29 



however, well out of any of the direct lines of volcanic energy, 

 though we are, in common with the rest of the world, always liable 

 to the minor earthquakes caused by the falling in of subterranean 

 cavities, and the fractures and crushings of rocks which must 

 necessarily be caused by the shrinking of the earth's crust. As 

 you all know, the earth, which was at one time a molten and glow- 

 ing mass, has been for countless ages gradually cooling, and is, of 

 course, doing so still. It is calculated by Sir William Thompson 

 that our globe loses each year sufficient heat, or caloric, to melt 

 777 cubic miles of ice, or to raise an equal bulk of water from 69 

 deg. Fahrenheit, to boiling point, 212 deg. This cooling is neces- 

 sarily attended with contraction, and it is this shrinking of the 

 earth's surface which has thrust up the various mountain ranges, 

 just as a withering apple in contracting thrusts up its skin. The same 

 process of contraction must be eternally going on beneath the 

 earth's surface, and must assuredly produce prodigious effects in 

 the shape of heat and motion, accompanied necessarily by earth 

 tremors and vibrations ; in fact by minor earthquakes. As regards 

 volcanic eruptions, however, we bave already had our day, in 

 long bygone geological (miocene) times, as evidenced by the vol- 

 canic deposits in Wales, the North of Ireland, and several parts of 

 England and Scotland. Snowdon and Cader Idris are both of 

 volcanic origin, as is also the celebrated Giant's Causeway. The 

 way in which volcanic action shifts to different points of the earth's 

 surface is most curious and unaccountable. There is scarcely a 

 spot which has not been visited, while those centres where it is 

 now most active were, in bygone ages, quite tranquil. The whole 

 submarine strata, for example, on which Etna now stands, were in 

 times not geologically very remote, not existent. Nor is any 

 instance known in which volcanic energy, after becoming extinct, 

 has again returned to the same spot. So that, humanly speaking, 

 we may be said to be tolerably safe, being, as I before remarked. 

 out of the direct line of still active volcanoes. A glance at the 

 map I have sketched will show you the curious linear arrange- 

 ments of volcanoes, which run in a more or less north and south 

 direction. As I have already pointed out, one would expect them 

 to occur on the lines of greatest weakness of the earth's crust, 

 namely, where the ocean bed is depressed and the adjoining land 

 elevated. And you will observe volcano outlets all more or less 

 directly follow the outlines of the two great continents of America 

 and A sia. If we imagine long cracks or fissures running, generally, 

 in a north and south direction, and connected here and there by 

 cross lines (just as might happen in a ball of drying clay), we 

 shall readily understand this linear arrangement of volcanoes, 

 more especially if we accept the theory of water finding its 



