tains many blunders. The Emperor has been unfortunate in the representatives of geological science whom he has sent to the American continent. M. Laur, who visited California some ten years ago, and made a report on its mines, showed a remarkable tact for misapprehending, the plainest and most important facts, and drawing erroneous conclusions; as, for instance, when he announced that the yield of the Comstock Lode would never exceed three millions of dollars a year, whereas, in reality, it soon after reached twelve millions. About half of the volume of MM. Dollfus and Mont-Serrat is taken up with remarks on the volcanoes of Central America, and it is astonishing how little there is of original and valuable matter to be found in it. One is more annoyed still, on examining the beautifully engraved illustrations, to find that they bear evident marks of the sensational style; the slopes of the cones are all enormously exaggerated, and no data are given by which these errors can be corrected. A few simple outlines plotted from actual measurements would have been worth more than the whole dozen and a half of costly steel plates which are given, the style of which takes us back to the dark ages of the illustrations to Humboldt's "New Spain." One should compare them with the drawings and sections illustrating M. Hartung's books on the Azores, Madeira, and Porto Santo, to see the difference between fancy and real work.

Baron Richthofen's quarto pamphlet of a little less than a hundred pages, with no illustrations, is entirely different from most of the works already cited, since it addresses itself exclusively to the professional geologist. It is the result of long observation and of much study bestowed on the volcanic rocks by an able and experienced observer in different parts of the world. In it many of the most difficult points in the theory of volcanoes are discussed in such a manner as to make its study imperative on all who desire to form an original opinion in regard to the subjects with which it deals. We shall refer to it further on, or at a future time, when the theory of volcanoes and earthquakes is under discussion.

In a previous article we endeavored to give a systematic view of the present condition of our knowledge of earthquake phenomena, so far as their external manifestations are con

cerned. We discussed the data of the earthquake catalogues with reference to the geographical distribution of seismic areas, to the relations of time of earthquake shocks, and to their connection with movements and conditions of the atmosphere. We had occasion to refer more than once to the relations between volcanoes and earthquakes both in time and space, and thus prepared the way for a discussion of the causes of these truly wonderful and most closely connected phenomena.

Before entering on this discussion, however, we must become more fully acquainted with the facts concerning volcanoes, and it is with these that this article will be occupied, leaving for a third and final one of the series, an attempt to show how far science is able, at the present day, to throw light on those workings of unseen forces which are manifested in the earthquake shock, the volcanic eruption, the rising and falling of the land, and the formation of mountain chains, for all these are effects of one and the same cause, or, at least, of one set of causes so intimately allied with each other that the discussion of any one of them must necessarily include that of all the others.

In pursuance of this plan, then, we purpose, in this article, to give an outline of what is known in regard to volcanoes, having reference chiefly to their external manifestations, such as form, geographical distribution, and their different phases of repose and action. This will prepare the way for us to get some idea of the nature of the forces at work below; for a volcano is a sort of happy accident, which lets us into some of nature's secrets, a peep-hole through which we may get a glimpse of the interior of the earth. It is evident that, if a great smelting establishment were buried so that no part of it should be visible except the top of the tall chimney, from which gases were issuing, and some piles of slags accumulated on the outside, and we had to report on the nature of the processes going on below from these imperfect data, the investigation would require no little scientific knowledge and ingenuity, and probably some time would elapse before a guess could be hazarded as to the. character of the work of which these gaseous exhalations and slags were the only tangible result. So it is with volcanoes: we collect and analyze their products, whether solid, fluid, or

gaseous; we note the times and places of these manifestations of the internal forces and their correlations with other natural phenomena; we avail ourselves of every conceivable source of information touching the subject, and reason to the best of our ability on the whole mass of evidence thus obtained. And yet the result, it must be confessed, is far from satisfactory. There are many obscure points in the theory of volcanoes and earthquakes; and if the general cause of the phenomena of volcanism is in the opinion of most geologists correctly determined, yet in regard to the precise mode of operation of the internal forces there is great discrepancy of opinion, even among those who have devoted most time to this branch of inquiry.

A volcano is a mountain, hill, or area of the earth's surface, connected with some more or less deeply seated portion of the interior by a canal or passage, through which solid or gaseous materials are brought to the surface. It is almost invariably the case that the substances thus ejected are intensely hot, the rocky material often pouring forth in a condition of igneous fluidity, and the term "lava" is applied to anything which has flowed in this way and which in cooling consolidates into rock. Elevations which would, according to the definition just given, be included under the head of volcanoes, but which emit only water with paroxysmal violence, are usually called "geysers." These are rare and on a small scale as compared with proper volcanoes. Orifices from which mud is thrown out, called "mud-volcanoes," are not uncommon, but are usually of small dimensions, and the temperature of the substances they eject is in many instances raised but little above their ordinary temperature.

Volcanoes are called "active" if they have within a comparatively recent period given indications of eruptive action. The term "dormant" may be used to designate that peculiar condition when the internal forces have remained quiet for a great length of time, so that only faint traces of activity are still visible; and if all chemical action has ceased, and there is no record in history of any outbreak, the volcano or volcanic region is considered and called "extinct." Yet it is not an easy thing to draw the line between dormant and extinct volcanoes.

Thus Epomeo, on the island of Ischia, remained entirely inactive for seventeen hundred years. So Vesuvius was never known in history as an active volcano until A. D. 79. A great saucer-like depression, overgrown with wild grapes, in which Spartacus once camped with ten thousand men, marked the position of its crater, and Herculaneum and Pompeii were two populous towns at its base. By the well-known eruption of that year, these two towns were overwhelmed, -— greatly to the inconvenience of their inhabitants, no doubt, but immensely to our advantage, the whole adjacent region devastated, and the mountain built up into an entirely different shape from that which it had had before. From this time on, the eruptions continued, without any long periods of repose between them, until the fourteenth century, after which there was quiet for nearly three hundred years. During this period of repose the crater became filled anew with a forest vegetation, and only a couple of hot-springs gave evidence of the forces slumbering beneath. All of a sudden, again, in 1631, a furious eruption took place, and seven streams of lava flowed down the slopes of the mountain at one time. Since that, Vesuvius has almost always been uneasy, there being rarely an interval of rest of more than ten years, and, of late, the eruptions have been very violent and frequent. The Gunung Gelungung, one of the great volcanoes of Java, was, and had been from time immemorial, perfectly quiescent, and the site of the present crater was a broad valley, the inhabitants of which had never dreamed of anything but the most peaceful security. But suddenly, in the middle of a fine day in October, 1822, they received notice to quit, in the form of a violent explosion beneath their feet, which proved to be the commencement of one of the most fearfully destructive volcanic eruptions on record.

There are but few volcanoes which are permanently active, and those which are thus in constant eruption are usually far from violent. Paroxysmal, powerful action occurs only occasionally, sometimes recurring, after short intervals, then slackening and perhaps ceasing altogether, or, after a long period of repose, say hundreds or perhaps thousands of years, beginning again.

We have in the moon the best possible specimen of thor

oughly played out volcanism. The most careful watching of the surface with powerful telescopes seems, thus far, to have failed to reveal any evidence of changes taking place there. And since there is neither water nor air to produce erosion or disintegration of the volcanic surface, it seems pretty clear that it will remain as it now is for an indefinite length of time.

In dividing terrestrial volcanoes into extinct, dormant, and active, it must be understood, then, that these terms are used to express our general opinion with regard to their condition, based on a variety of circumstances, and not as indicating any positively established criterion by which the different classes can be distinguished from each other. We speak of the volcanic region of Central France, as "extinct," because we know that a long time has elapsed since any indications of activity have occurred there; this has been ascertained by studying the amount of erosion which has taken place in the lava currents and in other ways. Yet the pouring out of a portion, at least, of the vast mass of volcanic material there visible took place, in all probability, after the appearance of man on the earth, although at an epoch immensely remote as compared with historical time. Neither can any conclusive reason be given why volcanic activity should not again manifest itself in this region.

A volcano may be considered as only dormant, and not extinct, when in the so-called "solfataric condition." This name is derived from the Solfatara, near Naples, where there has been no eruption since 1198, but where vapors and gases are constantly issuing from the region of the old crater. These vapors consist mainly of steam, mixed to some extent with sulphuretted hydrogen, and also with sulphurous acid, chlorohydric acid, carbonic acid, and nitrogen gases. The abundance of the sulphuretted hydrogen is usually testified to by the deposits of sulphur, so often met with in the craters of old volcanoes, and undoubtedly formed by the decomposition of this gas; besides, the nose has no difficulty, if no satisfaction, in detecting its presence. Steam and sulphuretted hydrogen usually predominate largely among the products of solfataric action. The other gases mentioned generally, but not always, occur in smaller quantity. Boracic acid, petroleum,