The conclusions arrived at by astronomers and physicists is one quite in accord with those which geologists have reached by totally different methods. It is now very generally admitted that if the earth were not a rigid mass, its behaviour under the attract live influences of the surrounding members of the solar system would be very different to what is found to be the case. [Sidenote: ARGUMENTS AGAINST LIQUID INTERIOR.] That the earth is in a solid condition to a great depth from the surface, and possibly quite to the centre, is a conclusion concerning which there can be little doubt; and in the next chapter we shall endeavour to show that such a condition of thirds is by no means incompatible with those manifestations of internal energy, the phenomena of which we are considering in this work. The question, therefore, of the complete solidity of our globe, or of its consisting of a solid and a liquid portion, is one of speculative interest only, and is in no way involved in our investigations concerning the nature and origin of volcanic activity. We may conclude this chapter by enumerating the several hypotheses which have at different times been maintained concerning the nature of the interior of our globe. _First._ It has been suggested that the earth consists of a fluid or semi-fluid nucleus surrounded and enclosed in a solid shell. Some have maintained this shell to be of such insignificant thickness, as compared with the bulk of the interior liquid mass, that portions of the latter are able to reach the earth's surface through movements and fractures of the outer shell, and that in this manner volcanic manifestations originate. Others, impressed with the general stability and rigidity of the globe as a whole, have maintained that the outer solid shell must have a very considerable thickness, amounting probably to not less than several hundreds of miles. But through a shell of such thickness it is difficult to conceive of the liquid masses of the interior finding their way to the surface, and those who have held this view are driven to suggest some other means by which local developments of volcanic action might be brought about.