below: and what is more remarkable in the lower half of the alluvial clay, they are sometimes filled with spar and the usual contents of the rents, and in the upper half, with clay deeply tinged with iron; and sometimes opposite the whole height of the alluvial matter they are filled with iron tinged clay: in both instances the rents are covered with only thin strata of soil. In the Shropshire and Cumberland coal formations I have seen rents so circumstanced at the earth's surface; and at Lead Hills in Scotland, in company with Mr. Martin of that place, I met with two such rents, that are situated in the north side of the valley and to the west of the Susannah vein. Other rents that reach to only a few inches below the surface are as wide in the alluvial clay just above the top of the hard rock as they are below; but upwards, they increase in width in such a ratio that each side deviates from 20° to 30° from a perpendicular line. Opposite the alluvial matter they contain clay, mixed throughout with large cobbles, which last are very numerous at the bottom. The contents in these parts appear as if they had been washed into the rents. I have seen such rents in Cornwall. Rents reaching through the alluvial matter exist most abundantly in low and smooth mountainous districts, such as Cornwall and Lead Hills.

The existence of rents in alluvial matter, though new to men of science, is a very important fact. It shows us that the alluvial matter must have been formed before these rents; otherwise, after reaching the surface of the present rocks, the rents could not have passed through the alluvial matter. It also shows us that the alluviał matter was formed from the matter below, when this matter was the least able to resist a disintegrating force: and by it we know that the alluvial matter has not been removed since then. Hence the rocks or strata underneath such parts have not been in the least wasted by the elements.

II. ON STRATIFICATION.

I have said that the phenomenon of stratification, in one point of view, is an effect of the unequal contraction of the earth's matter. I will now give my reasons for this assertion. But perhaps it may be previously necessary to give a definition of the term. Stratification consists in that assemblage of tabular masses, wherein any one mass is parallel to that next above, and to that next below it. A formation that is entitled to be called stratified must have this arrangement of parts every where. According to this definition, all, or nearly all, the red and white sand-stone, and some of the limestone formations, are stratified; but the formations of granite, micaslate, &c. are not stratified, unless they lie in hollows, as they sometimes do, on the primitive and unstratified mass. Mountains divided, in a few places, into tabular distinct concretions, have sometimes been called stratified; but to possess this structure they must be every where divided into tabular masses, which have the

1

same relation to one another as I have shown to be necessary to constitute stratification.

After the matter on which all stratified formations rest had assumed a small degree of solidity, it contracted unequally. Hence one part of its surface sunk lower than another, and gradually formed a number of hollows, into which as gradually entered originally fluid matter, and matter greatly comminuted and mechanically suspended in water.

Let us endeavour to follow the formation of a hollow through a few stages. Let A, fig. 3, be the first stage. Here a hollow, say of a few feet in depth, is observed, which has been gradually formed by the sinking of one part a lower than the part de. Let B be the second stage. The hollow has now got an additional area marked b, and is twice the depth that it was at the end of the first stage, with an equal increase of dimensions sideways. Between this stage and the first the hollow has been gradually increasing in dimensions by the sinking of the part a b more than the part de. C is the third stage; in which another space c is added to the hollow. In this manner the extension of the hollow would continue as long as the matter continued to contract unequally, or till the earth had acquired its present degree of solidity. Some hollows are filled with matter of one denomination, as white sand-stone, &c.; others with that of various denominations, as in the coal formations. The matter in the former instance has proceeded from one source; in the latter, from different sources. Some hollows, again, were filled with matter while forming; others not till after they were totally, or at least nearly, formed. But all hollows so produced, and filled, and such spaces only, except a few rents, contain matter having the stratified structure. The slow but gradual entry into hollows of matter either fluid or mechanically suspended in water, is certainly necessary to give to such matter the stratified structure; but if these hollows had not been formed by the unequal contraction of the matter below them, the present stratified matter would have remained for ever in its original situation. Stratification, then, in this point of view, is an effect of the unequal contraction of the earth's matter.

:

ARTICLE X.

Magnetical Observations at Hackney Wick. By Col. Beaufoy.

45 24 23 52 105 24 25 40

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25 24 21 10

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In deducing the mean of the observations in July, the morning and noon observations are rejected, on account of the great variation.

July 30.-The needle, after being steady for several weeks, vibrated 2' 15". The wind blew fresh from the north, and the needle has continued unsteady.

1.509 inch.

Rain fallen
Between noon of the 1st July }
Between noon of the 1st Aug.S
Evaporation during the same period ..........3.65

Since the instrument was constructed with which these observations were made, Mr. George Dollond, of St. Paul's Church Yard, has so much improved the construction, that the instrument which he now makes combines the advantages of a theodolite, transit, and equal altitude instrument and variation compass, and is equally portable with mine.

ARTICLE ΧΙ.

ANALYSES OF Books.

I. Philosophical Transactions of the Royal Society of London for 1815, Part I.

This volume contains the nine following papers :

1. Additional Observations on the Optical Properties and Structure of heated Glass and unannealed Glass Drops. By David Brewster, LL.D. F.R.S. Edin. and F.A.S. Edin-In a former. paper the author had shown that glass, when heated, acted on light like crystallized bodies; and that Prince Rupert's drops possessed a similar property. On examining these drops carefully, lines were visible in them, forming imperfect cleavages, and rendering th

crystalline structure more evident. The specific gravity of the unannealed and annealed drops was found to be nearly the same, 3.276, allowance being made for the cavities contained in the unannealed drops. These vacuities are occasioned by the contraction of the internal parts of the drop while cooling. They disappear when the drop is heated to redness. It appears, then, that heat produces a crystalline structure in glass, which vanishes as the glass cools.

2. Description of a new Instrument for performing mechanically the Involution and Evolution of Numbers. By Peter M. Roget, M.D. This instrument consists in a very convenient and ingeniously contrived sliding rule, which must be useful in a great variety of cases.

3. Experiments on the Depolarization of Light, as exhibited by various Mineral, Animal, and Vegetable Substances, with a reference of the Phenomena to the general Principles of Polarization. By Dr. Brewster.-In this paper Dr. Brewster gives a list of 58 substances, animal, vegetable, and mineral, which depolarize light; and of 53 substances, which have no effect in depolarizing light. He then gives what he calls a theory of the depolarization of light. The various modes in which bodies depolarize light may be reduced to seven. 1. When the crystal possesses neutral axes, and forms two images which are capable of being rendered visible, as in calcareous spar, topaz, &c. In this case he shows that the apparent depolarization of the pencil is nothing more than the polarizing of it in a new plane. 2. When the crystal possesses neutral axes, and exhibits only a single image, as in the human hair, and various transparent films. This he considers as exactly the same with the first case, excepting that the two images formed by the human hair, &c. being produced by the same, or nearly the same, refractive power cannot be rendered visible by any contrivance. 3. When the crystal has no neutral axes, but depolarizes light in every position, as in gum arabic, caoutchouc, tortoise-shell, &c. These bodies are composed of thin plates lying above each other. Each of these plates possesses neutral axes, and depolarizing axes. But as these different axes do not coincide with each other in the different plates, the consequence is, that the compound body depolarizes in every direction. 4. When there is an approach to a neutral axis, as in gold-beaters' skin, &c. In this case the body is composed of thin films, like the preceding; but the neutral axes of each are nearly coincident. 5. When the crystal depolarizes or restores only a part of the polarized image, as in a film from sea weed, and a film from the partan (crab). He considers that this case is owing to the bodies which possess this property being partly crystallized and partly uncrystallized. 6. When the crystal depolarizes luminous sectors of nebulous light, as the oil of mace.. How the halo in this case is produced, he does not attempt to explain: but he conceives that it necessarily follows from the phenomena that there are two halos or nebulous images, the one lying exactly above the other, and having