RECORD: Darwin, C. R. Geological diary: Mauritius. (5.1836) CUL-DAR38.882-901 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)

REVISION HISTORY: Transcribed by Kees Rookmaaker, corrections and edited by John van Wyhe 9.2009, corrections and additions by Gordon Chancellor. RN5

NOTE: This document, part of the largest scientific document composed by Darwin during the voyage of the Beagle, is written mostly in ink. Where pencil was used instead this is noted in the textual notes. Marginal notes are here integrated into the text. The paper size is uniform, 20 x 26 cm.

Editorial symbols used in the transcription:
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[some text] 'some text' is the conjectured reading of an ambiguous word or passage
[some text] 'some text' is a description of a word or passage that cannot be transcribed
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<some text> 'some text' is a description of a destroyed word or passage
Text in small red font is a hyperlink or notes added by the editors.

Reproduced with the permission of the Syndics of Cambridge University Library and William Huxley Darwin.

See the introduction to the Geological Diary by Gordon Chancellor.


882

1

1836 May Mauritius

Before describing the few facts, which I have collected concerning the coral reefs, which encircle this island, it will be necessary to give a slight sketch of its geological structure. M. Lesson in his the Zoologie of the voyage of the Coquille,1 states that the whole central plain of the island appears once to have been the bottom of a great crater. The detached hills which encircle it, have a usual elevation of between 2 & 3000 ft above the level of the sea. On the NW & W sides of the islands, I observed a quaqua versal dip, pointing from a common centre may be observed. These hills are composed of lavas 3618, which are Basalts, & black trachytes 3625: the solid strata alternate with firmly cemented fragmentary rocks. — If M. Lessons view is correct, as certainly appears possible, the crater, when perfect

1 Duperrey 1826-30.

882 verso

Bory St Vincent states that some dikes vertical in the marginal groups of mountains and that Peter Bot Mountain1 has general inclination towards shore.

Account from Peron R.N. p. 118

Talk of calcareous matter all over Pamplemousses

Have wood cut chart of Mauritius

M. Quoy (Freycinet Voyage)2 does not believe in great crater & says there are many craters & that common opinion of mountains having escarpment towards interior & slope to sea "But this observation is far from general." —

Holman Vol III p 98 Crocodiles said to be common on Seychelles3

1 Pieter Both Mountain. Bory de Saint-Vincent 1822-1831.

2 Quoy 1824.

3 Holman 1835.

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1836 Mauritius

must have been about nearly five leagues in diameter. The encircling hills, the picturesque & jagged forms of which are so well known by the descriptions of so many voyagers, stand detached from each other. The Corps du garde is 3 miles distant from a branch of La Puce, the smooth intervening space slopes gradually to seaward; its constitution, as seen in the [profound] ravine of the Great rivers consists of a multitude of these lave streams injected by scoriaceous masses. These probably flowed from the central platform, long after the grand crater had been breached. We must believe, that the only power, which could have removed masses several miles long of the border of the Crater, must have been the sea; hence at one time the detached hills formed a circle

884

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1836 Mauritius

in the same manner, as some of the so called craters of elevation now do in different parts of the ocean. Within the limits of the grand crater, there are hills, which without doubt, formerly existed as smaller orifices & in a similar manner, the chief of which is the Piton du Milieu, in a like manner on the flanks & in the outlying Islands, at the northern extremity, we find traces of a similar structure. The peaked hills, which form the remnants of the great wall, are situated generally at the distance of some miles we from the sea coast. Everywhere the land slopes from the base of these hills to the beach, with a most gradual & very most even inclination. This is especially conspicuous when viewed from above, & the wide extent of (such as the fertile plain of pamplemousses) & when viewing the extremities of the Island from a distance on board a

884 verso

NB. In the SW extremity of the Island I saw some quartz siliceous rocks, which perhaps bespeak the existence of some quite distinct formation, in the possession of Capt. Floyd.

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1836 Mauritius

vessel. This view at once suggests the idea, that the slope is has been a regular accumulation of matter beneath the sea: How far this is correct I do not know, for the greater part [word deleted] is composed of lava, but with respect to the lower height of 40 or 50 ft there can be no doubt, the rocks at no very distant period were covered by the waters of the sea. — On the NW, W & SW sides of the island, coral rock such as now is forming in the reefs, as are is commonly found in masses elevated considerably above the reach of any the highest tides. I will describe in detail some of these cases. To the northward of Port Luis, between it & Tonet Bat in many parts the surface of the country to the a height of 30 or 40 ft, & to a considerable distance inland, there was is coated by a bed of partially cemented fragments of strong branching corals. Some of the layers were are hard & compact & contained some contain

886

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1836 Mauritius

ferrugineous matter 3626. The rock is composed of precisely the same materials such as now the on the beach ; these are chiefly calcareous sand & small [commonest] fragments of the [series beds] of more delicate stony corals, which flourish on this protected & quiet side of the island, are [might] with calcareous sand. The land rises with a very gradual slope, from the line of high water mark, hence so that if the country should now again be elevated a few feet the same kind of superficial covering would be continued down to the beach.
Near the mouth of a small brook, but yet some distance within the running fresh water, several of the rounded blocks of were coated with serpulae, it would appear from this that the rise of the land had been very recent, but as earthquakes are said never to be experienced here, the epoch probably goes back to beyond historical records.

887

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1836 Mauritius

To the southward of Port Louis, between the Callit river & Tamarin Bay, large masses of Coral are found inland. This fact is mentioned in the excellent account of the Isle of france, in the voyage to this island, "par un Officier du Roi."1

? quoted right
quote more extensively

he adds, "J'observai, que là, on la pres étale independenment des rescifs du large. il y a à terre une espece d'effoncement, un chemin couvert naturel, on y pourroit mettre du canon, &c." — vol I P 272 quoted

Again further a few miles to the South between Tamarin Bay, & great Black river the surveyor general Capt. Floyd showed me an extensive flat entirely composed of coral rock & two remarkable little hillocks of a nearly similar constitution. copied. The first of these is situated about 200 yards from the beach, & almost nearly 30 ft: above its level; it consists of a conical mass, about 18 or 20 ft high, nearly circular its sides steep & much water worn. its form is nearly circular with steep & water worn sides, its height is about eighteen or 20 ft. —

1 Bernardin de Saint Pierre 1773. These quotations were later used in Coral reefs.

888

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1836 Mauritius

The mass is roughly stratified, the inclination being about 8˚ to seaward. The lower half is entirely composed a coarse but hard calcareous sandstone 3627; the minute rounded fragments being cemented into a hard rock. — The upper half is much less [finely] aggregated & consists of large blocks of various species of Astrea, & Madrepora 3628: 29 &c &c &c; Mingled with them there were are a few large & rounded fragments masses of basalt. The I could not ascertain with certainty, but I thought the corals appeared to It to me that the greater part of the Corals, whether or not these corals had grown in their present position have grown in their its present position; if such was the has been is the case it is interesting because it would show that we see it more in which a living reef might by the may extend over a sandy bottom. The other hillock is a little further inland & of a similar nature; its strata however appear are inclined at an of 18˚. — Both of them these singular little paps of calcareous rock rise abruptly out of a flat from a lower surface of

889

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Mauritius

broken Coral. comments To account for the origin We must suppose that banks were formed on the reef, & that during the rise of the land, the sea return wore away the outer parts, leaving only only left these remaining points. I was told, that at a few miles distance, there is a little islet, composed of a similar rock, standing on one of the reefs; if another elevation of the ba should happen this it also would might form a hillock on a arising out of a grassy flat plain. of coral rock.
Near the SW extremity, the Officier du Roi says vol I P 284 "Avant de passer le Cap, on remarque un gros bane, eleve de plus de quinze pieds de Corail; c'est une espece de rescif que la mer a abandonné: il regna au pied une longue flaque d'eau, dont on poussoit faire un bassin pour de petits vaisseaux" There can be little doubt from all these facts (as the above officer remarks) but that the land has

890

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Mauritius

undergone a small elevation within the recent period. date? This conclusion did not escape (although 1768 years since) the Officier du Roi; moreover from in the Island of Bourbon. vol II P 20 16 of my edition. from the presence of rounded blocks inshore & certain forms of the land he believes the same change has likewise there also taken place.
I have attempted to prove in detail that the land, within the Recent period, but at a date probably some centuries past, has been elevated, because in examining the structure of the coral reefs, we may feel assured that their present configuration is not dependent on any movement (such as subsidence) in the relative level of land & water. The Island is circumference 45 leagues in circuit, with the exception of one or two short spaces, the whole coast is fronted by living coral reefs. The Officier du Roi talking of the defence of the islands, remarks that where reefs are deficient, in coast is there steep & inaccessible; I have already

circumference] added pencil.

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Mauritius

said that generally the land slopes down to the water with a gradual inclination, as the depth of the neighbouring sea, is well known generally to correspond to the forms of the coasts, we may presume that where the coral reefs are absent, the depth is too great for the lithophytes to begin lay the foundations of their edifices. Dampier
In front of all the rivers there is a channel; this which probably is owing to the line of mud, deposited by the mountain torrents, as is seen to be the case at Port Louis which must present the growth of corals. The reefs in two or three places are of great dimensions, extending seaward for a space of at three miles in a direct line from the land; commonly they are about half a mile wide the living coral very rarely grows attached to the shore The highest part of the reef, is on the outer edge where the sea breaks; at the outer edge; on the windward side of the Island at low water a few small points may [word deleted] be seen uncovered I am told, that at low water on the windward side a large part reef at such time is quite dry, & generally in every part of few small points patches of Coral rock may be then seen uncovered. Between this outer line & the shore beach the water is so shallow, so that people, avoiding the deep holes & channels, can commonly wade out at low water to fish. — generally at low water in most parts the tide falls at low water

lay] added pencil.

I agree] added pencil.

which probably is owing] 'which' added pencil.

which must present the growth of corals.] added pencil.

the living coral very rarely grows attached to the shore] added pencil.

on the outer edge] added pencil.

at low water a few...seen uncovered] added pencil.

on the windward side] added pencil.

reef at such time] added pencil.

points] added pencil.

generally at low water in most parts the tide falls at low water] added pencil.

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Mauritius

The living coral very rarely grows attached to the shore, where [illeg] the beaches, as commonly happens, are sandy. the want of foundation appears a sufficient cause, & where the rocks of lava enter the sea, the growth of the coral is probably prevented by the sand which is temporarily cast over them during heavy gales of wind. —

On the NE side of the island, the channels are said to be a little deeper. The reefs at the Grand Port differ from the ge all the others, in having a broad channel, with 5-15 fathoms water, running for four miles parallel to the land; but as towards each end of it, extremity there are ordinary channels leading to seaward, & connected caused by two principal rivers, its origin likewise may be connected with the same circumstances. From these causes The channels of shoal water which thus skirt nearly the whole coast line are protected from the outer breakers, run along the shore, & so afford to canoes & boats a safe navigation. It would result from this general structure of the reefs, that if

where [illeg] the beaches] '[illeg]' added pencil.

extremity] added pencil.

leading] added pencil.

& connected] added pencil.

which thus skirt nearly the whole coast line are] added pencil.

The channels of shoal] 'The' added pencil.

& so afford to canoes] 'so' added pencil.

structure of the reefs] 'structure' added pencil.

893

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Mauritius

they were uplifted into dry land, the outer margin would form a ridge higher than the internal parts. It has been from oblong [illeg, faded] affected. The Can we doubt but that this affords an explanation to the "espece de effoncement ou chemin couvert naturel" & the long pools of water mentioned by the Officier du Roi? [sketch] lumps of coral elevated The outer margin of reef to seaward appears terminated to separate at by a defined line; but beyond it the slope of the bottom is by no means rapid: at the distance of half to three quarters of a mile measured from the reef this line the bottom depth is commonly about 20 fathoms; at [on m &c] between Port Louis & Tomb Bay, at one mile & a third of a mile from the reef I found bottom with 86 fathoms. This relating to east coast would be called Easter shore, but as [illeg] to by pass it, in due course Personally I only examined the reef on the west side of the island, where the sea is tranquil, almost like the water of a lake: (on the opposite & windward side, where a heavy swell constantly breaks, I understand that the structure of the reef differs in several circumstances aspects. Where I saw the reef there the shallow flat expanse was partly composed of sand & patches of delicate living (a)

It has been from oblong [illeg, faded] affected. The] added pencil.

lumps of coral elevated] and sketch added pencil.

to separate at] added pencil.

beyond it] added pencil.

this line the bottom] 'line' added pencil.

measured] added pencil.

at [on m &c]] added pencil.

This relating to east...in due course] added pencil.

aspects.] added pencil.

expanse] added pencil.

893 verso

(a) My term of Madrepora must be ascertained to be correct; the great flat pieces my memory will [sketch] like Madrepora.

page in pencil.

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Mauritius

[Madrepora] corals. Legon beach On the outer margin, great & very irregular masses, chiefly composed of strong branching Madrepores, reached to the height, of the level of the water at the lowest tides; these masses were separated by hole & channel of deeper water. Pulling out to seaward the water gradually deepened; to the depth of 8 fathom I sounded repeatedly with a lead, the face of which was formed like a saucer with a diameter of four inches. The arming invariably came up deeply cut by the branching corals Madrepores [sketch] & marked with the impressions of Astreas*; its surface was also without a single exception perfectly clean, not bringing up a particle of sand. (a) As at each cast especially where I could see the bottom we pounded the bottom with the lead, & as the sand, if present, would have adhered from any of the blows, I think it is pretty certain, that where coral is most abundant, the bottom is quite clean. (C) This fact would afford a useful aid in ascertaining the depth at which coral flourishes. from 8-15 fathoms occasionally there was a little sand, but most

[Madrepora]] added pencil.

Legon beach] added pencil.

94 verso

same fact observed at Keeling

useful indication

Corals like associated [grisses] hides the soil

page in pencil

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Mauritius

frequently the arming was quite clean & most beautifully marked with impressions of Astreas, the species of which resembled (3628, 3629). Hence we see that the limit of 25-30 ft fixed upon by M. Quoy & Gaimard as the extreme depth at which the genus Astrea grows, is three times too little. With the Astreas, there were some species of Madrepore, Seriatopora & fragments of branching Millepora (3583) & I think Porites as figured by Lamouroux. I amy add also that on the beach there were plenty of fragments like (3612 & 3573) Meandrina, & some of Astrea (3614). Parts of the beach were also nearly composed of small bits of an inflexible branched Corallina (like 3616). I do not know, where this flourishes, but probably, from what I have seen on other reefs near amongst the line of brakers. — To return to the soundings from 15-20 fathoms sandy bottom & very great quantities of Seriatopora (3633). This delicate coral must form most extensive beds; I believe it is the same

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Mauritius

species, but stunted in its growth, as that one which grows near the surface of the water in the shallow lagoon in the Keeling Islands; this is a singular fact, because it shows that some corals can grow at very different depth. — One sounding also at 20 fathom brought up a large piece of a strong stony branching Andrepore (like 3613) which is common close to the breakers. Between 20 & 33 fathoms, various more [several] sounding with sandy bottom; one patch however at 30 fathom was composed of some large coral, an indentation in the awning was fluted as if from the margin of a very large Caryophillia. Beyond this I only had one sounding at 86 fathoms, the bottom composed of small rounded bits of dead coral & a volcanic pebble. — (These soundings of course apply to one small space alone, but I feel little doubt that those corals, which are efficient in forming a reed do not abound at a greater depth then 15 fathom.) The slope of the bottom was much more rapid beyond them

shallow] added pencil.

897

[top of page torn off]

[illeg]
windward side of the <cut off>

(a) of the reef, consists of a tolerably level flat of solid coral rock, which at low water is covered by one or two feet of water: from the descriptions of the officers it would appear that the surface is coated over with encrusting Corallinas, & that living corals, which are of the delicate branching kinds only grow in the gullies & little pools of water. The outer margin, where the sea violently breaks, is steep, & is the highest part of the whole reef; this it is dry for about two hours at the bottom of spring tides. This elevation above the mean level of the sea, appears considerably to exceed that of

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Mauritius

the reefs in the Pacifick; hence I suspect it is owing to that rising of the land, which has affected the whole Island. On the inner side, the reef slopes down into the main channel. They state that the corals which grow on the exposed parts, differ entirely from those in the more protected situations. (Tahiti facts) It will hereafter be seen how closely this description agrees with the great reefs of the coral islands in other parts of the world. — A solid reef, such as this is described to be may be in vain sought for on the leeward side of the island; the solid massive corals appear only to flourish in a turbulent sea. It is the opinion of the officers, that the amount of matter in the reefs, is at least equal, if not more abundant on the windward than the leeward side. In the charts of the Island, the reefs at Green Port & on

A general discussion might be here introduced on the date of growth of windw & Leeward &c

(Tahiti facts)] added pencil.

A general discussion...Leeward &c] added pencil.

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The NE extremity are the most extensive & they are on the windward side; near Cape Brabant however, which is well protected, there is a very broad reef. No where could there be found a greater contract than on this island, between the turbulence of the water on the one side side as compared to the other side, tranquility on the other, yet here & as we see in the former situation the dimensions of the reef at least equal to those in the latter & its structure far more solid & durable. we may feel certain, that a violent agitation in the water of the sea, is far from being un favourable to the production of coral. — The most important conclusions which I deduce as probable from the examination of these reefs, are, 1st that the coral does not usually grow attached to the shores, a fact which probably originated from the want of a solid foundation & the injurious tendency of the loose matter washed about by the sea; to these causes some others, which will hereafter be mentioned,

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Mauritius

might be added: 2d that the highest part of the reef is situated at the outer margin; this must be owing either to the greater motion of the water, or to its greater purity: 3d that reefs composed of solid stone are only formed in a turbulent sea: 4th That efficient corals grow at the depth of 15 fathom & probably not commonly at a greater depth than 20: — 5th that the distance of the outer margin of the reef from the shore depends on the original inclination of the bottom: — it is also probable that from calcareous sand accumulating on the external slope, the reef of living coral may extend outwards, but necessarily at a very slow rate of increase.

The configuration of the reef of this island, differs from the encircling reeds of the many of the islands in the Pacifick in the following respects: the far greater depth & width of the channels between the reefs & the land: —

900 verso

The reef in front of the Grand Port, alone resembles those of the Society Isles. —

901

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Mauritius [pen or nib tests]

the much greater depth of the water at a short distance beyond the breakers; & lastly in this circumstance. — Taking for granted that generally the slope of the land & of the bottom of the neighbouring sea, bear a relation to each other (as has been remarked by Dampier & others), then if the land comes down to the beach coast at a high angle, & the reef is very broard, there must exist at the outer margin a great thickness of coral; such happens in the Pacifick, & is shown in the imaginary section (A). — On the other hand, in this island, the thickness of the coral (as shown in section B) need not much exceed the depth at which it is believed coral can spring up from the bottom. — We shall afterwards attempt to show that the relation of the breadth of the reef to the general angle of inclination is very important with respect to the theoretical origin of the coral reefs of the Pacifick. —

901v [not microfilmed, transcribed by Gordon Chancellor]

Important fact of attachment at end.— & causes of rivers lagoons aided by rivers and streams.— Confirmed with fact of the most [differ] corals outside; & greater absolute height which latter observations, applicable to original inequality in the lagoon Islds.— Effect of form of land.— examples in Pacifick


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