The foregoing description of the first-formed tabular masses, and their mode of arrangement, is of a general application: but many modifying circumstances are to be taken into the question in giving an account of all the variations in their arrangement; but a description of them would be too long for this essay.

Of the Second-Formed Earthy-Tabular Masses.

The second-formed tabular masses are known by the names of sticking, dowk, flookans, &c. These masses always resemble the softer parts of the matter contiguous to the bended-tabular rents; and consist chiefly of that part of it which is the easiest suspended mechanically in water. They, therefore, generally contain a large portion of clay. In the coal formation they resemble slate-clay, alum-slate, &c.; but in general they are much softer, and sometimes have the consistence of common clay. I have never met with them in lime-stone but in Derbyshire, and there they have proceeded from the slate-clay which lies on the lime-stone. In granite, gneis, and sienite, they consist of soft, greyish, greenish, or yellowish-white clayey matter, just as the colour of the felspar of these rocks may vary. In serpentine they sometimes constitute the substance called soap-rock. In Cornwall they are called flookans; and Price describes a flookan, in his Mineralogia Cornubiensis, "as a tenacious and glutinous earth or clay, that sometimes runs withoutside some veins, immediately between either wall of the lode and the lode itself, and more frequently adhering to the hanging or superior wall, and sometimes mixed in and throughout the lode itself."

The second-formed tabular masses are generally situated between the upper side of the rent and the first-formed tabular masses, and sometimes between these masses and the lower side of the rent. They lie parallel and close to the sides, and never stretch across the rent, like the first-formed tabular masses, except when they change from one side to another, which is very seldom. The unshaded parts a, b, c, d, e, fig. 3, represent these masses. They do not continue the whole length of a rent, but are frequently interrupted by the first-formed masses, and by the closing of the sides of a rent. Hence the spaces in the rents which contain these masses are of tabular shapes, which are of various figures and dimensions. These spaces are generally connected together by channels, which are sometimes very small. But although the greatest part of the matter is arranged in tabular masses which lie near the sides of these rents, yet the rest has adapted itself to every inequality in the surfaces of the first-formed tabular masses, and to those in the sides of the rents; and has also completely filled every cavity in the firstformed masses, and in the matter contiguous to the rents, to which they could gain access.

As the nature and position of the second-formed are so different to those of the first-formed tabular masses, these masses could not both acquire their situations at the same time; but as the former

have adapted themselves to the inequalities of the latter, those must have been in a fluid state when these were solid. Hence the first obtained their situations after the second sort of masses. With the help of these observations, it is easy to account for the forming cause of the second-formed tabular masses. As the bended tabular rent continued to increase in height and width, fluid matter entered it, and the first-formed tabular masses were produced in the manner already described; but after a certain length of time, no more matter was separated from that contiguous to the rent; yet by the contraction of this matter, the rent continued to widen, and the first-formed masses contracted also, so that hollow spaces were formed in it. These spaces could not lie without some interruptions as to their existence opposite the whole of the superficial area of a rent; because some of the first-formed masses must lie against the sides of the rent, to retain their situations, and to support others which did not rest against the sides. The spaces so formed were gradually and slowly filled up in the manner of a clayey sediment, and thus produced the second-formed tabular masses.

2. Of the Matter in Rents containing Earthy and MetallicTabular Masses.

The arrangement of the earthy-tabular masses in rents which contain metallic-tabular masses is the same in every respect an in rents, filled only with earthy-tabular masses, with the addition of being associated with the metallic masses which are found sometimes on the sides and middle of the rent, and which are subject to the same variation of thickness, and want of continuity, as the earthy-tabular masses. There is one circumstance in the arrangement of metallic matter which is peculiar to formations consisting of alternating strata of different denominations. The metallic masses are not found in rents opposite strata of every denomination, but only opposite one or two. Thus, in the mining district round Alston, in Cumberland, the strata consist of white sand-stone, lime-stone, coal, slate-clay, &c.; but the lead ore is only found in lime-stone and sand-stone, and never opposite the rest; except when the first-formed masses of lime-stone or sand-stone are situated in a rent, a little lower than the strata from which they proceed; then they sometimes have masses of lead ore adhering to them; and when the lead ore is found in small masses mixed throughout similar masses of lime-stone, sand-stone, and clay-slate, it is sometimes lying opposite the clay-slate strata; but here it is evident, from the lime-stone and sand-stone masses which accompany it, that it has fallen from opposite the lime-stone and sand-stone strata. The same peculiarity takes place in the Derbyshire mines. There the lead ore is found only opposite the limestone, and not opposite the amygdaloid, or the slate-clay strata, except when accompanied with lime-stone masses.

As there are no metallic strata opposite most of the bended

tabular rents from which the metallic matter could proceed, and as the arrangement of this matter in these rents is such as to show most distinctly that it has not been deposited from a solvent which might once cover the earth, nor has been forced into its present situation from below in a fluid state; so its existence in these rents embarrasses very much the speculator on the source of the matter in them. But as the metallic masses are associated with the earthy, which I have shown to proceed from the contiguous matter, I would be very much inclined to suppose that the metallic matter has the same source. But although the mechanical part of the process admits of proof, yet the chemical part is difficult to reconcile with the present principles of chemistry. Still, however, from my knowledge of the mutual arrangements of the earthy and metallictabular masses, I will venture to hazard the conjecture, that some profound chemist will discover much nearer relations between the internal properties of metallic and earthy matter than we are at present aware of.

SECOND DIVISION.

The Arrangement of that Matter in Bended-Tabular Rents which entered them at the Earth's Surface.

Although the matter in the most of bended-tabular rents is similar to, and has proceeded from, that in which they are situated, yet into some of them it has entered from the earth's surface, in either a fluid or a solid state.

The matter which entered these rents from above in a fluid state is green-stone, basalt, porphyry-slate, &c. Many of the rents so filled contain throughout matter of one denomination; but a few of them of two or three denominations. Rents filled from above with the matter of green-stone, basalt, &c. are frequently found in the coal, red, and white sand-stone formations, and are abundant in many parts of Scotland, in these and some other formations.

When the rents so filled were formed, the matter in which they are situated was so near its present state of solidity that no matter from that source could be forced into them by the incumbent weight. They, therefore, remained empty, till fluid substances, by a part of which they are filled, travelled over them to take their situations in lower parts of the same formations, or in others which are on lower levels; and in doing so, matter entered, and then filled them to the earth's surface.

Bended-tabular rents, filled with matter which entered them in a solid state, contain clay, sand, gravel, and stones, either separate or mixed together. A few of them are found in almost every formation of coal. Such rents were formed at the same time as those containing green-stone, &c.; but as these fluid substances did not travel over them, they remained empty till filled by the process which produced the alluvial matter, by a part of which they are filled.

Of the Straight-Tabular Rent.

This rent is straight in both directions. Hence its name. Its position is always at right angles to that of the strata. Hence it is generally either perpendicular, or angular with a great ratio of angularity, or approaching nearly to a perpendicular line. There is no alteration in the position of the strata contiguous to it, as any stratum on one side is opposite its corresponding part on the other. This is a circumstance that at first sight distinguishes it from the bended-tabular rent. It is filled with matter that entered it from above; such as the matter of green-stone, basalt, &c. or clay, stones, sand, and gravel.

This rent was formed when the earth's matter was so near its present state of solidity, that the inequality in its contraction was so very small as not to be discernible. Hence we find the strata on the same level on both sides of it.

The cylindrical and ovalar rents I will not at present describe, as. they are not very important phenomena in a geognostic point of view. They are very well described in William's Mineral Kingdom, under the names of pipe and flat veins.

In my next communication I will describe the junctions of tabular rents one with another.

Errata in Mr. Longmire's first Communication on Rents.

Read as follows the sentence which begins on the 42d line of the 84th page:— "And Professor Jameson mentions two near Freyberg, in Germany; one of which is more than four miles in its horizontal direction, and the other more than ten miles in this direction."

The letter w in fig. 1, Plate XXIX., ought to have been placed close to the highest extremity of the rent, v, w, h.

In fig. 3 of the same plate some letters are misplaced: these, however, will be best corrected by repeating the figure in a future plate.

ARTICLE V.

Experiments upon Green Uran Mica, with a view to its Chemical Analysis. By the Rev. W. Gregor.

ANY general account of the external character and appearance of a mineral that is so well known would be superfluous. A scientific description, that can be useful or interesting, must be left to the experienced crystallographer.

The subject of the following observations was raised in the copper-mine called Gunnis Lake, in the eastern extremity of the county of Cornwall.

The crystalline laminæ are, in general, very free from extraneous substances. Small fragments of quartz, however, and a light ochrey

matter, easily separable by washing, are sometimes found adhering in small quantities to them.

The purest pieces were carefully selected for experiment.

The specific gravity of this fossil, taken at the temp. 52 of Fahr.

was 3.3.

A portion of it, reduced to a fine powder, was exposed, in a small glass retort, to an open fire. A dew soon collected on the neck of the retort, and passed into the receiver. It was pure

water.

100 grains, that had been exposed to a low red heat for ten minutes in a platinum crucible, weighed whilst still warm = 84.6. The mineral, therefore, had lost 15-4 per cent. Another portion exposed to a stronger heat for one hour was diminished, in respect of weight, very nearly in the same proportion.

The mineral, after ignition had lost its brilliancy, and had assumed the appearance of fine brass filings. 100 grains, which had not been ignited, in the state of a fine powder, were put into a matrass, and covered with pure nitric acid, moderately diluted with water. The acid produced no effervescence, and whilst it was unaided by heat, it seemed to act very feebly upon the mineral for many hours. The vessel was then placed in a digesting heat. Yet the solution proceeded very slowly, and was frequently interrupted by the deposition of a green crust, which from time to time covered the bottom of the vessel, and protected the mass from the action of the acid. Boiling water produced little or no change on this crust. It became necessary, therefore, repeatedly to add fresh portions of nitric acid in order to dissolve it. At last, by means of an excess of acid, a solution of the mineral was effected, except a very small quantity of residuary matter, which after ignition = 01 gr. It was a mixture of quartz and oxide of iron; and as it was extraneous, I dissolved an equal quantity (0-1 gr.) of pure uran mica in nitric acid, and added the solution to the former one.

The solution was light green. I now poured into it ammonia in excess, which separated a yellowish-white matter. The blue supernatant fluid was carefully poured off from the subsided precipitate, and fresh portions of ammonia were repeatedly supplied, and the matrass containing it was from time to time shaken, and at last exposed to a digesting heat.

When the last effusions of ammonia, aided by the heat of the sand-bath, showed not the slightest indications of copper, but were poured off absolutely colourless, the undissolved matter was mixed with distilled water, and thrown upon a filter, and sufficiently edulcorated. What was thus separated by the filter was gradually

When in a former analysis I operated upon the same quantity of uran mica which had been ignited, I found that the last portions of ammonia, when they had ceased to be tinged blue, became of an opál hue, and that a considerable quantity of distilled water passed through the filter before it became perfectly transparent. La this case nearly two per cent, of uran oxide were suspended by the ammonia.