J. P. Id dings — Origin of Quartz in Basalt. %Y{ 



glasses, such as tachylites, hydrotachylites, palagonites, etc. 

 Professor Lagorio states that the feldspars of the lime-soda 

 feldspar series behave like eutectic substances. 



Increased liquidity of magmas. — The increased fusibility of 

 hydrated glass has been demonstrated by M. Daubree (Etudes 

 Synthetiques de Geologie Experimentale. Paris, 1879, p. 161), 

 and may be easily recognized in the case of obsidian carrying 

 0'5 per cent, of water, like that from Obsidian Cliff. When 

 this is fused before an oxy hydrogen blowpipe it melts at some- 

 thing less than white heat to an inflated glass, which, at first, 

 is quite fluid and flows away rapidly from the strong current 

 of the blowpipe flame. After driving off all the water from 

 this glass and allowing it to cool, a colorless glass is obtained 

 which only melts before the same blowpipe flame at an intense 

 white heat, and then forms a very viscous glass which moves 

 sluggishly before the same blowpipe current. 



Retarded viscosity and greater crystallization. — If we con- 

 sider the influence of water-vapor in increasing the liquidity 

 of molten magmas as simply a physical one, it must tend to 

 increase the mobility of the molecules among themselves. One 

 effect of absorbed water, then, would be to retard the increas- 

 ing viscosity of a rapidly cooling magma, which might permit 

 the crystallization of the more hydrated portions, while the 

 less hydrated parts became too viscous and solidified as glass. 

 A condition of affairs which undoubtedly existed in the ob- 

 sidian magma at Obsidian Cliff, where holocrystalline areas 

 occur irregularly scattered through glassy ones. 



Different hind of crystallization. — It is to be remarked that 

 the crystallization which was in this instance specially induced 

 by the influence of superheated steam, differs from that which 

 usually takes place in acid lavas upon cooling, and that the 

 result was the production of extremely basic and acidic min- 

 erals by the side of one another, or the production of extremes. 

 A result, which is in a measure, analogous to the dissociation 

 of the base and acid of a chemical compound by heating. The 

 absorbed water-vapor apparently weakened the affinity between 

 the bases and the silica and permitted them to separate into 

 more basic silicates and quartz or tridymite. 



Potential mobility. — Exactly what influence absorbed water- 

 vapor has upon deeply seated molten magmas we do not know, 

 but it seems reasonable to assume that its physical influence is 

 of the same kind as that upon molten magmas oh the surface 

 of the earth, for in the two cases the most noticeable difference 

 is that of the pressure under which each exists. Hence we 

 may assume that its most potent influence lies in the increased 

 mobility, which it tends to impart to the molecules of the 

 mass. But in a confined magma this must be combined with 



