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broken off; for a dead fragment proves nothing. Even a strong impression upon the lead, shewing the form and character of the surface cells of a coral is not wholly satisfactory, as it may have been given by a mass not living. A living fragment, placed in water, will be seen to have a fleshy surface, even if the polyps do not expand. The best observations, with reference to this subject, would be made with a diving bell.
Much yet remains for further investigation. Mr Edward Forbes, in his Zoological Explorations of the Ægean, distinguished three separate regions of invertebrate species within 20 fathoms of the surface: the first, or littoral, extending to 2 fathoms in depth; the second from 2 to 10 fathoms; the third from 10 to 20 fathoms.* Similar subdivisions, or others on the same general principle, may yet be detected in the Pacific, indicated perhaps by zoophytes as well as molluses. There is no evidence, however, that there are successive beds, composed of a distinct set of species, as has been sometimes suggested. The upraised reefs of Metia afford no proofs of such a mode of formation; on the contrary, they shew that the process is continuous and uniform in character through the reef-growing depths. The species in the lower part of the 16 fathoms are probably different from many of those above ; but they pertain to the same genera in most instances, and moreover there are no abrupt transitions, consequently the resulting reefs should have a nearly uniform character, as here stated. This fact may be better appreciated after perusing the following chapter.
The Nullipore zone along the reef-line, between low and high tide, is clearly made out by Mr Darwin, and is one of the interesting results of his investigations. It performs a very important part, by the protection it gives the reef from abrasion. The exposed reef is thus gathering lime from the waters, and extending itself; when, if devoid of this protection, it would be constantly yielding to the sea. On the inner reefs, where the protection is not needed, it is not given.
* On the Ægean Invertebrata. E. Forbes, Rep. Brit. Assoc. for 1843, Some species of Nullipore, however, occur in these regions, and others are found at various depths.
As the Caryophyllia family extend into deeper waters than most other reef corals, it might be inferred that these at least may constitute a lower bed, or substratum. But this is by no means the case. As just stated, one species (the Dendrophyllia nigrescens) was found at 14} fathoms, and also of identical characters at low tide level. The Caryophylliæ are but sparingly distributed; the species are few, and mostly small, and not a dozen different kinds were detected in the Pacific. Their contributions to reefs are therefore inconsiderable.
4. Rate of Growth of Zoophytes. The rate of growth of zoophytes is a subject but little understood. We do not refer here to the progress of a reef in formation, which is another question complicated by many co-operating causes; but simply to the rapidity with which particular species of coral zoophytes increase in size. There is no doubt that the rate is different for different species. It is moreover probable that it corresponds with the rate of growth of other allied polyps that do not secrete lime. The rate of growth of Actiniæ might give us an approximation to the rate of growth in a Mussa, which are coral animals of like size and general characters ; for the additional function of secreting lime would not retard necessarily the maturing of the polyp; and from the rate of growth of the same animals in the young state we might perhaps draw some inferences as to the rate in polyps of corresponding size. But no observations on this point were made by us while abroad.
Although the rapidity is undoubtedly far less than was formerly reported, the following facts from different sources, seem to shew that the rate is still greater than has been of late believed. Mr Darwin, citing from a manuscript by Dr Allen of Forres, some experiments made on the east coast of Madagascar, states, that in December 1830 twenty corals were placed by this gentleman apart on a sandbank, in three feet water (low tide), and in the July following each had
nearly reached the surface, and was quite immovable, and some had grown over the others. Mr Stutchbury describes a specimen, consisting of a species of oyster, whose age could not be over two years, encrusted by an Agaricia, weighing two pounds nine ounces.* It is stated by M. Duchassaing, in a letter from Guadeloupe, that in two months some large individuals of the Madrepora prolifera, which he broke away, were restored to their original size.
Since the return of the expedition I have received a letter containing some facts on the growth of Actiniæ from Sir J. G. Dalyell, whose able observations in this department of science are highly curious and important. After speaking of the various conditions and sizes of the young at birth, and of the difference in the rapidity of growth depending on the amount of nutriment at hand, he says, speaking of a Scottish species of Actinia, “The dimensions will generally double in a fortnight from its birth. The diameter of the base being originally about an eighth of an inch, or hardly as much, will be five-eighths in six months; and the tentacles will occupy a circle of an inch and a half in diameter. In twelve or thirteen months the diameter of the base will reach an inch, and the expansion of the tentacles two inches between the tips. An Actinia, whose tentacula expanded a quarter of an inch three weeks after it was produced, enlarged so much in five months that they expanded an inch, and the body was then half an inch thick.” If we reason upon these data, and assume that the Madrepore polyps may increase lineally in six months as much as the young Actinia, we shall have an elongation of five-eighths, or three-fourths of an inch in six months. Taking the still more rapid rate of doubling in a fortnight, which might be more correct, since the Madrepore polyps are about the size of the Actinia in its earliest state, we should have a lengthening of a fourth of an inch in a month, and three inches a year. The data upon which this conclusion is based, though important, are uncertain, but would probably give too high rather than too low an estimate. And yet it is far below the rate apparently established by the experiments with corals cited in the preceding paragraph. We must admit that the subject requires more accurate investigation.
* West of England Journal, vol. i., p. 50. † L'Institut, No. 639, April 1, 1846, p. 111.
The stay of the expedition near any particular reef in the Pacific was too short for any examinations by us. They might easily be made by those residing in coral seas, either in the manner adopted by Mr Allan, or more definitely by placing marks upon particular species. By inserting slender glass pins a certain distance from the summit of a Madrepore, its growth might be accurately measured from month to month. Two such pins in the surface of an Astræa would in the same manner, by the enlarging distance between, shew the rate of increase in the circumference of the hemisphere; or, if four were placed so as to enclose an area, and the number of polyps counted, the numerical increase of polyps resulting from budding, might be ascertained. It is to be hoped that some of the foreign residents at the Sandwich, Society, Samoan, or Feejee Islands, will take this subject in hand. There are also many parts of the West Indies where these investigations might be conveniently made.
(To be continued.)
On the Colour of the Rocks in the Lake District of the North
of England. By JOHN DAVY, M.D., F.R.S., &c. Communicated by the Author.
The careful observer in passing through the lake district of Westnuoreland and the adjoining counties, can hardly fail of being struck by the prevailing dark hues of the rocks, often in so great a degree as to be almost black, and of having his curiosity excited as to the cause.
If he break a portion, he will commonly find the very dark hue to be superficial, or at least almost invariably of greater intensity at the surface than in the interior ; and he will probably arrive at the conclusion, that this darker hue is a stain or discolouration imparted by adhering adventitious matter.
This conclusion I believe to be the correct one. I shall mention some circumstances which appear to confirm it.
First, Of the rocks of the district. These, for the most part,
are varieties of clay-slate, more or less modified by metamorphic agencies. Their colour, as exhibited by a fresh fracture, is grey of different shades, owing chiefly to the presence of a minute portion of carbonaceous matter. Accordingly, when exposed to the action of an open fire, the grey hue is destroyed. Its colour is changed to shades, commonly light, of red or yellow, from the combustion of the carbonaceous matter, and the conversion of some protoxide of iron,-an oxide seldom entirely absent from the composition of these rocks,-into peroxide. And the effect of weathering—of long exposure to the atmosphere,-is similar, where the circumstances are such as to permit of its being witnessed without interference of adventitious causes, -as in the instance of rocks subjected to the action of torrents when the streams they skirt are flooded, and that so frequently as to prevent the growth of the lower order of plants on their surface.
Secondly, Of the nature of the discolouring matter. When abraded and placed under the microscope, it is found to be composed in most instances of granular matter like that of soot, and of fibres and cells like those afforded by the lower order of vegetables, such as the mucors, lichens, and mosses in a state of change, analogous probably to that by which peat is formed from the same class of plants. Further, ignited in the open air, the colouring matter burns, and is consumed, leaving only a very little ash.
Considering the composition of the rocks of the district, as adverted to, and these results, are we not justified in the conclusion that the discolouration is adventitious; and, moreover, that it is occasioned partly by a substance resembling soot, and partly by vegetable matter of a peaty character.
The source of the latter matter is easily found, the tendency to form peat being always to be observed in this climate wherever the local circumstances favour, such as moderate moisture, water more or less stagnant, shade, and lowness of atmospheric temperature. The rocks, the surface of which are most discoloured,--are of the darkest hues,-are so situated as to be under the influence of these circumstances, especially the “ watery rocks," an expressive term of our great poetrocks over which water oozing from the ground above, com