Earthquakes.

through the turf it can easily be pushed, though perhaps 15 or 20 feet long, to the bottom. The surface of the flow was at different places between 50 and 80 feet higher than the fine fertile plain between it and the river Esk. About the middle of the flow were the deepest quags, and there the moss was elevated higher above the plain than in any part of the neighbourhood. From this to the farm called the Gap, upon the plain, there was a broad gully, though not very deep, through which a brook used to run. In consequence of an uncommon fall of rain the moss being quite overcharged burst at these quags, about eleven o'clock at night; and, finding a descent at hand, poured its contents through the gully into the plain. It surprized the inhabitants of twelve farms in their beds. Nobody was lost, but many persons saved their lives with great difficulty. By the next morning 200 acres were entirely overwhelmed, and in a few days it overspread about 400 acres. The land covered was all enclosed with hedges, and bore excellent crops of wheat and turnips. The greatest part of the plain was covered with 15 feet of moss, and in some hollows the moss could not be less than 30 feet deep. The ridge of the flow itself subsided in consequence of this inundation about 25 feet.*

The existence of petrifactions in such abundance, incorporated with different rocks, is a demonstrative proof that this earth which we inhabit has undergone great changes since its original formation. Now what are the sources of these changes? There are three sources of changes which immediately present themselves to our view as soon as we consider the subject; namely, earthquakes, volcanoes, and inundations; and to one or other of these sources, or to all of them united, geologists have been in the habit of ascribing these changes. Let us consider each of them in succession, as far as the information contained in the Philosophical Transactions on the subject will enable us to go.

III. EARTHQUAKES. If we consider the violent effects which earthquakes produce, especially in hilly countries, where their effects are most perceptible, we cannot hesitate to admit that considerable changes in the position of some of the strata at the surface of the earth may have originated from them. The number of papers on earthquakes in the Philosophical Transactions amount to about 130. But, the greater number of these containing hardly any other information than the existence of an earthquake in a particular spot at a given period, I did not include them among the number of geognostical papers, because very little geognostical information can be deduced from them.

With the exception of a few descriptions of earthquakes contained in the earlier volumes of the Transactions, almost the whole of these papers refer to the period between 1750 and 1756 inclusive, a period remarkable for the frequency and the violence of the earthquakes which were felt in almost every part

* Phil. Trans. 1772. Vol. LXII. p. 123.

year of

of the civilized world. No fewer than 55 of them relate to the earthquakes which were felt all over the south-east of England during the year 1750, and which some of the writers distinguish emphatically by the name of the earthquakes. There are 45 papers which give an account of the earthquake which destroyed Lisbon on the 1st of November 1755. This appears, from all these descriptions, to have been one of the most extensive earthquakes ever felt, having extended itself every way not less than 4,000 miles.

As to the phenomena of earthquakes, they scarcely admit of a general description. It has been supposed by some that they only take place in very dry scasons; but this does not appear to be a well established fact. To the same place they seem always to come in the same direction. Thus every earthquake felt at Lisbon has come from the north-west. The agitation of the earth lasts only for a very short time, but it is often repeated two or three times within a very small interval. The effect has by most persons been compared to an undulation passing along the surface of the earth, and different persons affirm that they have traced the progress of this undulation for a considerable way by the nodding of the trees in succession. The earthquake is always attended by a loud noise.

As to the cause of earthquakes we are not in possession of any very satisfac- Cause of tory theory. Dr. Stukely, who has inserted a paper on the subject in the Philo- carthquakes. sophical Transactions, afterwards published separately, has endeavoured to account for them by means of the Franklinian theory of electricity. But his reasoning is so loose and unsatisfactory, that it is not entitled to a serious examination. Neither is the hypothesis of Dr. Hales more plausible or better supported. He conceives the phenomenon to be occasioned by a sudden vacuum formed in the atmosphere. In dry seasons he thought that sulphureous exhalations rose in abundance from the earth. These, he conceived, might make their way into the atmosphere enveloped in clouds; but the moment atmospherical air was admitted to them a condensation would take place, which might be so extensive as to occasion an earthquake. This reasoning was founded on an experiment which Dr. Hales had made many years before. By mixing together nitric acid and iron pyrites, an effervescence was produced, and a particular air generated, which remained without any alteration while confined over water; but, when mixed with atmospherical air, assumed a fine reddish brown colour, and diminished in bulk. The air which he had thus formed he conceived to be the same with the supposed sulphureous exhalations which rise from the earth. The mistake of Dr. Hales was afterwards clearly shown by the discoveries of Dr. Priestley and Mr. Lavoisier.

The air which Dr. Hales had generated is the same substance which is now distinguished by the name of nitrous gas. It is produced not from the pyrites, but from the decomposition of the nitric acid. When mixed with atmospherical air, it unites with the oxygen of that air, and immediately forms again nitric

For the nitric acid, as

acid. Hence the red fumes and the diminution of bulk. soon as formed, is absorbed by the water over which the experiment is made. But we have no proof that nitrous gas is emitted from the earth: on the contrary, we have every reason to believe that it is not. But even supposing nitrous gas by some unknown means to be introduced into the atmosphere in considerable quantity, and to be suddenly mixed with the atmospherical air, the consequence would, no doubt, be a violent tempest, the air rushing from all quarters to fill up the vacuum; but no reason can be adduced why such a vacuum should occasion any motion in the earth, far less a motion so extensive as to be felt over 4,000 miles of the earth's surface.

We must conceive the cause of earthquakes to be lodged within the earth's surface. And certainly no cause capable of producing the effect presents itself to us with so much facility as the sudden generation of an enormous quantity of elastic fluid within the bowels of the earth. Mr. Michell has written a very elaborate and ingenious dissertation on this subject,* in which he endeavours to convince his readers, that earthquakes are occasioned by the sudden inundation of water upon the fires which he conceives to exist in considerable numbers at various depths under the surface of the earth. The water thus heated being converted into steam, he supposes, forces its way between the different strata of the earth which he conceives to be horizontal, and, heaving them up in succession, may occasion an earthquake over a considerable extent of country. When the internal fire lies deep under the surface, the earthquake will move with greater velocity; when it is more superficial, it will move over a smaller tract of country, and with less velocity. Where the strata raised by this steam are covered by a smaller number of incumbent strata, the earthquake will be more violent. This he conceives to be the case with mountainous countries; and if the strata be very thin, the fire may even break out and constitute a volcano. This frequently happens on the summit of mountains. And the irruption of a volcano always puts an end to the earthquake for the time. Of this Mr. Michell brings several instances from Peru. The internal fire which occasioned the earthquake of Lisbon he conceives to exist under the Atlantic Ocean, in a direction north-west from Lisbon, and at the distance of about a degree. As to the depth, there are no data to determine that point, but he guesses it to be intermediate between a mile and a half and three miles.

Though this hypothesis of Mr. Michell is attended with many difficulties, which it is not possible at present to explain, yet it must be allowed to be the most plausible solution of earthquakes hitherto proposed. Werner conceives all volcanoes to be situated in a coal country; and explains them, by supposing the coal, at an immense depth, by some means or other to be set on fire. But

* Phil. Trans. 1760. Vol. LI. p. 566.

there are several circumstances which militate against this very simple and ingenious explanation. We have never heard of any coal occurring in tropical countries; yet it is well known that volcanoes are very common in these countries. The account which Humboldt gives of the composition of several volcanic mountains, in South America, is utterly irreconcileable with their focus being in coal, unless we admit that coal may exist below granite; a phenomenon which has never been observed. But, if coal be not the combustible substance, we can form no idea of what the matter can be which supplies so many internal fires with fuel for so many ages.

An idea has been thrown out that the combustible matter may be potassium, sodium, and the metallic bases of the earths; which would take fire by mere contact with water, and therefore might be conceived to burn any length of time, without the assistance of external air. That such a combustion is possible, cannot be denied; but it is scarcely reconcileable with the nature of the substances thrown out of volcanoes. We never find great quantities of potash, or soda, among the lavas of volcanoes, nor of perfect glass; as we ought to do, on the supposition that the bases of the alkalies, or alkaline earths, supplied the materials of the combustion. Hence there does not appear any reason to believe that these substances supply volcanoes with fewel. Nothing remains, therefore, but the bases of silica and alumina; which are still hypothetical, and concerning which, it would, consequently, be absurd to speculate. Besides, the quantity of sulphureous vapours, of sulphureous acid, and of sulphur, which are emitted from volcanoes, announces the presence of that combustible substance. But, if sulphur, iron, and charcoal, (as is most likely) be the food which supplies volcanoes with the means of burning, it is impossible to explain the existence of subterraneous fire at great depths under the earth, and quite unconnected with the external air. That they do in some cases exist, in such situations, must be admitted; but every attempt hitherto made to explain how, must be allowed to be unsatisfactory. Upon the whole, I feel rather inclined to adopt the opinion of Mr. Michell, respecting the cause of earthquakes; though that opinion still leaves much to account for.

Dr. Hutton, of Edinburgh, and his disciples Mr. Playfair and Sir James Hall, have got rid of these difficulties, by conceiving the whole central part of this globe to be a mass of solid fire. But this supposition, independent of its inexplicability, leads to consequences which are inconsistent with the phenomena of nature; and of course overturn it. To reconcile this red hot nucleus with the phenomena of earthquakes, it is obvious that it must extend to within a few miles of the surface of the earth. We are not sufficiently acquainted with the conducting powers of the earth to know what absolute effect such a central mass would have upon the temperature of the surface of the globe; but we are certain that it would have considerable effect. It is true, that we may conceive the waste of heat, from the surface, just to balance the quantity proceeding

Volcanoes.

from the centre. On such a supposition, the heat of the surface indeed would not increase, but it would certainly remain stationary. But this is not the case, we find the heat of the surface to vary, and to be entirely regulated by the sun, indicating irresistibly that the temperature is derived from that luminary, and not from the centre of the earth. Neither are the polar regions the hottest, as they ought to be on the supposition of a central fire; since they are much nearer the centre than the equatorial regions; but the coldest, because the sun's rays strike them most obliquely. Neither do we find the temperature of the earth increase, the deeper we dig in mines, as it must do, if the heat proceeded from a central fire. On the contrary, we speedily come to a certain temperature, (always the mean temperature of the country in which the mine is situated) which never varies, how deep soever we proceed. These, and various other phenomena which will readily occur to the reader, appear to me so utterly irreconcilable with the hypothesis of a central fire, as to oblige every person who weighs them with sufficient care to abandon that hypothesis entirely. IV. VOLCANOES. Volcanoes constitute, without doubt, the most striking and formidable geognostic phenomenon which nature has presented to our view. They are not, indeed, so destructive to the lives of the human race as earthquakes; but they present the eye with something much more terrific. Their number is very considerable, nearly two hundred having been reckoned by different writers. There is an immense range of them running from north to south, on the continent of America, and occupying the summits of many of the Andes, and the mountains of Mexico and California. There is a considerable number along the east coast of Asia, and in the Indian islands. Iceland alone contains eight volcanoes. The two volcanoes with which we are best acquainted, are those of Ætna and Vesuvius. Ætna has been burning as far back as the records of history go. We have an account of an eruption during the expedition of the Argonauts; which took place at least 12 centuries before the commencement of the Christian æra. The following dates of remarkable eruptions of this volcano are given in a paper published in an early volume of the Philosophical Transactions: