Lavis — On the Structure of Bocks. 137 



cases to a great thickness. Prof. Judd, who fully appreciates this 

 fact, gives a striking example of this side by side with an equally 

 interesting illustration of the opposite condition. On the contrary, 

 should a basic magma be remarkably devoid of dissolved water, its 

 surface will not be broken up, and it will assume forms like any 

 other viscid body in movement. In the case of a water-bearing 

 acid lava, the scoria surface will be much thicker, on account of 

 the difficulty with which the gaseous materials escape in con- 

 sequence of the viscidity of the paste ; whilst in nearly non- 

 aquiferous acid lavas the surface figures that result will be more 

 marked, and more characteristic of an intensely viscous magma, 

 as illustrated in the mammelon volcanoes, such as the islands of 

 Reunion, Hawaii, the obsidian stream of Vulcano, or some of the 

 central French groups. On the other hand, the Yesuvian lavas of 

 1858 and 1872, as pointed out by Judd, are respectively typical 

 of aquiferous and non- aquiferous magmas, which may be further 

 illustrated by the trachyte of Monte Olibano, and the Lava del' 

 Arso of Ischia. 



From the mode of formation of scoria we must expect it to 

 exhibit two very marked differences in structure and mineral com- 

 position from pumice. In the latter the vesicular cavities are of 

 all sizes, ranging down to the mimitest dimensions, which are the 

 most abundant and marked characteristic of pumice. This is due 

 to the intermolecular separation of steam and its union sopra loco 

 into vesicles of varying dimensions. In the case of the scoria, 

 the gases are derived from the whole thickness of the subjacent 

 lava, which, in rising in the mass, further unite together, so that 

 the cavities are rarely of microscopic size, and may reach very great 

 dimensions ; and unless the bubbles be of a certain size, the large 

 area of their surface in proportion to their volume increases, so 

 that the friction is so much that they could not rise in the viscid 

 mass. In the case of pumice we have the vesicular structure 

 developing in a complete or nearly vitreous magma, which is the 

 principal cause of rapid solidification ; but in scoria the bubbles of 

 hot gas that rise from the bottom, which, from being more pro- 

 tected, is the hottest part, through a magma already far advanced 

 in crystallization, would help to prevent or ward off the cooling of 

 the surface. Besides, the scoria will cool slowly, resting as it does 

 on the surface of a highly-heated mass. We therefore may sum 



