only remarking, that as we have examples of both these movements in countries subject to earthquakes, and beset with volcanos, it seems more philosophical to attribute the gradual elevation of continents in all cases to volcanic action, than to assume the existence of two independent causes to explain operations differing only in these subordinate particulars *.

See however the remarks of Darwin, Lyell, and others on this point. I would observe that Mr. Darwin, although inclined to attribute the elevation of mountain-chains generally to a slow and gradual movement, does not appear prepared to deny the reality of paroxysmal elevations. See Geol. Trans. vol. v. p. 624.

CHAPTER XXXIV.

THERMAL SPRINGS-THEIR GEOLOGICAL POSITION.

SALSES OR MUD-VOLCANOS-improperly designated by the latter name— not being amongst the primary effects referable to the action of volcanic forces.

THERMAL SPRINGS-afford indications of languid volcanic action.-This proved by their immediate connexion with volcanos-active-or extinct -or where not so circumstanced, by being placed at the foot, or in the midst, of some chain of mountains that has been elevated.-Instances of the latter in the Pyrenees-in the Alps.-Where not connected with any great system of mountains, they often proceed from rocks which show evidences of dislocation.-Examples of this, in the thermal springs of Bristol-of Matlock-of Carlsbad-of St. Paul near Carcassone-of Pfeffers-in those of Virginia, &c.

In the two preceding chapters I considered a class of phanomena which seem to be connected with volcanic operations whilst in a condition of intense activity; I next proceed to those indications of their existence which can be discovered when they are in a state of apparent quiescence, or when the processes that occasion them are going on in a slow and languid manner.

But before entering upon this extensive subject, it may be right to make a few remarks upon a class of phænomena, which, from the name of mud-volcanos frequently assigned to them, may be assumed to have some connexion with the Agent under consideration.

As they are seen only in a very low degree of development in the only volcanic region with which those geologists, whose researches have been circumscribed within the confines of Europe, can be familiar, their real nature seems to be involved in much uncertainty.

In the chapter on Sicily I have related the appearances that present themselves at Macaluba, which I can only regard as secondary effects of volcanic agency, originating in local accumulations of materials which may have been brought

are

together perhaps by the operation of volcanos, but do not proceed directly from so deeply-seated a cause.

The mud-eruptions of South America, described by Humboldt, have clearly no connexion with such phænomena as these, so that for this as well as for other reasons, the name mud-volcano should be discarded, and that of salses, by which they are also known, adhered to.

That they are in certain parts of the globe important geological agents, will not be disputed after the statement given in my 21st chapter of the physical changes produced by their operation in the Crimea, the Black Sea, and the Caspian.

But to ascribe them to the same cause as volcanos would be obviously premature,-at least until some attempt shall have been made to explain, why many of their phænomena

so entirely distinct from those proceeding from the former.

True volcanos, as we have seen, generate sulphuretted hydrogen and muriatic acid, upheave tracts of land, and emit streams of melted felspathic materials. Salses, on the contrary, disengage little else but carburetted hydrogen, together with bitumen and other products of the distillation of coal, and pour forth no other torrents, except of mud, or argillaceous materials mixed up with water.

It is nevertheless far from improbable, from the extensive scale in which they appear to be developed in the regions explored by Sir Roderick Murchison, M. de Verneuil, and Dubois de Montpereux, that the reports which have been circulated respecting the existence of burning mountains in Central Asia, and other regions remote from the sea, which in other cases seems an essential condition to the putting forth of genuine volcanic forces, may have originated in this class of phænomena.

Such, as we have seen, is the opinion of one of the individuals most conversant with the physical structure of these remote and little-known regions of the earth—I mean the traveller and geologist Erman; and with his authority on the other side, no one in the present state of our knowledge can be justified, either in building a theory of volcanos on facts derived from so apocryphal a source as that of Chinese or

Indian writers, or in alleging the latter as objections to a theory based upon the concurrent evidence of those naturalists who have explored parts of the globe more open to investigation.

Whilst therefore I would recommend the subject of Salses to the consideration of my readers, and would refer them to this work for a summary of what is known concerning their history, I forbear to bring them forward in the present department of this volume, as furnishing facts that can be safely relied upon in illustration of the primary phænomena of vol

canos.

Of the effects then resulting from a more tranquil and subdued action of the forces we are considering, the most remarkable are Thermal Waters, or natural Springs possessing a temperature exceeding that which would be imparted to them by the mean of the climate in the country in which they occur. But here it will be necessary to pause, in order in the first place to inquire, whether every minute excess of temperature beyond that point is sufficient to make a spring rank as thermal.

Gustavus Bischoff* indeed asserts, that all springs of constant temperature are somewhat warmer than the mean of the climate, and according to Humboldt, this excess of temperature in springs increases with the latitude; in Paris, lat. 48°-50, the mean temperature of the climate being 51°6, that of the springs 52°7-excess 1·1; at Berlin, lat. 52°31, the atmospheric temperature being 46°4, terrestrial 50°2—excess 3°8, indicating a rate of progression equal to about 0·7 of a degree of temperature for 1° of latitude. But, on the other hand, the accurate observations of Playfair have shown, that at Edinburgh, in a still higher latitude than either, namely 55°58, the temperature of the springs is identical with that of the atmosphere; so that this supposed progression, if it exists at all, would seem to be at least limited to still higher latitudes even than the latter locality.

See his valuable memoirs on the Laws of Temperature (Wärmelehre), contained in the Edinb. New Phil. Journ. for 1836, but now published a a separate volume.

Neither are the observations of Wahlenberg on the Scandinavian peninsula *, nor those of Kupffer on the Ural ranget, absolutely conclusive as to the generality of the supposed law, even in those still higher latitudes to which they respectively refer.

The elevation of temperature which these writers show to exist, may, for aught we know, be confined to the neighbourhood of uplifted chains of mountains; it may be the consequence of those great natural events to which are owing the disturbances there experienced, and consequently it may not extend to the great plains of Russia or Siberia, where no such local influences manifest themselves.

It may however be said, that the high temperature which is proved to pervade the interior of the earth, and which, judging by the very limited range of human observations, would seem to go on progressively increasing, sufficiently explains the temperature of springs, which will always exceed that of the external air in a greater or lesser ratio according to the depth from which they have arisen.

According to the view thus taken, all springs that possess a constant temperature must have been derived from a point where the heat is permanently greater than on the surface, and the only reason why one spring is hotter than another will be, that it has proceeded from a greater depth within the earth.

These two propositions however ought to be kept distinct, as the first does not imply the second ; and whilst I am disposed to concede, that the higher temperature of the interior of the earth may sufficiently account for the law which Kupffer and Humboldt have attempted to establish, supposing it to be substantiated by subsequent observations, I should protest against the latter conclusion, on grounds connected with the phænomena of thermal waters which will be afterwards discussed. Hence, removing from the class of “thermal” all those springs whose temperature exceeds by no more than a few degrees that of the atmosphere, I shall proceed to consider such only as exhibit some marked or abnormal elevation above that standard ; and where this occurs, inquire