CHAP. Ill OUR MOLTEN GLOBE 41 



complex one, and the evidence of so varied a nature and 

 often so difficult to interpret, that the conclusions reached 

 have been usually doubtful and often conflicting. This 

 seems to have been due, in part, to the fact that no pro- 

 perly qualified person had, till quite recently, devoted 

 himself to a thorough study of the whole subject, taking 

 full account of all the materials available for arriving at a 

 definite conclusion, as well as of the various groups of phe- 

 nomena which such a conclusion must harmonize and 

 explain. But for many years past a good practical geo- 

 logist, who is also an advanced mathematician — the Rev. 

 Osmond Fisher — has made this subject his speciality, and 

 in a most interesting volume, of which a second and care- 

 fully revised edition, with an appendix, has been recently 

 published, he has brought together all the facts bearing on 

 the problem, and has arrived at certain definite conclu- 

 sions of the greatest interest. The object of this 

 chapter is to give a popular account of so much of his work 

 as bears upon the question of the thickness and density of 

 the earth's crust and the constitution of the interior.^ 



The Argument from Internal Heat. 



We will first consider the nature of the evidence in 

 favour of the view that, below a superficial crust, there is 

 a molten or highly heated substratum. The existence of 

 volcanoes, geysers, and hot springs irregularly scattered 

 over the whole surface of the globe, and continually eject- 

 ing molten rock, ashes, mud, steam or hot water, is an 

 obvious indication of some very widespread source of heat 

 within the earth, but of the nature or origin of that heat 

 they give little positive information. The heat thus indi- 

 cated has been supposed to be due to many causes, such 

 as the pressure and friction caused by contraction of the 

 cooling crust, chemical action at great depths beneath the 

 surface, isolated lakes of molten rock due to these or to 

 unknown causes, or to a molten interior, or at least a 



^ Physics of the Earth's Crust, by the Rev. Osmond Fisher, M. A., 

 F.G.S. Second edition, altered and enlarged. Macmillan and Co., 

 1889. With an Appendix, 1891. 



