IN this chapter we shall learn what we can as to the physical condition of the interior of our earth so far as it may be reasonably inferred from the facts of observation. We have already explained in the last chapter that a very high temperature must be found at the depth of even a small fraction of the earth’s radius, and we have pointed out that the excessively high pressure characteristic of the earth’s interior must be borne in mind in any consideration as to the condition of the matter there found.
Let us take, for instance, that primary question in terrestrial physics, as to whether the interior of the earth is liquid or solid. If we were to judge merely from the temperatures reasonably believed to exist at a depth of some twenty miles, and if we might overlook the question of pressure, we should certainly say that the earth’s interior must be in a fluid state. It seems at least certain that the temperatures to be found at depths of two score miles, and still more at greater depths, must be so high that the most refractory solids, whether metals or minerals, would at once yield if we could subject them to such temperatures in our laboratories. At such temperatures every metal would become fluid, even if it were not transformed into a cloud of vapour. But none of our laboratory experiments can tell us whether, under the pressure of thousands of tons on the square inch, the application of any heat whatever would be adequate to transform solids into liquids. It may indeed be reasonably doubted whether the terms solids and liquids are applicable, in the sense in which we understand them, to the materials forming the interior of the earth.
It was my good fortune some years ago to enjoy a most interesting trip to Norway, in company with a distinguished geologist. Under his guidance I there saw evidence which demonstrates conclusively that, when subjected to great pressure, solids, as we should call them, behave in a manner which, if not that of actual liquids, resembles at all events in some of its characteristics the behaviour of liquids. These rocks in some places are conglomerates, of which the leading constituents are water-worn pebbles of granite. These pebbles are of various sizes, from marbles to paving-stones. In some parts of the country these granite pebbles remain in the form which they acquired on the beach on which they were rolled by the primæval ocean; in other parts of the same interesting region the form of the pebbles has been greatly changed from what it was originally. For in the course of geological periods, and after the pebbles had become consolidated into the conglomerate, the rock so formed had been in some cases submitted to enormous pressure. This may have been lateral pressure, such as is found to have occurred in many other places, where it has produced the well-known geological phenomenon of strata crumpled into folds. In the present case, however, it seemed more probable that it was the actual weight of the superincumbent rocks, which once lay over these beds of conglomerate, which produced the surprising transformation. It seems to be not at all improbable that at one time these beds of conglomerate must have been covered with strata of which the thickness is so great that it may actually be estimated by miles. There has, however, been immense denudation of the superficial rocks in this part, at all events, of Norway, so that in the course of ages these strata, overlying the conglomerate for ages, have been so far worn away, and indeed removed, by the action of ice and the action of water that the conglomerate is now exposed to view. It offers for our examination striking indications of the enormous pressure to which it was subjected during the incalculable ages of geological time.