sited before many of those unevennesses which give' rise to our present hills and dales were carved out upon the surface, and often where even a deep and extensive valley intervenes, the same strata running in the same direction, and at the same levels, are found continuous on both its sides; except in some particular cases, where the valley is very narrow and abrupt, and its sides perpendicular, and where it appears to have been formed by the dislocation and fracture of the ground, once continuous, in which it occurs.

In consequence of the peculiar arrangement of the coal strata, a section of them often gives the idea of a basin, or boat-shaped concavity, which has been successively filled with the various substances that occur in it; the seams of coal vary in number and in thickness, as well as in quality, and the upper seams are generally imperfect.

In consequence of the dip of the strata, it not unfrequently happens that we have an opportunity of examining and ascertaining the nature of the lowest seams, which, though deep and out of reach in one part of the coal-field, are superficial at another. This is shown at Cross-fell, where the crow-coal rises to-day, which, in consequence of the inclination of the beds, is considered to be nearly 460 fathoms below the lowest of the Newcastle beds, a little to the east of that town.

The description of one coal formation applies, in general, to others; but there are certain circumstances which give peculiarities to some of our coal districts, among which the iron-works are deserving attention. The ore which is here worked is the clay iron-stone; an ore poor in itself, but deriving value from the abundance of coal that attends it.

Some writers have amused themselves with speculations respecting the exhaustion of our coal-mines, and have calculated the number of years, or centuries, that the stock on hand is likely to last. When, indeed, we reflect upon the vast importance of this species of fuel in a country dependent not merely for its prosperity, but even for its very existence, upon its manufactures and consequent commerce; when we remember its enormous and increasing consumption; when we consider that the metropolis only

swallows up annually considerably more than a million of chaldrons exclusively from the Tine and Wear districts; it might appear that the apprehensions of some worthy persons upon this score were not altogether without foundation. It is however admitted, on the other hand, that the Newcastle mines only are capable of continuing their supply for another thousand years; and if this reflection is insufficient to satisfy the disquieted minds of those who are still uneasy, they may console themselves with the reflection that there are many other districts which have only been, as it were, begun upon, and probably numerous deposits of which we are as yet ignorant, but which will be searched for and found when wanted. Besides which, it may, I think, be calculated, that of every chaldron of coals consumed in our ordinary fires, about one-eighth part is lost in the character of soot, smoke, and other unburnt matters; so that in London only, upwards of 100,000 chaldrons of coals are thus dissipated and unprofitably applied to the contamination of our atmosphere, which smoke, by improved methods of combustion, might be turned to profitable account.

In speaking of the general arrangement of the coal strata, I have said nothing of the dislocations to which they are subject, in consequence of what are called troubles, or slips, and dikes; that is, the strata are cut through, broken off, and sometimes thrown up on one side, and depressed on the other; and by the fissures and cracks thus produced are filled with broken stones and fragments of the strata, or with a hard species of rock called a dike, near which the coal is converted into a cinder, and from its cavities emits those tremendous torrents of inflammable gas, techni cally called blowers.

To the probable origin of these faults, or dikes, I shall afterwards advert more at full; they record one fact, namely, that the coal strata, subsequently to having assumed their present disposition and arrangement, have been subject to various disturbing causes, breaking their continuity merely in some instances, but in others affecting the whole district, and throwing it for hundreds of acres together out of its original position.

Lastly, as to the sources and origin of coal. Upon these subjects geologists, as usual, have amply indulged their inventive faculties. Every thing tends to show the vegetable origin of coal, and a regular succession might be shown, commencing with wood, little changed, and ending with coal, in which all traces of organic texture are lost. Yet even in the most perfect coal we frequently find some relic, some trace of a vegetable, or some remains of fibrous texture that announces its ligneous origin. In the leaves that occur in Bovey coal, Mr. Hatchett, to whom we owe many important observations and experiments upon this subject, found resin and extractive matter; and what is more to the purpose, he found a substance having properties intermediate between resin and bitumen, and therefore partaking partly of vegetable and partly of mineral characters; and more lately the same substance has been found in the principal coal-field of Staffordshire. Perhaps, therefore, antediluvian timber and peat bog may have been the parent of our coal strata, but then, how has its conversion been effected; is it merely by the agency of water, a kind of decay and rotting down of the wood; or has fire been called into action, torrefying the vegetable matter, and has the pressure under which this heat has operated prevented the escape of volatile matters, and caused them to assume the form of bitumen; and are those reservoirs of compressed carburetted hydrogen which I have mentioned as causing blowers, to be ascribed to such mode of formation? The discussion of these subjects might be prolonged, but it would end in nothing satisfactory. The theories that have been invented to account for our coal formations are full of weak and assailable points; the further we pursue them, the less do they satisfy us, and the more discordant do they seem with the phenomena they are intended to explain.

We should almost conclude, from the dogmatical air of some writers upon this subject, that they had seen the agents they speak of in active operation; that they had fathomed the depths of the globe, and measured its central heat; but if we compare our planet to an orange, and remember that we have not as yet penetrated its rind; if we compare it to the pasteboard globe of

the instrument-maker, and remember that we have scarcely peeled the paper from its surface; these considerations should alone be sufficient to check the presumption of the theorist, and set bounds to the arrogance of hypothesis.

ART. VI. On a peculiar Appearance exhibited by Hoar Frost. By J. Mac Culloch, M.D., F.R.S.E., &c.

[Communicated by the Author.]

It has long been known that dew, as well as hoar frost, has a tendency to attach itself to particular bodies, rather than to others, and to rough surfaces in preference to smooth. Hence also it is found that hoar frost, which, from its permanence, is more easily examined than dew, is frequently deposited on acute edges and points, when the flat surfaces of the same bodies continue bare. I need not notice the speculations which have been entertained respecting the connexion of this phenomenon with electrical agency; but it is also obvious that the attachment of hoar frosts to strings, edges, and points, bears a striking analogy to that which occurs in many cases of the ordinary crystallization of salts from solution in water, where the preference is so often given to bodies of these forms as the first bases of attachment.

The causes which influence this mode of disposition are as much unknown as is every thing that relates to this mysterious process. It is to little purpose to form conjectures, or to propose hypotheses on this subject; but it is not useless to record any facts, which, by their accumulation and ultimate comparison, may tend to throw light on it; and, with this view, I transmit to you the enclosed sketch of a crystallization, or rather a deposit, of hoar frost, which is remarkable for its singularity, and which has not, as far as I know, been hitherto noticed by those who have paid attention to these subjects. As it will tend to save much explanation in words, and will at the same time render the appearance in question much more intelligible, I send you the sketch precisely as it was made on the iron railing of the door-way where I at first observed it.

The crystallization
magnified.

Section of a bar, with the true position of the crystallization.

The temperature was little less than the freezing point, and there was a moderate fog with a high barometer, and an easterly wind; but no other meteorological phenomena were observed, nor had I any opportunity of examining the state of the atmospheric electricity, being far from home, and casually visiting in Portlandplace, where this sketch was made.

Although the general effect of the distribution of this deposit of ice is sufficiently visible in the drawing, it will require a few words of explanation. The points did not consist of single crystals, but of pyramidal bodies, formed of crystals so minute and entangled, that their forms could not be discovered by the lens. They were about the sixth of an inch in length, and distant from