pavements, from the lower strata; building stones, of moderate value, from the red and yellow strata; stones for macadamizing, from the trap; and agates and other precious stones from the amygdaloid variety of the latter. At the hill of Kinnoul, near Perth, there is a rock of this kind full of fine specimens.

The Mountain Limestone

Consists, in great part, of what was once life.-Perhaps there is no fact in geology so utterly beyond our power to realize even in idea the truth of, as the formation of the mountain limestone, which is found in our country eight hundred yards deep, almost its entire substance being the remains of animal life we can in many instances trace distinct relics of shells, corals, and crinoidea to the extent of three-fourths of the mass. Need we say, after this, what an astonishing development of life must have characterized the present era, and left such evidences of itself?

Position, Geographical Distribution, &c.-The mountain limestone is found sometimes in beds, divided by layers of argillaceous matter, or of calcareous sandstone and shale, and surmounted often by the millstone grit of the north of England. At other times we find it flanking or even crowning the trap hills in masses of enormous size, when it has been likened to a coral reef surrounding the island which formed its base. Although generally the Coal measures are above the mountain limestone, beds of coal, of the harder and less bituminous kind, called anthracite, descend as it were below the great mass, and occasionally alternate with the various stratą composing the limestone. On the other hand, the limestone seems to ascend beyond its own proper limits, and to alternate with the greater coal beds, and with sandstones, shales, and ironstones. The words "mountain limestone are applied directly to the thick masses that are found beneath the Coal

measures.

Caverns, &c.-A noticeable peculiarity of this limestone is its tendency to divide into rents, or "backs," as they are called, which are perpendicular to the line of stratification, and into other partings which are parallel with the same line. Caverns are frequent; the most magnificent caverns and grottos of the world are found in this rock-those of Derbyshire, for example.

Decided Development of Land Plants.-At last land plants begin to be visible in the earth, now represented by the mountain limestone; for it was unquestionably a terrestrial vegetation that was gradually transformed into the thin seams of coal found in the system. This fact, the great characteristic of the carboniferous group, will be better dealt with in detail when we reach the Coal measures.

Its Animal Life.—The animal life is still marine. The corals are now of large size, and exhibit a very marked advance upon those of previous eras. For instance, in the Silurian rock the corals were chiefly of a sessile kind, that is, sitting or supported in some way or other on or above the ground; but in the mountain limestone many of them are free independent animals, able to rove at their pleasure, and unlike any before or since existing.

The family of the encrinites or crinoids the stone lilies-is very remarkable. "We may judge," says Dr. Buckland, "of the degree to which the individuals of these species multiplied among the first inhabitants of the sea

from the countless myriads of their petrified remains, which fill so many limestone beds of the transition formations, and compose vast strata of entrochal (expressive of the wheel-like joints of the stem of the animals) marble, extending over large tracts of country in Northern Europe and North America. The substance of this marble is often almost as entirely made up of the petrified bones of encrinites as a corn-rick is of straws. Man applies it to construct his palace and adorn his sepulchre; but there are few who know, and fewer still who duly appreciate the surprising fact, that much of this marble is composed of the skeletons of millions of organized beings, once endowed with life and susceptible of enjoyment, which, after performing the part that was assigned to them in living nature, have contributed their remains toward the composition of the mountain masses of the earth." Let us present a portrait of one of the members of this noticeable family: we cannot, perhaps, select a more interesting example than the Apiocrinites rotundus.

The

It is here shown as restored from the mutilated fossils, and greatly reduced from the natural size. In Fig. 1 we see the animal with its fingers open, ready to catch any of the smaller fry that might come within their grasp; whilst in Fig. 2 we behold them shut, while, possibly, the process of digestion is going on in the remarkable stomach revealed in Fig. 3. thickened part marked a shows that an injury to the stem has been repaired. This animal was fixed at the bottom of the sea, but could reach a considerable distance around it, through the flexure of its wonderful stem, of which we will speak presently. Others were able to float singly through the water; and yet others were accustomed to attach themselves to floating pieces of wood, &c. The stem is composed of joints, often called wheelstones, and also St. Cuthbert's beads, as they were used in monkish times, upon strings, as beads for a rosary. Hence the lines

"On a rock by Lindisfarn

Saint Cuthbert sits, and toils to frame

The sea-borne beads that bear his name."

APIOCRINITES ROTUNDUS.

"Each of these joints presents a similar series of articulations, varying as we ascend upwards through the body of the animal, every joint being exactly adjusted to give the requisite amount of flexibility and strength. From one extremity of the vertebral column to the other, and throughout the hands and fingers, the surface of each bone articulates with that adjacent to it with the most perfect regularity and nicety of adjustment. So exact and methodical is this arrangement, even to the extremity of its minutest tentacula, that it is just as improbable that the metals which compose the wheels of a chronometer should for themselves have calculated and arranged the form of the teeth of each respective wheel, and that these wheels should have placed themselves in the precise position fitted to attain the end resulting from the combined action of them all, as for the successive hundreds and thousands of little bones that comprise an encrinite to have arranged them

selves in a position subordinate to the end produced by the combined effect of their united mechanism, each acting its peculiar part in harmonious subordination to the rest, and all conjointly producing a result which no single series of them, acting separately, could possibly have effected."*

The shell-fish of the mountain limestone are also exceedingly numerous, and present many curious and gigantic forms; and we may conclude, from their superior development as regards their predecessors, that their ocean home had already experienced a more genial temperature. It is further clear that calcareous matter must have abounded in the water, to supply the material of which so large a part of their bodies consists.

a

BUOMPHALUS PENTAGULATUS. a. Exterior view. b. Section showing chambers. In this animal, which abounded in the sea during the era of the mountain limestone, the shell internally is divided into chambers, and the animal is supposed to have retreated at different periods of its growth from the compartment previously formed, and then to have cut off all communication with it.

The fishes of the era now grow to a gigantic size, and in some cases resemble reptiles so strongly that they have been called Sauroid fishes-from the reptile class, Saurians. Teeth have been found belonging to them four inches long-terrible instruments for the humbler neighbours which were destined to perish beneath their operation. We know the nature of these fishes' food perfectly, for their excrements-coprolites-have become fossilized; and we see mixed up with the latter fish scales and bones. And if there be something startling in this kind of familiar glimpse of the life of such distant eras, the feeling is increased when on close examination we can even perceive the unmistakable traces of the actions of the intestines, in the convoluted form of the coprolites.

The Uses of the Mountain Limestone are important. Valuable building stone is obtained from the sandstone beneath the mountain limestone, and from the millstone grit. The mountain limestone itself is our great storehouse for that most valuable article-lime. The encrinal beds already spoken of furnish an extremely pretty marble, in which, as in a picture, may be seen the various members of individuals of that interesting and abundant family. The ornamental spars of Derbyshire are well known: they belong to the mountain limestone. This rock also contains the principal lead mines of our country. With the lead, silver and gold in small quantities are not unfrequently associated.

* Dr. Buckland's Bridgewater Treatise.

CHAPTER VII.

SECONDARY STRATA-THE COAL MEASURES.

Meaning of the Coal Measures.-Under this name we recognize that series of strata composed of coals, sandstones, shales (or mud), bands of ironstone, fire-clay, and impure limestone, which overlie the mountain limestone, and alternate with each other in irregular succession. Coal is the peculiarly distinctive and interesting feature of this series. At first it only appears in thin seams at the bottom of the coal measures, while the shales and sandstones, on the contrary, occur in thick beds. But as we ascend, the coal increases, and its companion minerals decrease, till about the centre of the "measures we find the culminating point of the coal, for there it is at once found in the largest masses, and of the best quality. Above the middle it again begins to decrease, and once more the shales, but of lighter colour than the previous ones, and the sandstones prevail, until the next system is reached-that of the New Red Sandstone. A curious sort of regularity is often perceptible in the midst of the irregularities of the alternating strata. For instance, Professor Sedgwick tells us that at Cross Pits, in the Valley of Dent, Yorkshire, the coal seam under the twelve-fathoms limestone is divided by a band of clay, half an inch thick, into two parts, with distinct mineral characters; and the same coal seam, with exactly the same subdivisions, has been found in the mountains on the opposite side of the valley, at the. distance of three or four miles, measured in a straight line. This seems to prove that a bed, not more than a fraction of an inch thick, was originally continuous throughout an area probably several miles in diameter.

The Ironstone of previous formations, it will be remembered, occurred in veins, or merely as a thin colouring matter diffused through the whole structure of the bed in which it is found; in the Coal measures we find it in the form of an argillaceous carbonate, massed in thin layers, from an inch or so up to a foot thick, and in irregular nodules, called septaria, from the nodules being divided into septa, or partitions. The formation of the nodules is worthy of remark. Each consists of a nucleus of the remains of animals and plants, such as fish-spines, coprolites, teeth, scales, leaves, &c., round which the ironstone has been deposited, till the whole took the forms here represented, and which we borrow from Mr. David Page's excellent Rudiments of Geology.

IRONSTONE NODULES.

In No. 1 we see imbedded a fragment of a plant; in 2, a fish-tooth; in 3,

a fossil coprolite, or excrement; and in 4, the internal divisions spoken of. These are formed by white carbonate of lime, which produces an effect somewhat resembling that of a beetle-hence the name among the peasantry, the beetle stones.

Varieties of Coal.-Coal is found in various states, and by comparing the whole of these together, the circumstances of its formation are made tolerably clear. We will briefly review the varieties.

Lignite, or brown coal, or wood coal (for it is known by all these names), represents the first step in the process of the conversion of vegetable into bituminous matter. There can be no doubt as to the origin of lignite, for the woody structure is still clearly to be seen in it. But our chemists have not been content with this evidence; they have made elaborate experiments, which show that if wood and vegetable matter are buried in the earth, exposed to moisture, and excluded from the external air, they decompose slowly, giving forth the while carbonic acid gas, thus losing some portions of their original oxygen, until, at last, the residue becomes lignite. It is by a continuance of this process of decomposition, and the accompanying discharge of carburetted hydrogen (the gas which we burn in our shops and streets), that Nature forms

Common Coal, which includes caking and cubic coal. Caking coal is so denominated on account of the tendency of its lumps to cake together during combustion-a quality doubtless owing to its highly bituminous nature. It contains forty per cent. of bitumen. This is the prevailing sort in the mines of Durham and Northumberland. Cubic coal is not so full of bitumen, and in breaking divides into cubical-shaped masses.

Cannel Coal is the most striking of all the strictly coal forms; and well we remember, in our younger days, when we lived in Devonshire, hoarding small pieces of it for the sake of its glossy, lustrous beauty. It makes most brilliant fires. Cannel coal contains about twenty per cent. of bitumen.

Jet is even more compact and lustrous. It is found in Saxony, and also in detached fragments in the amber mines of Prussia.

Anthracite. Many of the fatal accidents that occur in mines are owing to the escape of various inflammable gases from mineral coal. These gases include carbonic acid, carburetted hydrogen, nitrogen, and olefant gas. After a long period of continual discharge of this kind, the common coal ceases to present its original characteristics, and is, in fact, transformed into anthracite, known also by the names of blind coal, from its burning without flame; glance coal, from its shiny surface; culm, &c. The word anthracite is derived from the Greek anthrax, charcoal, which expresses the distinguishing quality of the thing, for this kind of coal is little else than a mineral charcoal. In composition it is closely allied to the ordinary blacklead of our pencils. It is almost or entirely destitute of bitumen.

Formation of Coal.-While it is clear that the origin of coal is to be found in the decay and conversion of vegetable matter, it is a much more difficult question to determine the particular circumstances that contributed to its formation in the places where we now find it. Did the plants grow where the coal formed from them now lies? If not, how could such vast masses of vegetable matter have been brought together? Why, again, do we find the coal formations so continually interrupted by strata of shale or sandstones? And when we examine the nature of the plants of the coal formation, as evidenced by their fossil remains, we shall find reason to ask