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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.f

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 à 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, commonly peaty and mossy, not being absorbed, flows so slowly as barely to keep the surface wet; and also the rocks on the shores of the lakes and mountain tarns, more or less washed and kept wet by the waves breaking on them, or by drifting spray, and likewise the rocks in the beds, or on the sides of streams which, from fluctuations of level, are rarely long under water, and yet seldom perfectly dry. In these latter situations, it may be remarked, the remains of aquatic plants and of infusoria are to be detected in the colouring matter, as well as soot-like granules.

The source of the soot-like matter is not so obvious. In a former number of this Journal, that for July 1844, I have given an account of “a carbonaceous deposit or film, on the Lakes of Westmoreland.” That deposit was shewn to be of the nature of soot, and was inferred to be derived from the adjoining manufacturing districts, wafted here by the wind, and falling with mist or gentle rain with a lull of wind. The observations which I have since made, have been confirmatory of this inference, and tending to prove that such precipitation of soot is an ordinary, rather than an uncommon or accidental occurrence here, and adequate therefore to the effect supposed. One confirmatory instance may suffice,-it is that of the discolouration of the sheep of the country, especially after exposure for weeks or months on the higher fells. Seen on their mountain pastures, or when driven into the lower meadows in the early spring, their coats are of so dark a hue as to resemble closely those of their fellows fed in the most smoky precincts of our great towns; and, on examination, the colouring matter staining the fleece, is found to be similar to that of the black film of the lakes and tarns; and in brief, essentially soot.

What I have described, may seem disparaging to the lake district, “ the staple" of which has been poetically defined to be “ its beauty.” This I can hardly admit; contrast conduces to pleasing effects; dark rocks, and bright verdure are not incongruous on the mountain fell, no more than are the black shores and blue waters of the mountain tarns, or the pure white of the new born lamb by the side of its dark mother. The contrast almost surprises, and surprise com

monly pleases ; moreover, it ought not to pass unobserved, that the discolouring effects described, however prevalent, are not universal, and that they are often intermixed with brighter hues, tints of red, orange, and white, derived from the lichens which spread over the rocks of drier surfaces, and enhance the variety and beauty of the colouring.

In another point of view, in relation to the influence of atmosphere, it may be a question, whether the salubrity of air of this district may not be impaired by the cause adverted to. If it be, it is probably in so small a degree, as not to be appreciable. The good to the town or crowded district from which the fuliginous vapours are removed no doubt preponderates greatly over any possible evil to the country. Benefit may even result to the latter, from the precipitation of the matter of these vapours, in their fertilizing effects on the upland pastures. Thus considered may they not be viewed as an example of the equalizing, moderating tendencies, which belong to the economy of nature, in which, when sufficiently scrutinized no real waste of means can be detected, nor loss of substance? LESKETH How, AMBLESIDE,

September 26, 1851.

Some vienos respecting the source of Light, Sc. By JAMES

NASMYTH, Esq., F.R.A.S. Impressed with the conviction that the progress of science has often been most importantly advanced by the setting forth of hypothetical views as to the nature of those causes which result in great phenomena, I am, for these reasons, induced to hazard and venture forth with some views on the subject of the nature of the solar light, more especially in reference to the well known but most remarkable phenomena occurring in the case of stars of variable and transitory brightness, as also in reference to those wonderful results of geological research, namely, the unquestionable evidence of the existence of an arctic or glacial climate in regions where such cannot now naturally exist; thus giving evidence of the existence of a condition of climate,


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