Part 15 (2/2)
We have also to recollect that the abrading action of water has been absent from the moon; so that, while acc.u.mulations of matter had been proceeding throughout a prolonged period over its surface, there was no counteracting agency of denudation at work to modify or lessen the effects of the ruptive forces.
[1] Correctly speaking, each attracts the other towards its centre of gravity with a force proportionate to its ma.s.s, and inversely as the square of the distance; but the earth being by much the larger body, its attraction is far greater than that of the moon.
[2] The variation in the distance is only under rare circ.u.mstances 40,000 miles, but ordinarily about 13,000 miles.
[3] _Story of the Heavens_, 2nd edition, p. 525, _et seq._
[4] A series of researches made by Zollner, of Leipzig, led him to a.s.sign to the light-reflecting capacity of the full-moon a result intermediate between that obtained by Bouguer, which gave a brightness equal to 1/300000 part of that of the sun, and of Wollaston, which gave 1/801070 part. We may accept 1/618000 of Zollner as sufficiently close; so that it would require 600,000 full moons to give the same amount of light as that of the sun.
[5] Schroter, however, came to the conclusion that the moon has an atmosphere.
[6] A chart of the moon's surface, with the names of the princ.i.p.al physical features, will be found in Ball's _Story of the Heavens_, 2nd edit., p. 60. It must be remembered that the moon as seen through a telescope appears in reversed position.
[7] _Ibid._, p. 66.
[8] As represented by Nasmyth's models in plaster.
[9] Ball, _loc. cit._, p. 67.
[10] Ball, _loc. cit._, p. 69.
[11] _Astronomy_, p. 78.
[12] _Outlines of Astronomy_, p. 285.
[13] At rare intervals a few crescent-shaped ridges are discernible on the lunar sphere, but it is very doubtful if they are to be regarded as breached craters.
[14] The number of ”spots” on the moon was considered to be 244 until Schroter increased it to 6,000, and accurately described many of them.
Schroter seems to have been the earliest observer who identified the circular hollows on the moon's surface as volcanic craters.
[15] Drawings of these very curious forms are given by Judd, _Volcanoes_, p. 127.
CHAPTER III.
ARE WE LIVING IN AN EPOCH OF SPECIAL VOLCANIC ACTIVITY?
The question which we are about to discuss in the concluding chapter of this volume is one to which we ought to be able to offer a definite answer. This can only be arrived at by a comparison of the violence and extent of volcanic and seismic phenomena within the period of history with those of pre-historic periods.
At first sight we might be disposed to give to the question an affirmative reply when we remember the eruptions of the last few years, and add to these the volcanic outbursts and earthquake shocks which history records. The cases of the earthquake and eruption in j.a.pan of November, 1891, where in one province alone two thousand people lost their lives and many thousand houses were levelled[1]; that of Krakatoa, in 1883; of Vesuvius, in 1872; and many others of recent date which might be named, added to those which history records;--the recollection of such cases might lead us to conclude that our epoch is one in which the subterranean volcanic forces had broken out with extraordinary energy over the earth's surface. Still, when we come to examine into the cases of recorded eruptions--especially those of great violence--we find that they are limited to very special districts; and even if we extend our retrospect into the later centuries of our era, we shall find that the exceptionally great eruptions have been confined to certain permanently volcanic regions, such as the chain of the Andes, that of the Aleutian, Kurile, j.a.panese, and Philippine and Sunda Islands, lying for the most part along the remarkable volcanic girdle of the world to which I have referred in a previous page. Add to these the cases of Iceland and the volcanic islands of the Pacific, and we have almost the whole of the very active volcanoes of the world.
Then for the purposes of our inquiry we have to ascertain how these active vents of eruption compare, as regards the magnitude of their operations, with those of the pre-historic and later Tertiary times. But before entering into this question it maybe observed, in the first place, that a large number of the vents of eruption, even along the chain of the earth's volcanic girdle, are dormant or extinct. This observation applies to many of the great cones and domes of the Andes, including Chimborazo and other colossal mountains in Ecuador, Columbia, Chili, Peru, and Mexico. The region between the eastern Rocky Mountains and the western coast of North America was, as we have seen, one over which volcanic eruptions took place on a vast scale in later Tertiary times; but one in which only the after-effects of volcanic action are at present in operation. We have also seen that the chain of volcanoes of j.a.pan and of the Kurile Islands are only active to a slight extent as compared with former times, and the same observation applies to those of New Zealand. Out of 130 volcanoes in the j.a.panese islands, only 48 are now believed to be active.
Again, if we turn to other districts we have been considering, we find that in the Indian Peninsula, in Arabia, in Syria and the Holy Land, in Persia, in Abyssinia and Asia Minor--regions where volcanic operations were exhibited on a grand scale throughout the Tertiary period, and in some cases almost down into recent times--we are met by similar evidences either of decaying volcanic energy, or of an energy which, as far as surface phenomena are concerned, is a thing of the past. Lastly, turning our attention to the European area, notwithstanding the still active condition of Etna, Vesuvius, and a few adjoining islands, we see in all directions throughout Southern Italy evidences of volcanic operations of a past time,--such as extinct crater-cones, lakes occupying the craters of former volcanoes, and extensive deposits of tuff or streams of lava--all concurring in giving evidence of a period now past, when vulcanicity was widespread over regions where its presence is now never felt except when some earthquake shock, like that of the Riviera, brings home to our minds the fact that the motive force is still beneath our feet, though under restrained conditions as compared with a former period.
Similar conclusions are applicable with even greater force to other parts of the European area. The region of the Lower Rhine and Moselle, of Hungary and the Carpathians, of Central France, of the North of Ireland and the Inner Hebrides, all afford evidence of volcanic operations at a former period on an extensive scale; and the contrast between the present physically silent and peaceful condition of these regions, as regards any outward manifestations of sub-terrestrial forces, compared with those which were formerly prevalent, cannot fail to impress our minds irresistibly with the idea that volcanic energy has well-nigh exhausted itself over these tracts of the earth's surface.
From this general survey of the present condition of the earth's surface, as regards the volcanic operations going on over it, and a comparison with those of a preceding period, we are driven to the conclusion that, however violent and often disastrous are the volcanic and seismic phenomena of the present day, they are restricted to comparatively narrow limits; and that even within these limits the volcanic forces are less powerful than they were in pre-historic times.
The middle part of the Tertiary period appears, in fact, to have been one of extraordinary volcanic activity, whether we regard the wide area over which this activity manifested itself, or the results as shown by the great amount of the erupted materials. Many of the still active volcanic chains, or groups, probably had their first beginnings at the period referred to; but in the majority of cases the eruptive forces have become dormant or extinct. With the exception of the lavas of the Indian-Peninsular area, which appear, at least partially, to belong to the close of the Cretaceous epoch, the specially volcanic period may be considered to extend from the beginning of the Miocene down to the close of the Pliocene stage. During the Eocene stage, volcanic energy appears to have been to a great degree dormant; but plutonic energy was gathering strength for the great effort of the Miocene epoch, when the volcanic forces broke out with extraordinary violence over Europe, the British Isles, and other regions, and continued to develop throughout the succeeding Pliocene epoch, until the whole globe was surrounded by a girdle of fire.
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