454 PROF. J. W. JTJDD AND G. A. J. COLE ON THE 



the former being of more acid composition than the latter. It is 

 conceivable that in a dyke the crystals brought up from below may 

 be made to congregate in the central part of the mass, while its 

 sides contain more than their due share of the fused liquid material 

 in which these crystals are entangled. In this way, since we now 

 know, from the researches of Rosenbusch and others, that the glassy 

 magma of a rock is often of far more acid composition than the 

 crystals imbedded in it, the selvages of some dykes may have a 

 higher silica-percentage than the rock in their central portions. 

 But that this is ever the case we have as yet no definite proof ; and, 

 as we shall now proceed to indicate, there is another and a more 

 probable mode of accounting for the more acid composition of most 

 basalt-glasses. 



It is a well-recognized fact that the acid rocks more readily pass 

 into a glassy condition than those of basic composition. Obsidians 

 or rhyolite-glasses are common and widely distributed rocks ; 

 trachyte-, andesite-, and phonolite-glasses are less common ; and 

 basalt-glasses, as we have seen, are comparatively rare. This fact is 

 probably accounted for by the circumstance, which has been recog- 

 nized by chemists *, that mixtures of silicates in which silicates of 

 the alkalies abound more readily assume the vitreous condition than 

 those in which silicates of lime, magnesia, iron, and alumina pre- 

 dominate ; and the acid rocks usually contain a large proportion of 

 the silicates of the alkalies. 



Now, such being the case, we might expect that among basic 

 rocks, like basalts, those varieties would be more likely to assume a 

 glassy condition in which the silica-percentage is high, especially if 

 the proportion of the alkalies at the same time were excessive. 



A comparison of the various analyses of basalt-glass, such as 

 those brought together by Mohlf, Zirkel+, and others, shows this 

 to be the case, the percentage of silica in these rocks being seen to 

 vary from 50 to more than 56. The proportion of the alkalies is in 

 most cases considerably in excess of what is commonly found in 

 basalts, varying from 3 or 4 to 10 or 12 per cent. The quantity of 

 water in basalt-glass appears to be small and variable ; its presence 

 may probably be regarded as accidental. In this respect basalt- 

 glass strikingly differs from the palagonites, which contain from 11 

 to 25 per cent, of water. The rock known as hydrotachylyte, 

 which is regarded by most petrographers as a nepheline-basalt 

 glass, contains, it is true, nearly 13 per cent, of water, according 

 to the analysis of Petersen §. Possibly, however, this rock must be 

 considered to some extent a product of alteration, as was believed by 

 Mohl[|. 



We may perhaps conclude that as a general rule the basalts 

 which show a tendency to assume the vitreous condition are those 



« 

 * Miller, Elements of Chemistry, part ii., oth edition, p. 416 (1874). 

 t Die Gesteine der Sababurg in Hessen, (Cassel) p. 41. 

 | Lehrbuch der Petrographie, vol. ii. p. 305. 

 § Neues Jahrbuch fur Min. &c. 18 69, p. 32. 

 || Die siidwestlicnen Auslaufer des Vogelsgebirges, Theil 1, p. 21. 



