222 J. LOGAN LOBLEY, F.G.S., F.R.G.S.^ ON VOLCANIC ACTION 



thus it was concluded a fused mass would remain in the 

 interior enclosed in a solid crust. There is, besides, a general 

 similarity in the character of volcanic ejectamenta in all parts 

 of the world. 



It has, however, now been ascertained by physical research 

 and astronomical observation, that the rigidity of the earth as a 

 planet is so great that it must either be a solid sphere or have 

 a solid crust of very great thickness, from 400 to 800 miles at 

 least, a thickness of massive cooler rocks obviously far too great 

 for lava to pass through. Lord Kelvin is of opinion that the 

 earth is solid to the centre. The increase of heat with descent 

 will in its fusing effect be counteracted by the enormous 

 pressure of the exterior rocks, and thus solidity may be main- 

 tained at a very high temperature. The general similarity of 

 volcanic ejectamenta may be explained by the general similarity 

 of the crystalline rocks which form the platform, as it were, 

 upon which the clastic or sedimentary rocks are superimposed, 

 and which, therefore, we must conclude, underlie the derived 

 rocks everywhere around the lithosphere of the globe. These 

 primordial rocks are made up of minerals that are combinations 

 of but a few chemical elements in the aggregate, and so on 

 being melted will give generally similar products. And it is 

 only a general similarity that exists, since there is a considerable 

 divergence in specific chemical composition to be found in the 

 outputs of different volcanoes. 



From these considerations, the central source hypothesis must 

 be regarded as quite inadmissible, and ought now to be looked 

 upon .by all writers on the subject as obsolete, and yet this 

 hypothesis is implied in much that is written on volcanoes in 

 recent works and especially in newspapers. 



The wide-spread opinion, however, tliat lava is derived from 

 a distance of about 30 miles from the surface, is not dependent 

 on the central source hypothesis, since it is compatible both 

 with separated reservoirs of fused rock matter in a thick crust, 

 or in an otherwise solid globe, and with an intermediate Ocean 

 of fluid lava between a solid central mass and a solid crust. 



Lateral pressure relieves and lessens vertical pressure, and so 

 at places in a thick earth's crust it was said the great vertical 

 and fusion preventing pressure of the exterior rocks may be so 

 much lessened by lateral pressure that the internal heat may 

 exert its fusing effect, with the result of convertiug solid rock- 

 masses into subterranean lakes of molten rock-matter, from 

 which the surface volcanoes receive their supplies of lava. 



To this it may be replied, that a thickness of 30 miles of 



