lates to the formation of limestone rocks and strata. Whence was the calcareous matter derived? Some limestone rocks are chiefly composed of shells, or other calcareous remains of marine animals, and in such instances we can have little hesitation in ascribing their formation to animal secretion, similar to what is taking place in the numerous coral reefs in the Pacific ocean. There are other beds, however, such as chalk, to which a similar formation cannot be ascribed; for though they contain numerous organic fossils, these do not bear the proportion of one to one hundred millions, when compared to the whole mass, and the chalk does not appear to have undergone any chemical change, from heat or other causes, that could have obliterated the traces of organic existence. In no formation are the most delicate organic textures of animals better preserved. In Mr. Mantell's splendid collection of chalk fossils at Lewes, there are specimens of fish, in which the body is entire and the air-bladder is uncompressed -and the beautiful forms of many shells covered with spines, prove that they could not have been drifted from a distance, or deposited in an agitated ocean. I have never been able to comprehend, why any peculiar difficulties should be supposed to attend the enquiries respecting the origin of calcareous or magnesian earths; or of the carbon and sulphur occurring in rocks, in the state of carbonic or sulphuric acids. It would be equally proper to institute an enquiry into the origin of silex or alumine. I hold the earth itself, and its ancient atmosphere, to have been the great chemical laboratories, in which all the solid and fluid parts of the surface were originally prepared and formed. This opinion I stated at some length in Chap. XVI. of the second edition of this work in 1815, and also in the third Edition, in a chapter on the agency of subterranean fire in the formation of rocks, and on igneous and aqueous eruptions of earthy matter. It has been too much the fashion to consider all the secondary strata as mechanical depositions; but the siliceous strata in the Paris basin, the layers of flint in chalk, and the beds of chert or hornstone in transition limestone, are certainly as much original formations as granite itself. In referring to the vast magnitude of ancient volcanoes, I have stated that they had doubtless an important office to perform in nature: and can it be unreasonable to believe, that the earth itself is the great laboratory and storehouse, where the materials that form its surface were prepared, and from whence they were thrown out upon the surface in an igneous, aqueous, or gaseous state, either as melted lava, or in aqueous solution, or in mechanical admixture with water in the form of mud, or in the comminuted state of powder or sand? Inflammable and more volatile substances may have been emitted in a gaseous state, and become concrete on the surface. These primeval eruptions, judging from the size of the ancient fissures and craters, may have been sufficient to cover a large portion of the globe. Nor can it be deemed improbable, that still larger and more ancient craters have been entirely covered by succeeding eruptions. In proportion as the formation of the surface advanced, these eruptions might decline, and be more and more limited in their operation. It is not necessary to suppose, that these subterranean eruptions consisted only of lava in a state of fusion. The largest active volcanoes at present existing, throw out the different earths intermixed with water in the form of mud. Nor should we limit the eruptions of earthy matter in solution or suspension, to volcanic craters: the vast fissures or rents which intersect the different rocks, may have served for the passage of siliceous solutions to the surface. We know no instances in nature of siliceous earth being held in aqueous solution, except in the waters of hot or boiling springs; and hence it seems reasonable to infer, that many siliceous ** rocks and veins have been deposited from subterranean waters, at a high temperature. Calcareous or cretaceous matter is also ejected during aqueous volcanic eruptions. According to Ferrara, streams of liquid chalk, or chalk in the state of mud, were ejected from the mud volcano of Macaluba, in Sicily, in 1777, which, in a short space, formed a bed several feet in thickness. Beds of limestone may have been formed by similar calcareous eruptions, in which the lime might be sometimes in solution, and sometimes mechanically suspended; and the numerous remains of testaceous animals in limestone appear to indicate, that the calcareous solutions were favourable to the growth of animals, whose coverings contain so much calcareous matter. Nor is it necessary to suppose, that these aqueous eruptions were always sudden, and attended with violent convulsions, for when a passage was once opened, they may have risen slowly, and have been diffused in a tranquil state, and by gradual deposition, or condensation, may have enveloped the most delicate animals or vegetables, without injuring their external form. - Second edition, 1815. If the geologist can admit such a condition of the ancient world as above described, a condition which, on a smaller scale, might be proved to have existed since the period of authentic history: if he will further admit, that, before the formation of chalk, a great portion of what is now England, and the northern Continent of Europe, was covered by a deep ocean, interspersed with islands, and surrounded by ancient continents, and this few modern geologists will deny. Then, if we allow submarine aqueous eruptions of calcareous matter, and siliceous solutions from thermal waters, to have been poured into this deep *M. Brogniart, to whom I sent the edition of 1815, subsequently admitted a similar formation of the siliceous beds and millstone in the Paris basin, that they were deposited by thermal waters holding silex in solution. ancient ocean, we shall have all the circumstances required, to form thick beds of chalk, interspersed with nodules of flint. In an experiment on clay formed into a stiff paste, by admixture with a saturated solution of alum, it was found, on breaking the clay when dry, that alum was interspersed through the mass in distinct crystals and concretions. In the same manner, we may suppose that the silex in the siliceous solutions, spread through the calcareous matter, would separate into distinct concretions, filling the cavities and pores of zoophytes such as sponges and alcyonia, or of shells deposited in the chalk. Every fact connected with the history of chalk, proves that it was formed in a very tranquil sea, and not by the drift or detritus of more ancient rocks. Mr. Mantell, whose almost daily observations on the chalk formation scarcely suffer an important fact to escape his notice, says, that, in the whole of these immense beds that he has examined, the occurrence of a single fragment or pebble of more ancient rocks in chalk is extremely rare; a fact decisive against its being formed by mechanical deposition of drift, or detritus of older limestones. The preservation of the most delicate textures of animals before referred to, proves beyond doubt, that those organic bodies had not been transported from a distance, or subjected to the violent action of inundations or currents. The fossil fish found in chalk with the body preserving the natural form, and with the air bladder uncompressed, proves beyond doubt, that the animal was encased in mineral matter, before the putrefactive process had effected the destruction of the fleshy parts. A sudden eruption of thermal water holding calcareous earth in solution or suspension, might instantly deprive the animal of life, and protect the body from decay. The matter, called creta by Ferrara, erupted from Macaluba, was certainly a soft limestone, analogous to chalk; and though the eruption lasted only part of a day, it formed a stratum many feet in thickness. Had this eruption taken place under water, the earthy matter would have been more widely diffused, and the stratum of limestone deposited, would have been proportionably thinner. In the case of the fossil fish before stated, we are not obliged to suppose the deposition to be so rapid several days might elapse before the body was entirely buried under calcareous earth. If we say seven days, and estimate the thickness of the fish at three inches, we shall have a chronometer to measure the time required to form a stratum of chalk three inches in depth, which is one week. This is equal to one foot in a month, or twelve feet in a year; and could we suppose the deposition to proceed without interruption, it would not require more than ninety years, to form a mass of chalk beds, one thousand feet in thickness; which is more than that of all the chalk beds in England. It is by no means intended to support the opinion, that the chalk beds were all deposited in so short a period; long intervals of repose might pass between different eruptions. My object in calling the attention of geologists to this subject is, to show that strata may be formed more rapidly than they are generally disposed to believe, and that the feeble operations of natural causes in our own times, however similar in kind, bear no proportion, in their intensity, to the mighty agents that have formed the ancient crust of the globe. The deposition of a bed of calcareous earth, a few feet in thickness in some of the Scottish lakes, as described by Mr. Lyell, would appear to have required many centuries for its completion. In some of the beds of oolite, the quantity of animal remains bears a considerable proportion to the whole mass, and the beds of encrinal limestone in some of our mountain limestones, are formed principally of the stems and branches of encrinites, probably broken by the violent action of the sea; but it is not improbable, that the interstices have been filled by calcareous depositions. It is obvious, that limestone strata of |