THE INTERNAL CONDITION OF THE EARTH; AS TO TEMPERA- [Being Paper read before the Geological Society of Glasgow, February 14, 1878.] ON previous occasions I have referred to the various arguments that have been adduced with reference to the interior condition of the earth, and to the different grounds on which it may be concluded that, instead of being a liquid mass enclosed by a mere thin shell of solid material, the earth must be, on the whole, solid. That there is some liquid within the earth is not to be denied. Immediately before lava breaks forth in the eruption of a volcano there is liquid in the interior; and, from volcanic eruptions, we know that there is sometimes, and that there may be always, some liquid in the earth's interior. How much liquid there may be-how much it is necessary to assume in order to account for the known phenomena of volcanic eruptions cannot be estimated with any degree of definiteness; but I hope to be able to put before you, this evening, arguments which will convince you that we cannot admit that there is any great liquid ocean under the earth's surface, and that we are forced to look to local causes for the explanation of volcanic eruptions. The main reason given for supposing the interior of the earth to be fluid is not, however, the existence of volcanic phenomena. From considerations regarding underground temperature, many geologists have been led to hold as a geological truth that the interior of the earth is molten throughout. The evidences of a high internal temperature are well known. It is found by observation that passing downwards from the surface, we meet first with a thin stratum in which variations of tempera ture, due to the conduction inward of summer heat and winter cold, are perceptible. This stratum is on the average about 10 metres thick. Its thickness, however, is different in different localities. It depends on the thermal conductivity of the rocks composing it. At greater depths than about 10 metres the effect of the variations of external temperature with day and night and with the seasons is not sensible. As we go down lower and lower we find that the temperature of the strata increases steadily. Speaking roughly, we find that, on the average, the increase of underground temperature in ordinary localities is about 1° C. per 30 metres of descent, or about 1° F. per 50 feet. There are localities, such as Kreuznach in Rhenish Prussia, in which the rate of increase of underground temperature, as we proceed downwards, is much greater than this. These are places, no doubt, where disturbances, in the way of great outbursts of lava, have taken place in comparatively recent times; and I may suggest, that careful observation of the rate of increase of underground temperature, in places where the date is known of a recent disturbance, would be of great importance, and would give us the means of inferring something regarding the geological history of places where an extraordinary rate of increase is observed. It is not, however, my purpose, at present, to enter into any speculation regarding geological dates in cases such as these, but rather to put before you considerations with respect to underground temperatures in general, which in the first place seem to indicate internal fluidity, but which, when further studied, do not really lead us to any such conclusion, but rather lead us to suspend our judgment, and to look in other directions for evidences of the internal condition of the earth as to whether it is fluid or solid. From the observed increase of temperature downwards below ground in all localities, we must conclude that the earth had, at some time more or less remote, its whole surface at some very high temperature. It may have been red-hot and solid all round; but it is far more probable that the surface, at least, was molten all over; and it is quite likely that the hypothesis more commonly held-namely, that it was molten throughout—is true. The most probable hypothesis that has been given for the early history of the earth is, that it was built up by the falling in of meteoric matter, and that the matter falling in with great velocity, acquired through gravitation, became heated by collision. If this hypothesis is true, the heat caused by the collisions, after the earth had grown to be anything approaching to its present size, would be far more than sufficient to melt the material falling in. Let us now consider a red-hot liquid globe, and suppose it to cool as does lava. The result of the cooling is that as soon as any portion of the matter reaches a temperature below the melting point, it freezes and becomes solid. Instead of molten lava, it becomes solid rock. And now comes the great question-will the solid rock thus formed sink or float? I wish this question could be answered. We have at present very little information on the subject-none of a definite kind, so far as I know, |