All these rocks have received the name of "primary," they have no appearance of being stratified or deposited in layers, which appearance seems to be in all cases the result of having at some time been suspended in water and thence deposited gradually as mud, sand, &c., time, pressure, and heat having afterwards altered their consistence. All these consist of the earths described as composing granite, but combined in different proportions. Above these rocks there is a formation of a totally different composition, namely crystalline limestone, commonly known as marble; this consists wholly of carbonate of lime or lime in union with carbonic acid, and its crystalline state appears to have been produced by long-continued heat and pressure. This limestone is by many ranked amongst the primary rocks, although it differs so entirely in its composition which exactly resembles a formation presently to be described (the chalk); however, if this be classed with the primary rocks, then it may be fairly said that all above have resulted from their disintegration and reunion in different forms, by the action of water and vital forces of different kinds. The state of the earth at the close of this the "primary period" may now be considered. The early history of our globe forms one of the great problems of geology, but there is evidence enough upon which to form conclusions and show that the earth had undergone some great and varied changes embracing immense periods of time, and if the sciences of astronomy, chemistry, mathematics, &c., be brought in to assist the inquiry, some plain facts become evident. Astronomy shows us orbs circulating round the sun (the planets), strictly analogous to our earth, and furnishes us with their densities; these are found to be various, ranging from about six times the density of water to half its density. The planets appear to be at different stages of condensation, and it is not unreasonable to suppose that our earth was once of no greater density than the lightest of them. The inner ring of Saturn is probably in a liquid state, for it is transparent. The substance of which comets are composed is a vapour, so rare and thin that it cannot be compared in density to our atmosphere even, and yet these comets preserve their identity, circulating through immense realms of space with prodigious rapidity. Thus astronomy presents analogies in favour of the supposition that the earth was once gaseous, or at all events of much less density than at present; but mathematical inquiries go much further, and furnish almost proof that the earth was once (if not now) in a liquid state, for the exact form which a liquid ball would assume upon rotating at the rate which the earth does, is exactly that which the globe is found by measurement to possess.