to the sloth, the other from the bear to the opossum. Farther inquiries would likewise disclose other intermediate series. Such interruptions of the principal series are, however, quite correspondent to the laws of natural affinity. They would only transgress these if interposed members gave rise to recurrent series in one of the two principal; if, for instance, an animal which, according to its natural affinities, succeeds to the sloth, should, from the proportion of its brain and spinal cord, stand above it. But I do not believe that such a genus can be found.

As every genus and species, not only of the mammalia, but also of the other vertebrated animals, has a certain proportion between its brain and spinal cord, in the same manner each part of the brain, especially the cerebellum, bears a definite proportion to the other parts. Hence we might deduce further specific and generic marks; and one day we may be able to distinguish the whole of the vertebrata by the numerical masses and dimensions of the parts of the encephalon, and range them from this character according to their natural affinities. The relation, it is true, varies in different individuals of each species, but there are limits which it never exceeds.

Analysis of the Labradorite Felspar found in the Trap-Rocks of Scotland. By Captain LE HUNTE. Communicated by the Author.

The specimen analyzed was found near Campsie, in a porphyritic greenstone. It was in the form of long narrow crystals, nearly transparent and colourless, of a foliated structure, and vitreous lustre. Its specific gravity was 2,689. The larger crystals frequently present a flat conchoidal fracture, which first

induced me to suspect that it was not felspar. From the great difficulty that I experienced in procuring the mineral in a pure state, the analysis was made upon a small scale; but it was repeated with nearly the same result. It appears, then, that this mineral is labradorite, with the best analysis of which the foregoing nearly agrees. The large crystals of labradorite that are imbedded in trap-rocks are very much cracked, and so impure that they cannot be employed for analysis.

About two miles to the west of the village of Milngavie, near the road between Glasgow and Strathblane, there is a very remarkable brown porphyritic trap, that contains large and beautiful crystals of yellow labradorite. These crystals are cracked, and when heated, present numerous brown spots, which show that they are not pure. The following is their composition:Silica, Alumina,

Lime,

Soda,

Potash,

Perox. of Iron,

52.341

29.968

12.103

3.974

0.301

0.866

99.553

All the analyses of labradorite that have hitherto been made differ a little from each other; some agree with my first analysis, while one of Klaproth's differs but little from the last. It is probable, that, owing to its structure, labradorite has seldom been examined in a pure state.

The chemical characters of labradorite enable us to distinguish it from felspar, even when the quantity for examination does not exceed a grain. For this purpose the mineral, in the state of an impalpable powder, is treated with weak muriatic acid in a watch glass, and gently heated for an hour. The solution is then evaporated to dryness, and the saline residue is heated until the excess of acid be expelled when it is redissolved in water. To the clear solution, when warm, a few drops of oxalate of ammonia are added, which produces a precipitate of oxalate of lime if the mineral be labradorite, but does not produce a precipitate if it be pure felspar.

Dr Macculloch mentions, that the glassy felspar which he found

at Sky frequently resembled labradorite. I have seen many specimens from that island to which the latter name must be applied. The mineral, which gives a porphyritic aspect to some of the pitchstone of Arran, appears to be glassy felspar; as far, at least, as I can judge, from having examined a very small fragment of it. The porphyroidal traps, on the western boundary of the Scotch coal district, more frequently owe their structure to the presence of labradorite than to that of felspar. This last mineral, with its usual characters, very rarely occurs in them. We frequently indeed find a white, opaque, foliated mineral, destitute of lustre, which has generally been called felspar; but I have procured both lime and soda from this substance; therefore it has, in some cases at least, been improperly named. The trap-rocks appear to contain other minerals which resemble felspar in many of their characters, but differ from it in composition. One of these, which I found in the neighbourhood of Stirling, is of a red bright colour, opaque, foliated, and has a silky lustre; from the small quantity of it which I could procure I could only determine, that it contained a good deal of peroxide of iron, and a little lime. The analysis of another was published some time ago by Dr Thomson, who called it Mornite. It is very remarkable that the composition of mornite is precisely the same as that of the first specimen of labradorite that I examined, excepting that it does not contain an alkali, but it contains a quantity of protoxide of iron, exactly equivalent to the soda in labradorite. The external characters of mornite and labradorite must also be very similar, for the mineral dealer from whom Dr Thomson procured the former, when I showed him my specimens of the latter, immediately called them mornite. I have not seen this mineral, which was found in a trap-rock in the north of Ireland. The granitoidal traps of the Scotch coal district, on the contrary, very frequently present well characterised crystals of felspar, and rarely labradorite. They often, however, contain a white mineral, which, through a great extent of rock, does not present any of the characters of felspar, but rather resembles a zeolite; and these differ very much, in all their characters, from those which contain felspar. It would be easy to arrange the granitoids, from their external characters, into three

classes, and to give them very expressive names, if the minerals which they contain were well known. The unicoloured crystalline traps occasionally contain labradorite, and the presence of this mineral may account for the soda that has generally been found in them, as well as in basalt, to which they are closely allied; but it does not, in some cases, account for the whole of the soda, so that they probably contain other minerals, of which this alkali forms a constituent part. I have analyzed two of these rocks, that contained labradorite, and the analysis differed but little from those of some basalts. The neighbourhood of Glasgow affords some very fine specimens of columnar trap, which all contain labradorite. The range of large and regular columns that stands on the side of a wooded hill, a little to the south of Strathblane, is well known. It is composed of a dark green trap or greenstone, containing crystals of that mineral. The same may be said of the black basalt, that forms the remarkable horizontal columns, near Altmarry turnpike, on the road from Glasgow to Drymen; and also of the columns that are found in more than one place on the Kilpatrick and Cathkin Hills. So that labradorite is probably one of the minerals of which basalt is often composed.

I have made these remarks upon the situations in which I have found labradorite, because I believe that we cannot attempt to arrange the trap-rocks, or give them expressive names, until we are better acquainted with their composition. The present nomenclature of these rocks is very defective, and, not being formed upon any fixed principle, it cannot be improved. The names must either be multiplied to such an extent as to render them very burdensome to the memory, or be applied so loosely, as always to require a definition. The short and simple terms now in use have not the flexibility which names, applied to objects that are constantly varying in their characters, should possess. It would be better to employ in geology, as often as possible, compound descriptive names, that would indicate the structure and composition of rocks. Such names would, in many cases, be shorter than those now employed, as they never would require a definition; and, when formed upon a fixed principle, they might be multiplied without loading the memory. As far

as the trap-rocks are concerned, they can be accurately named in this manner only; for we frequently find among them rocks of the same composition, but differing in structure; and, on the contrary, their structure is often the same, when their composition is different. They are easily arranged in a few classes, by their external characters; and the class to which the rock belongs may be expressed in a single word, while its composition may be denoted by two or three. There are some cases in which we should still be obliged to use arbitrary terms; but this should not induce us to reject the only principle upon which an expressive and convenient nomenclature can be formed for the greater number of these rocks. I have long been in the habit of using those compound terms in a district which presents trap under a great variety of forms; and I should mention the mode that I have adopted, to render them as short and expressive as possible, but that my observations and experiments on the subject are far from being completed.

Physiological Investigations arising from the Mechanical Effects of Atmospherical Pressure on the Animal Frame. By JOHN DALTON, F. R. S.

A PERIOD of a century and a half has elapsed since the inventions of the barometer and the air-pump. In this time the weight of the atmosphere, its elasticity, its specific gravity, and many other properties, have been ascertained experimentally with almost mathematical precision. The weight of the atmosphere, that quality we have more particularly to consider in the present essay, is not constantly the same, as is proved by the rising and falling of the barometer. It varies in this part of the earth from

th toth of the whole weight at certain times; but those variations are gradual, so that it requires some days or weeks before the weight passes from one extreme to the other. On an average the weight or pressure of the atmosphere amounts to 14 lb. on each square inch of surface of the earth; and, as fluids press equally in all directions, every square inch of surface, whatever may be its position, must be subject to the same pressure. The surface of the human body, as well as that of animals in general,