existing groups of atolls
(excepting as many of the highest islands as there now ordinarily occur
encircling barrier-reefs in the existing groups of atolls)? and likewise
what is the height of the single scattered islands standing between such
groups of islands? Subsidence sufficient to bury all these islands (with
the exception of as many of the highest as there are encircled islands in
the present groups of atolls) my theory absolutely requires, but no more.
To say what amount of subsidence would be required for this end, one ought
to know the height of all existing islands, both single ones and those in
groups, on the face of the globe--and, indeed, of half a dozen worlds like
ours. The reefs may be of much greater [thickness] than that just
sufficient on an average to bury groups of islands; and the probability of
the thickness being greater seems to resolve itself into the average rate
of subsidence allowing upward growth, and average duration of reefs on the
same spot. Who will say what this rate and what this duration is? but till
both are known, we cannot, I think, tell whether we ought to look for
upraised coral formations (putting on one side denudation) above the
unknown limit, say between 3,000 and 5,000 feet, necessary to submerge
groups of common islands. How wretchedly involved do these speculations
become.

LETTER 534. TO E. VON MOJSISOVICS.
Down, January 29th, 1879.

I thank you cordially for the continuation of your fine work on the
Tyrolese Dolomites (534/1. "Dolomitriffe Sudtirols und Venetiens": Wien,
1878.), with its striking engravings and the maps, which are quite
wonderful from the amount of labour which they exhibit, and its extreme
difficulty. I well remember more than forty years ago examining a section
of Silurian limestone containing many corals, and thinking to myself that
it would be for ever impossible to discover whether the ancient corals had
formed atolls or barrier reefs; so you may well believe that your work will
interest me greatly as soon as I can find time to read it. I am much
obliged for your photograph, and from its appearance rejoice to see that
much more good work may be expected from you.

I enclose my own photograph, in case you should like to possess a copy.

LETTER 535. TO A. AGASSIZ.

(535/1. Part of this letter is published in "Life and Letters," III.,
pages 183, 184.)

Down, May 5th, 1881.

It was very good of you to write to me from Tortugas, as I always feel much
interested in hearing what you are about, and in reading your many
discoveries. It is a surprising fact that the peninsula of Florida should
have remained at the same level for the immense period requisite for the
accumulation of so vast a pile of debris. (535/2. Alexander Agassiz
published a paper on "The Tortugas and Florida Reefs" in the "Mem. Amer.
Acad. Arts and Sci." XI., page 107, 1885. See also his "Three Cruises of
the 'Blake,'" Volume I., 1888.)

You will have seen Mr. Murray's views on the formation of atolls and
barrier reefs. (535/3. "On the Structure and Origin of Coral Reefs and
Islands," "Proc. R. Soc. Edin." Volume X., page 505, 1880. Prof. Bonney
has given a summary of Sir John Murray's views in Appendix II. of the third
edition of Darwin's "Coral Reefs," 1889.) Before publishing my book, I
thought long over the same view, but only as far as ordinary marine
organisms are concerned, for at that time little was known of the multitude
of minute oceanic organisms. I rejected this view, as from the few
dredgings made in the 'Beagle' in the S. Temperate regions, I concluded
that shells, the smaller corals, etc., etc., decayed and were dissolved
when not protected by the deposition of sediment; and sediment could not
accumulate in the open ocean. Certainly shells, etc., were in several
cases completely rotten, and crumbled into mud between my fingers; but you
will know well whether this is in any degree common. I have expressly said
that a bank at the proper depth would give rise to an atoll, which could
not be distinguished from one formed during subsidence. I can, however,
hardly believe, in the former presence of as many banks (there having been
no subsidence) as there are atolls in the great oceans, within a reasonable
depth, on which minute oceanic organisms could have accumulated to the
thickness of many hundred feet. I think that it has been shown that the
oscillations from great waves extend down to a considerable depth, and if
so the oscillating water would tend to lift up (according to an old
doctrine propounded by Playfair) minute particles lying at the bottom, and
allow them to be slowly drifted away from the submarine bank by the
slightest current. Lastly, I cannot understand Mr. Murray, who admits that
small calcareous organisms are dissolved by the carbonic acid in the water
at great depths, and that coral reefs, etc., etc., are likewise dissolved
near the surface, but that this does not occur at intermediate depths,
where he believes that the minute oceanic calcareous organisms accumulate
until the bank reaches within the reef-building depth. But I suppose that
I must have misunderstood him.