to Mr. Scott Russell's investigations with respect to waves, already referred to. (Book VIII.) The wave in this case would be the wave of translation, in which the motion of the water is as great at the bottom as at the top; and it has hence been asserted that by paroxysmal elevations of 100 or 200 feet, a current of 25 or 30 miles an hour might be accounted for. But I think it has not been sufficiently noted that at each point this 'current' is transient: it lasts only while the wave is passing over the point, and therefore it would only either carry a single mass the whole way with its own velocity, or move through a short distance a series of masses over which it successively passed. It does not appear, therefore, that we have here a complete account of the transport of a collection of materials, in which each part is transferred through great distances:-except, indeed, we were to suppose a numerous succession of paroxysmal elevations. Such a battery might, by successive shocks, transmitting their force through the water, diffuse the fragments of the central mass over any area, however wide.

The fact that the erratic blocks are found to rest on the lower drift, is well explained by supposing the latter to have been spread on the sea bottom while rock-bearing ice masses floated on the surface till they deposited their lading.

Sir R. Murchison has pointed out another operation of ice in producing mounds of rocky masses; namely, the effects of rivers and lakes, in climates where, as in Russia, the waters carry rocky fragments entangled in the winter ice, and leave them in heaps at the highest level which the waters attain.

The extent to which the effects of glaciers, now vanished, are apparent in many places, especially in Switzerland and in England, and other phenomena of the like tendency, have led some of the most eminent geologists to the conviction that, anterior to the period of our present temperature, there was a Glacial Period, at which the temperature of Europe was lower than it now is.]

Although the study of the common operations of

water may give the geologist such an acquaintance with the laws of his subject as may much aid his judgment respecting the extent to which such effects may proceed, a long course of observation and thought must be requisite before such operations can be analysed into their fundamental principles, and become the subjects of calculation, or of rigorous reasoning in any manner which is as precise and certain as calculation. Various portions of Hydraulics have an important bearing upon these subjects, including some researches which have been pursued with no small labour by engineers and mathematicians; as the effects of currents and waves, the laws of tides and of rivers, and many similar problems. In truth, however, such subjects have not hitherto been treated by mathematicians with much success; and probably several generations must elapse before this portion of geological dynamics can become an exact science.

Sect. 3.-Igneous Causes of Change.-Motions of the Earth's Surface.

THE effects of volcanoes have long been noted as important and striking features in the physical history of our globe; and the probability of their connexion with many geological phenomena, had not escaped notice at an early period. But it was not till more recent times, that the full import of these phenomena was apprehended. The person who first looked at such operations with that commanding general view which showed their extensive connexion with physical geology, was Alexander von Humboldt, who explored the volcanic phenomena of the New World, from 1799 to 1804. He remarked1 the linear distribution of volcanic domes, considering them as vents placed along the edge of vast fissures communicating with reservoirs of igneous matter, and extending across whole continents. He observed, also, the frequent sympathy of volcanic and terremotive action in remote districts of

1 Humboldt, Relation Historique; and his other works.

the earth's surface, thus showing how deeply seated must be the cause of these convulsions. These views strongly excited and influenced the speculations of geologists; and since then, phenomena of this kind. have been collected into a general view as parts of a natural-historical science. Von Hoff, in the second volume of the work already mentioned, was one of the first who did this; At least,' he himself says,2 (1824) 'it was not known to him that any one before him had endeavoured to combine so large a mass of facts with the general ideas of the natural philosopher, so as to form a whole.' Other attempts were, however, soon made. In 1825, M. von Ungern-Sternberg published his book On the Nature and Origin of Volcanoes, in which, he says, his object is, to give an empirical representation of these phenomena. In the same year, Mr. Poulett Scrope published a work in which he described the known facts of volcanic action; not, however, confining himself to description; his purpose being, as his title states, to consider the probable causes of their phenomena, the laws which determine their march, the disposition of their products, and their connexion with the present state and past history of the globe; leading to the establishment of a new theory of the earth.' And in 1826, Dr. Daubeny, of Oxford, produced A Description of Active and Extinct Volcanoes, including in the latter phrase, the volcanic rocks of central France, of the Rhine, of northern and central Italy, and many other countries. Indeed, the near connexion between the volcanic effects now going on, and those by which the basaltic rocks of Auvergne and many other places had been produced, was, by this time, no longer doubted by any; and therefore the line which here separates the study of existing causes from that of past effects may seem to melt away. But yet it is manifest that the assumption of an identity of scale and mechanism between volcanoes now active, and the igneous catastrophes of which the

2 Vol. ii. Prop. 5.

3 Werden und Seyn des Vulkanischen Gebirges. Carlsruhe, 1825.

products have survived great revolutions on the earth's surface, is hypothetical; and all which depends on this assumption belongs to theoretical geology.

Confining ourselves, then, to volcanic effects, which have been produced, certainly or probably, since the earth's surface assumed its present form, we have still an ample exhibition of powerful causes of change, in the streams of lava and other materials emitted in eruptions; and still more in the earthquakes which, as men easily satisfied themselves, are produced by the same causes as the eruptions of volcanic fire.

Mr. Lyell's work was important in this as in other portions of this subject. He extended the conceptions previously entertained of the effects which such causes may produce, not only by showing how great these operations are historically known to have been, and how constantly they are going on, if we take into our survey the whole surface of the earth; but still more, by urging the consequences which would follow in a long course of time from the constant repetition of operations in themselves of no extraordinary amount. A lava-stream many miles long and wide, and several yards deep, a subsidence or elevation of a portion of the earth's surface of a few feet, are by no means extraordinary facts. Let these operations, said Mr. Lyell, be repeated thousands of times; and we have results of the same order with the changes which geology discloses.

The most mitigated earthquakes have, however, a character of violence. But it has been thought by many philosophers that there is evidence of a change of level of the land in cases where none of these violent operations are going on. The most celebrated of these cases is Sweden; the whole of the land from Gottenburg to the north of the Gulf of Bothnia has been supposed in the act of rising, slowly and insensibly, from the surrounding waters. The opinion of such a change of level has long been the belief of the inhabitants; and was maintained by Celsius in the beginning of the eighteenth century. It has since been conceived to be confirmed by various observations of marks cut on the

face of the rock; beds of shells, such as now live in the neighbouring seas, raised to a considerable height; and other indications. Some of these proofs appear doubtful; but Mr. Lyell, after examining the facts upon the spot in 1834, says, 'In regard to the proposition that the land, in certain parts of Sweden, is gradually rising, I have no hesitation in assenting to it, after my visit to the districts above alluded to.' 4 If this conclusion be generally accepted by geologists, we have here a daily example of the operation of some powerful agent which belongs to geological dynamics; and which for the purposes of the geological theorist, does the work of the earthquake upon a very large scale, without assuming its terrors.

[2nd Ed.] [Examples of changes of level of large districts occurring at periods when the country has been agitated by earthquakes are well ascertained, as the rising of the coast of Chili in 1822, and the subsidence of the district of Cutch, in the delta of the Indus, in 1819. (Lyell, B. II. c. xv.) But the cases of more slow and tranquil movement seem also to be established. The gradual secular rise of the shore of the Baltic, mentioned in the text, has been confirmed by subsequent investigation. It appears that the rate of elevation increases from Stockholm, where it is only a few inches in a century, to the North Cape, where it is several feet. It appears also that several other regions are in a like state of secular change. The coast of Greenland is sinking. (Lyell, B. II. c. xviii.) And the existence of 'raised beaches' along various coasts is now generally accepted among geologists. Such beaches, anciently forming the margin of the sea, but now far above it, exist in many places; for instance, along a great part of the Scotch coast; and among the raised beaches of that country we ought probably, with Mr. Darwin, to include the 'parallel roads' of Glenroy, the subject, in former days, of so much controversy among geologists and antiquaries.

Connected with the secular rise and fall of large

4 Phil. Trans. 1835, p. 32.