258 



Prof. J. Prestwich. On the 



[Apr. 16, 



This lie considers to be dependent upon the difference between the 

 statical and the kinetical pressure of the column of lava on the sides 

 of the duct. In the change from the one state to the other, when 

 the lava begins to flow, and its lateral pressure is lessened, the equili- 

 brium with the surrounding elastic high pressure vapour becomes 

 destroyed, and the vapour forces its way into the ascending Java. 

 As this proceeds, the heated water further from the duct, and held 

 back by the pressure of the vapour, flashes into steam to supply its 

 place. If that water should be lodged in the joints of the surround- 

 ing rock, blocks of it will also be blown off, driven into, and ejected 

 with, the ascending lava, as have been the blocks in Somma and of 

 other volcanoes. 



It is the double action thus established between the inland- and 

 sea- waters that has probably prolonged the activity of the existing 

 volcanoes settled in ocean centres, or along coast lines, while the 

 great inland volcanic areas of Auvergne, the Eifel, Central Asia, 

 &c, have become dormant or extinct. 



But if water only plays a secondary part in volcanic eruptions, to 

 what is the motive power which causes the extravasation of the lava 

 to be attributed ? This involves questions connected with the solidity 

 of the globe far more hypothetical and difficult of proof. The 

 author first takes into consideration the probable thickness of 

 the earth's crust from a geological point of view, and shows, that 

 although the present stability of the earth's surface renders it evident 

 that the hypothesis of a thin crust resting on a fluid nucleus is un- 

 tenable, it is equally difficult to reconcile certain geological phenomena 

 with a globe solid throughout, or even with a very thick crust. The 

 geological phenomena on which he relies in proof of a crust of small 

 thickness, are : — 1. Its flexibility as exhibited down to the most 

 recent mountain uplifts, and in the elevation of continental areas. 

 2. The increase of temperature with depth. 3. The volcanic pheno- 

 mena of the present day, and the out-welling of the vast sheets of 

 trappean rocks during late geological periods. 



He considers that the squeezing and doubling up of the strata in 

 mountain chains — as, for example, the 200 miles of originally hori- 

 zontal strata in the Alps, crushed into a space of 130 miles (and in 

 some cases the compression is still greater) — can only be accounted for 

 on the assumption of a thin crust resting on a yielding substratum, 

 for the strata have bent as only a free surface plate could to the 

 deformation caused by lateral pressure. If the globe were solid, or 

 the crust of great thickness, there would have been crushing and 

 fracture, but not corrugations. Looking at the dimensions of these 

 folds, it is evident also that the plate could not be of any great 

 thickness. This in connexion with the increase of heat with depth, 

 and the rise of tne molten lava through volcanic ducts, which, if too 



