SECONDARY ROCKS.

The secondary rocks have, by some, been divided into the lower secondary, and upper secondary, the second being superincumbent on the first; but as it is difficult to determine where the lower series terminate, and the upper one commences, we shall follow the more simple method of considering the whole as merely secondary formations. The same difficulty, indeed, is applicable to the termination of the transition series, and the commencement of the secondary. The chief differences being, that the secondary is not so generally composed of fragments, shows less of the crystalline structure, and contains organic remains of known existing species; while the transition class is more fragmentary; more crystalline, and contains few, or no shells, known to be recent, or living. The principal secondary formations, are Coal, Secondary Limestone, Chalk, Oolite, and Sandstone.

The last named rock, we have placed among the transition series; and undoubtedly that which is composed, in considerable proportion of the fragments of other rocks, belongs there; but many sandstone formations appear more properly to be arranged as secondary rocks. The actual inquirer will often find himself at a loss to determine, from the position of strata, with respect to each other, which are the transition, and which the secondary; because, in many instances, the secondary, as well as the tertiary, to be next described, will be found lying immediately upon the primitive. This arises from the fact, that no formation of the secondary series extends to every part of the earth. Did the different formations cover the earth entirely, as the coats of an onion surround each other, there would exist neither doubt nor difficulty on this subject; for then the same characters would identify the different classes, in all parts of the earth, and each could be known, merely by its depth under the surface. But instead of this, it is quite common, even in countries of no considerable elevation, to observe the primitive rocks projecting above the surface, or lying only a few feet beneath the soil. It is, therefore, only in certain parts of the earth, that the relative positions of strata can be determined, as a whole, for it.is obvious, from what we have stated, that in some places, the newest formations overlay the oldest,

without the intervention of any other. In such situations, however, as afford opportunities for observing the several strata lying superincumbent, the same relative positions are found every where to exist, or to exist so uniformly as to lead to definite general conclusions.

Coal. This well known substance affords several varieties, differing in color, from dark brown, to jet black; and containing variable proportions of carbon and bitumen, with more or less impurities.

The English mineral coal, is stated by mineralogists to contain from fifteen to forty per cent. of bitumen, and from forty to eighty per cent. of charcoal.

Black, or common coal, is found in regular strata, or beds, from a few inches, to several yards in thickness. Several beds commonly occur under each other, being separated by strata of clay or sandstone. These series of strata are called coal fields, or coal measures.

Coal Fields. Every coal field has its peculiar series of strata, which vary in thickness from those of any other. The coal beds are also separated by deposites which differ in thickness, in kind, or in arrangement, from those of other formations. Hence each coal field is a distinct and independent deposite, and is in no way connected with any other coal field, with respect to the sources whence their materials were originally derived. Hence they are all of limited extent, and most commonly basin-shaped concavities, which have the forms, and so far as can be ascertained, the appearance of once having been lakes, or ponds of greater or less depth and extent. In some of the large coal fields, the original formation of the lake cannot be traced, but in many smaller ones, it is distinctly ascertained.

The number of coal beds, and the various intervening strata through which the shafts of some coal mines pass, often amount to great numbers. In a coal field belonging to Lord Dudley, in Staffordshire, a shaft was sunk to the depth of 939 feet. The beds passed through in this shaft, which the miners distinguish by different names, are sixty-five. The number of beds of coal, are eleven, of which five are above the principal bed, called the main coal, and five below it. The main coal is about three hundred feet below the surface, and consists of thirteen diffe

rent beds, lying close to each other, but separated by their layers of slate clay. Its thickness is about twenty-seven feet.

To convey an idea of the regularity of these strata, we here give the names of a few of them, and the succession in which they occur, beginning with the lowest.

15 Coal, (main,)

29

10

8 Coal,

16 Bituminous shale, 7

See "Origin of, and Searching for Coal."

Secondary Limestone. This is also called carboniferous, and mountain limestone. Its texture is compact, and not crystalline, like the primitive limestone before described. Its prevailing colors are grey, or yellowish white, but it is sometimes bluish, or black. This formation is sometimes extensive, underlaying large districts, and rising into considerable mountains. The hills of this formation, often present mural, or wall-like precipices, and rocky, uneven dales.

It is considered a more recent rock than transition limestone, and is often composed, almost entirely of marine shells, sometimes only slightly adhering together. It also contains the bones of animals, chiefly of extinct species, but sometimes of those now living, and which are never found in the transition class. It is often difficult, however, to distinguish this rock from transition limestone, into which it insensibly passes.

Rock Salt. Although this salt cannot properly be classed as a rock, yet as it forms considerable beds, and is, withal, an important article, it is proper to describe its geological bearings and associations.

In its impure state, as it is raised from the mine, rock salt is in large solid masses, of a crystalline structure, with a reddish, or bluish color. When pure, as it sometimes occurs in the mine, it is perfectly colorless, and transparent, like the best flint glass.

Rock salt is found at various depths below the surface. At Cheshire, in England, where vast quantities are raised, the first bed is one hundred and thirty feet deep, and sev-. enty-eight feet thick. This is separated from the next bed by a stratum of clay-stone, thirty feet thick. The lower bed has been penetrated one hundred and twenty feet, but has not been sunk through.

The principal known deposites of salt, are those of Cardona, in Spain; those of Hungary, and Poland; that of Caramania, in Asia; the extensive formations of Germany and Austria; those on each side of the Carpathian mountains, and those of South America.

According to the traveller Chardin, rock salt is so abundant in Caramania, and the atmosphere so dry, that the inhabitants sometimes build their houses of it.

Origin of Rock Salt. At Posa, in Castille, there is a deposite of rock salt, within the crater of an extinct volcano; and in the island of Sicily, there exists more or less of the same mineral, in such situations as to indicate that it has been formed by the evaporation of sea water, by volcanic heat. But if subterranean heat has in a few instances produced salt by evaporation, still the situation and appearances of these formations generally, are such as to preclude any rational supposition, that they have been formed in this manner.

The most natural hypothesis that has been offered, to account for the existence of this salt, especially in certain situations, is that which attributes it to the gradual evaporation of pools, or lakes of salt water left by the ocean, when it retired from the present continents, in consequence of their elevation. This theory, too, might be considered as receiving strong support from the fact, that in some of the Polish mines, sea shells, the claws of crabs, and vegetable impressions have been found.

But on the contrary, most salt mines are entirely without any organic, or other remains, by which any gleam of light is thrown upon the history of their origin. Were these formations the solid matter, left by the desiccation of salt lakes, we should suppose that fossil sea animals, as well as shells, ought to be found every where, and in abundance. Another, and still stronger objection to this hypothesis, is the great purity of subterranean salt, when compared with that obtained by the evaporation of sea

water.

With the exception of foreign impurities, such as clay and sand, rock salt is nearly pure muriate of soda; while sea water, by evaporation, produces, muriate of magnesia, and sulphate of soda, besides muriate of soda. The mode in which rock salt is disposed in the earth, is also against the hypothesis of evaporation. That of Cheshire, instead of being in strata, is found in distinct concretions.

To these difficulties, it may be added, that the depth of sea water required to produce some of the larger masses of rock salt, must not only have been unfathomable, but incomprehensible. The salt hill of Cardona is 663 feet in height, and is solid muriate of soda. Now, according to the experiments of Dr. Marcet, 500 grains of salt water, yielded 21 1-2 grains of solid matter, of which 13.3 parts were muriate of soda. From 10,000 parts of sea water, Dr. Murray obtained 220 parts of common salt. According to such data, if the salt of Cardona was formed in a lake, by evaporation, the water not only yielded pure muriate of soda, but must have been more than 27,000 feet, or more than five miles in depth.

Finally, this subject appears to be one of great difficulty, for although geologists have made the theory of these formations a matter of much interest and inquiry, no rational hypothesis concerning them has yet been proposed.

Gypsum. Sulphate of Lime. This is known under the name of Plaister of Paris, and is so common as to need no description. This substance, like rock salt, is seldom found in extensive formations. It occurs both with primitive and secondary rocks, and, from the species of shells it sometimes contains, has been considered a fresh water formation. Beds of gypsum commonly alternate with those of marl and limestone.

The greatest deposite of gypsum described, is that of Paris, which extends about twenty leagues. At Montmatre, near Paris, two formations of this substance may be observed; the lower is composed of alternate beds of little thickness consisting of gypsum, often crystalline, alternating with lime and clay-marls. The upper formation is the most important and remarkable. It is about sixty-five feet thick, and, in some places, lies immediately under vegetable mould. This is especially interesting, from the number and variety of organic relics it contains, and from its being the chief source whence the celebrated