Dr. Johnston-Lavis — Vesurius and Monte Somma. 305 



present active, as, for instance, Jorullo in Mexico, and some others. 

 It would also be worth studying the proportion of alkalies in any 

 given series of eruptive deposits, to endeavour to discover whether 

 the amount of water- vapour that escapes bore any relation to these. 

 This remark will be better understood when we remember the vast 

 amount of HCl and SO2 that escapes in the "smoke " of a volcano, 

 which, if derived from the decomposition of sea salts, must certainly 

 represent a considerable amount of alkalies and alkaline earths, 

 which should be found in the igneous rock. 



So far we have only studied the modifications brought about 

 between water and substances held in solution in it, and the igneous 

 magma ; there is therefore left the consideration of the interchanges 

 that may take place between that latter and various solid rocks. 



We should naturally expect that the delayed contact of an igneous 

 mass with the walls of its containing tube, if they should consist of 

 highly siliceous rocks, would not be the same as with walls of 

 magnesian limestone or other earthy carbonates. Neither should we 

 expect that either would result in the same way if inclosed in a 

 tube of argillaceous strata. We must remember that in all prob- 

 ability the tube may traverse all these materials, but still one 

 particular class of rocks may predominate. 



This certainly seems a more reasonable explanation than that of 

 Von Richthofen,^ which supposed one silicate floating on another. 

 If such were the case, we should expect distinct groups of igneous 

 rocks instead of those imperceptible gradations that puzzle all the 

 experience of our chemists and petrologists. Did any laboratory 

 experiment, any smelting-furnace or glass-factor}^ show that one 

 silicate floats on another like oil on water ? 1 think the most 

 extraordinary complexity of the composition of the crystallized 

 silicates is alone sufficient to prevent our supposing the separation 

 of bodies so nearly allied chemically. It is not astonishing to see 

 substances like oil floating on water, or the latter on chloroform, 

 when we remember what a very different arrangement of molecules 

 each of these bodies have when compared together, whereas glycerine, 

 alcohol, ether, chloroform, bodies nearly allied, mix with facility. 

 Nevertheless, these are not so similar as one silicate to another. 



Those geologists who have studied the late Tertiary volcanoes 

 of Italy have always remarked a great similarity between their 

 products. True it is that we have basalts, dolerites, leucilites, 

 andesites, trachytes, and rhyolites ; yet the great majority of them 

 range between the dolerite and non-quartziferous and rather basic 

 trachyte, whilst the basalts and quartziferous lavas are confined to 

 small and isolated patches. The remarkable similarity presented by 

 the leucitic rocks in a range of volcanoes parallel to the mountain 

 axis, and therefore bursting through similar strata, is worthy of 

 notice. Perhaps not less so than the fact that all the smaller 

 volcanoes that have been formed chiefly by spasmodic or paroxj'smal 



^ It will be seen that even were the earlier part of the mountain pumiceons. it 

 certainly was not trachytic with products whose silica percentage is beneath 55 per 

 cent. — Judd, Volcanoes, 1881, p. 201. 



DECADE III. VOL. II. — NO. VII. 20 



