584 Proceedings of Royal Society of Edinburgh. [sess. 
condensed, does not seem to suit these recent seismological dis- 
coveries so well as the theory of the solid nucleus. The very 
high temperatures required by Ritter’s theory would necessitate 
a gradient of temperature in the outer crust which is difficult to 
bring into accord with Clarence King’s recent calculations ; and 
there are other physical difficulties besides. But, from the 
seismological side, there is the difficulty of accounting for the two 
or more types of vibrations — the preliminary tremors, and the big 
waves. On the other hand, the assumption of a solid nucleus 
necessitates the existence of two such types at least. 
It is probable that at a certain depth below the earth’s surface 
materials are in a state bordering on fusion. Lower down the 
increasing pressure raises the melting point of the material to a 
higher value than the temperature existing there ; nearer the 
surface, the temperature falls off so rapidly as to be, at any place, 
well below the melting point of the material there under its 
particular pressure. We assume, of course, in accordance with 
Barus’s experiments on diabase, that the melting point of most 
earth-forming materials is raised by increase of pressure. 
It is possible, however, that in former ages this critical shell of 
material just .bordering on fusion was really in a state of fusion. 
The settling down process by which at a particular stage in its 
history the surface of the earth became cool enough to be habitable 
is well described by Kelvin in his last article on ‘‘The Age of the 
Earth as an Abode fitted for Life” (Phil. Mag., January 1899). 
Twenty-four million years ago, according to Kelvin’s and Clarence 
King’s calculations, there was no solid crust ; but when the solid 
crust began to form by a kind of crystallisation, it would rapidly 
cool over its surface. There would soon be an approximation to 
present conditions, but the crust would be subject to greater and 
more frequent ruptures and readjustments. Changes of level of the 
solid superstructure would occur more rapidly than at present, and 
underground displacements and explosions would occur more 
frequently and with greater violence. As the earth continued to 
cool to its present state, the conditions for the production of earth- 
quakes would become less favourable. The causes that operated 
five million years ago were the same as those that operate now ; 
but they would operate more rapidly and with greater vehemence 
