Yet another theory has found advocates. The mathematician Hopkins, whose analysis of the molten-nucleus theory was mainly effective in showing that theory to be untenable, suggested that there may be isolated subterranean lakes of fiery matter, and that these may be the true seat of volcanic energy. But such lakes could not maintain their heat for ages, if surrounded (as the theory requires) by cooler solid matter, especially as the theory also requires that water should have access to them. It will be observed also that none of the theories just described affords any direct account of those various features of the earth’s surface—mountain ranges, table-lands, volcanic regions, and so on—which are undoubtedly due to the action of subterranean forces. The theory advanced by Mr. Mallet is open to none of these objections. It seems, indeed, competent to explain all the facts which have hitherto appeared most perplexing.
It is recognized by physicists that our earth is gradually parting with its heat. As it cools it contracts. Now if this process of contraction took place uniformly, no subterranean action would result. But if the interior contracts more quickly than the crust, the latter must in some way or other force its way down to the retreating nucleus. Mr. Mallet shows that the hotter internal portion must contract faster than the relatively cool crust; and then he shows that the shrinkage of the crust is competent to occasion all the known phenomena of volcanic action. In the distant ages when the earth was still fashioning, the shrinkage produced the irregularities of level which we recognize in the elevation of the land and the depression of the ocean-bed. Then came the period when as the crust shrank it formed corrugations, in other words, when the foldings and elevations of the somewhat thickened crust gave rise to the mountain-ranges of the earth. Lastly, as the globe gradually lost its extremely high temperature, the continuance of the same process of shrinkage led no longer to the formation of ridges and table-lands, but to local crushing-down and dislocation. This process is still going on, and Mr. Mallet not only recognizes here the origin of earthquakes, and of the changes of level now in progress, but the true cause of volcanic heat. The modern theory of heat as a form of motion here comes into play. As the solid crust closes in upon the shrinking nucleus, the work expended in crushing down and dislocating the parts of the crust is transformed into heat, by which, at the places where the process goes on with greatest energy, “the materials of the rock so crushed and of that adjacent to it are heated even to fusion. The access of water to such points determines volcanic eruption.”
Now all this is not mere theorising. Mr. Mallet does not come before the scientific world with an ingenious speculation, which may or may not be confirmed by observation and experiment. He has measured and weighed the forces of which he speaks. He is able to tell precisely what proportion of the actual energy which must be developed as the earth contracts is necessary for the production of observed volcanic phenomena. It is probable that nine-tenths of those who have read these lines would be disposed to think that the contraction of the earth must be far too slow to produce effects so stupendous as those which we recognize in the volcano and the earthquake. But Mr. Mallet is able to show, by calculations which cannot be disputed, that less than one-fourth of the heat at present annually lost by the earth is sufficient to account for the total annual volcanic action, according to the best data at present in our possession.
As I have said, I do not propose to follow out Mr. Mallet’s admirable theory into all its details. I content myself with pointing out how excellently it accounts for certain peculiarities of the earth’s surface configuration. Few that have studied carefully drawn charts of the chief mountain-ranges can have failed to notice that the arrangement of these ranges does not accord with the idea of upheaval through the action of internal forces. But it will be at once recognized that the aspect of the mountain-ranges accords exactly with what would be expected to result from such a process of contraction as Mr. Mallet has indicated. The shrivelled skin of an apple affords no inapt representation of the corrugated surface of our earth, and according to the new theory, the shrivelling of such a skin is precisely analogous to the processes at work upon the earth when mountain-ranges were being formed. Again, there are few students of geology who have not found a source of perplexity in the foldings and overlappings of strata in mountainous regions. No forces of upheaval seem competent to produce this arrangement. But by the new theory this feature of the earth’s surface is at once explained; indeed, no other arrangement could be looked for.
It is worthy of notice that Mr. Mallet’s theory of Volcanic energy is completely opposed to ordinary ideas respecting earthquakes and volcanoes. We have been accustomed vaguely to regard these phenomena as due to the eruptive outbursting power of the earth’s interior; we shall now have to consider them as due to the subsidence and shrinkage of the earth’s exterior. Mountains have not been upheaved, but valleys have sunk down. And in another respect the new theory tends to modify views which have been generally entertained in recent times. Our most eminent geologists have taught that the earth’s internal forces may be as active now as in the epochs when the mountain-ranges were formed. But Mr. Mallet’s theory tends to show that the volcanic energy of the earth is a declining force. Its chief action had already been exerted when mountains began to be formed; what remains now is but the minutest fraction of the volcanic energy of the mountain-forming era; and each year, as the earth parts with more and more of its internal heat, the sources of her subterranean energy are more and more exhausted. The thought once entertained by astronomers that the earth might explode like a bomb, her scattered fragments producing a ring of bodies resembling the zone of asteroids, seems further than ever from probability; if ever there was any danger of such a catastrophe, the danger has long since passed away.
TOWARDS THE NORTH POLE.
The Arctic Expedition which returned to our shores in the autumn of 1876 may be regarded as having finally decided the question whether the North Pole of the earth is accessible by the route through Smith’s Sound—a route which may conveniently and properly be called the American route. Attacks may hereafter be made on the Polar fastness from other directions; but it is exceedingly unlikely that this country, at any rate, will again attempt to reach the Pole along the line of attack followed by Captain Nares’s expedition. I may be forgiven, perhaps, for regarding Arctic voyages made by the seamen of other nations as less likely to be successful than those made by my own countrymen. It is not mere national prejudice which suggests this opinion. It is the simple fact that hitherto the most successful approaches towards both the Northern and the Southern Poles have been made by British sailors. Nearly a quarter of a century has passed since Sir E. Parry made the nearest approach to the North Pole recorded up to that time; and although, in the interval between Parry’s expedition and Nares’s, no expedition had been sent out from our shores with the object of advancing towards the Pole, while America, Sweden, Russia, and Germany sent out several, Parry’s attempt still remained unsurpassed and unequalled. At length it has been surpassed, but it has been by his own countrymen. In like manner, no nation has yet succeeded in approaching the Antarctic Pole so nearly, within many miles, as did Captain Sir J. C. Ross in 1844. Considering these circumstances, and remembering the success which rewarded the efforts of Great Britain in the search for the North-West Passage, it cannot be regarded as national prejudice to assert that events indicate the seamen of this country as exceptionally fitted to contend successfully against the difficulties and the dangers of Arctic exploration. Should England, then, give up the attempt to reach the North Pole by way of Smith’s Sound and its northerly prolongation, it may fairly be considered unlikely that the Pole will ever be reached in that direction.
It may be well to examine the relative probable chances of success along other routes which have either not been so thoroughly tried, or have been tried under less favourable conditions.