LETTER IV. On the HISTORY OF THE EARTH, from the time of the formation of the calcareous strata, to the last period in which the Sea remained in its ancient Bed; a long space of time, which comprises, in particular, the Origin of Volcanic Eruptions and of Coal Beds; the formation of the strata of Chalk and Rock Salt; and the History of the QUADRUPEDS, of which we find the remains in our struta. IN SIR, my former letter I entered upon the detail of operations belonging to the fifth of the periods into which I have divided the ancient history of the earth: and I stopped at the point where I was about to speak of volcanic eruptions; with this phenomenon, therefore, I shall resume the series of the events that have happened on our globe. OF VOLCANIC ERUPTIONS. 1. Volcanic eruptions are amongst those terrestrial phenomena that have most excited the attention of former geologists, both from their indicating the operation of some great cause, and from their true cha will bear in mind that I am here tracing, from precise monuments, the foundation of the ancient history of mankind, since the subject of our discussion is their abode. I have the honour to be, &c. LETTER IV. On the HISTORY OF THE EARTH, from the time of the formation of the calcareous strata, to the last period in which the Sea remained in its ancient Bed; a long space of time, which comprises, in particular, the Origin of Volcanic Eruptions and of Coal Beds; the formation of the strata of Chalk and Rock Salt; and the History of the QUADRUPEDS, of which we find the remains in our struta. IN SIR, my former letter I entered upon the detail of operations belonging to the fifth of the periods into which I have divided the ancient history of the earth: and I stopped at the point where I was about to speak of volcanic eruptions; with this phenomenon, therefore, I shall resume the series of the events that have happened on our globe. OF VOLCANIC ERUPTIONS. 1. Volcanic eruptions are amongst those terrestrial phenomena that have most excited the attention of former geologists, both from their indicating the operation of some great cause, and from their true cha racters having been long misunderstood. The number of ancient volcanic cones which are to be met with on our continents, as well as of volcanic islands scattered in the sea, gave rise to many systems, in which it was attempted to explain the formation of our continents themselves, by materials forced up from the bottom of the ocean. I have treated this subject in my Letters on the history of the earth, &c. and subsequently, in the Journal de Physique; I shall therefore confine myself here to the consideration of it in a general point of view. 2. The phenomenon of volcanic eruptions is doubtless a very considerable one, when we regard it separately; but it is very slight, when we take in the whole assemblage of revolutions that must have happened to the surface of the globe; and its products bear but an insensible proportion to the mass of strata produced by chymical processes, in a liquid, where marine animals lived and propagated in great abundance, and the bottom of which consequently could not be hardened by fusion, or after the manner of bricks, as Dr. Hutton has of late conjectured, thus reviving an hypothesis which all attentive naturalists had abandoned. These strata, which constitute the greatest phenomenon on our globe, have certain characters that indisputably connect them with all the general causes that have acted on it; while the volcanic substances, raised here and there in patches on these same strata, indicate certain local causes only; and it is in this point of view that I shall consider them. 3. The problem concerning volcanic eruptions involves the three following questions.-1. Whence is it that these eruptions have issued?-2. By what power have the fused matters been driven out and accumulated on the surface ?-3. At what time did those eruptions happen, of which we have no accounts in history? 4. First question.-Many mineralogists have sought to discover, what the strata were that furnish the substances of lavas, and by what means they are brought into fusion; but I have reason to think, that our strata have no share in this phenomenon. The quantity of pyrites found in certain schistose and argillaceous strata, had led some naturalists, and particularly M. Pallas, to fix the focus of volcanos in these. But pyrites do not undergo decomposition and combustion, except when they are exposed to the air; and each mass of pyrites being perfectly enclosed in the part of the stratum it occupies, is entirely protected from that effect. The coal strata have also engaged the attention of some naturalists in this line of enquiry; but besides that they do not lie deep enough to answer the phenomena of volcanos, they are also surrounded by other substances, which prevent their communication with the air, without which they cannot burn. Accidents many times have set coals on fire, in certain mines, where they have even burned for many years. I have witnessed this fact in Staffordshire, and have been informed of all its circumstances: it has not occurred, nor indeed can occur, excepting in mines; the combustion takes place only in the rubbish, and in the pillars that are left to support the superincumbent soil. The conflagration, indeed, occurs, merely owing to precautions not having been taken in time to close all the openings which |