“O Lord, how manifold are thy works! in wisdom hast thou made them all.”—David.
Proceeding with our rapid sketch of the crust of the earth and its uses, we now leave the Lias, to enter upon a survey of the Oolite proper. Overlying the Lias, and underlying the Wealden, we find this deposit, which, though it occupies a comparatively narrow track in our own country, is remarkable for the peculiarity and beauty of its fossils, and for the commercial importance of the rocks of which it is composed. Its name is derived from ōŏn, an egg, and lithos, a stone,[[88]] from the remarkable resemblance many of the beds bear to the roe, or eggs of a fish. A good specimen of oolite and the hard roe of a red herring are unlike one another, mostly in the circumstance that one can be cooked, and the other cannot. These egg-like grains are mere agglomerations of calcareous matter, although sometimes a piece of coral, or a broken shell, or a grain of sand, is found to be the nucleus around which these deposits have arranged themselves. It may be as well here to say a word upon the general character of calcareous rocks, which are so largely to engage our attention in this and a subsequent chapter. “This division comprehends those rocks which, like chalk, are composed chiefly of lime and carbonic acid. Shells and corals are also formed of the same elements, with the addition of animal matter. To obtain pure lime it is necessary to calcine these calcareous substances,—that is to say, to expose them to heat of sufficient intensity to drive off the carbonic acid and other volatile matter, without vitrifying or melting the lime itself. White chalk is often pure carbonate of lime; and this rock, although usually in a soft and earthy state, is sometimes sufficiently solid to be used for building, and even passes into a compact stone, or a stone of which the separate parts are so minute as not to be distinguishable from each other by the naked eye.
“Many limestones are made up entirely of minute fragments of shells and corals, or of calcareous sand cemented together. These last might be called ‘calcareous sandstones;’ but that term is more properly applied to a rock in which the grains are partly calcareous and partly siliceous, or to quartz-ore sandstones having a cement of carbonate of lime.
“The variety of limestones called ‘oolite’ is composed of numerous small egg-like grains, resembling the roe of a fish, each of which has usually a small fragment of sand as a nucleus, around which concentric layers of calcareous matter have accumulated.
“Any limestone which is sufficiently hard to take a fine polish is called marble. Many of these are fossiliferous; but statuary marble, which is also called saccharine limestone, as having a texture resembling that of loaf-sugar, is devoid of fossils, and is in many cases a member of the metamorphic series.”[[89]] The geographical distribution of this group of rocks may be traced thus:—Commencing with the Bill of Portland, (for there is no isle of Portland,) it runs up through part of Dorsetshire, Somersetshire, Gloucestershire, Oxfordshire, Northamptonshire, part of Lincolnshire, terminating in Yorkshire, where the lias and oolite may be seen lying conformably or in sequence; the zone we have thus indicated being about thirty miles in width. The following tabular arrangement will supplement this by pointing out the divisions and subdivisions of the oolite; on which, however, we do not intend to dwell, as our only object, in this most preliminary treatise,—and we shall be pardoned again intruding this thought upon our readers,—is to assist in the investigation of our standard text-books on this science:—
OOLITE PROPER.
| 1. Upper. | 1. Portland stone, with underlying dirt-beds. |
| 2. Kimmeridge clay. | |
| 2. Middle. | 1. Coral rag. |
| 2. Oxford clay. | |
| 3. Lower. | 1. Cornbrash and Forest marble. |
| 2. Great Oolite and Stonesfield slate. | |
| 3. Fuller’s earth. | |
| 4. Inferior Oolite.—Lyell. |
In the upper oolites, it will be seen, is found the famous Portland stone, in which are some of the most remarkable specimens of the extinct fauna of this remote period. “They are found most plentifully in what is locally designated the ‘dirt-bed’ of Portland—a stratum of dark argillaceous mud, which must at one time have been the soil in which they and other vegetables flourished, but which, by a submergence of the land, was converted into the bottom of an estuary, over which other strata of clay, limestone, and sand were deposited. ‘At the distance of two feet,’ says Bakewell, ‘we find an entire change from marine strata to strata once supporting terrestrial plants; and should any doubt arise respecting the original place and position of these plants, there is, over the lower dirt-bed, a stratum of fresh-water limestone; and upon this a thicker dirt-bed, containing not only the cycadeæ, but stumps of trees from three to seven feet in height, in an erect position, with their roots extending beneath them. Stems of trees are found prostrate upon the same stratum; some of them are from twenty to twenty-five feet in length, and from one to two feet in diameter. The following section of a cliff in Dorset exhibits very clearly proofs of the alternation from marine strata to dry land covered with a forest, and of a subsequent submergence of the dry land under a river or lake which deposited fresh-water limestone.’”
a a a, Portland stone (marine formation); b, Dirt-bed, consisting of black mould and pebbles (temporary dry land); c, Burrstone, and d, Calcareous slate (both of fresh-water formation).—Chambers’ Geology, p. 128.