288 



DYNAMICAL GEOLOGY. 



field of aa is usually 20 to 30 feet higher in level than that of 2')alioelioe. 

 The breaking up is produced while the stream is slowly moving. Some 

 cause, acting beneath, half chills the mass, and the lava, thus rendered 

 brittle, is readily broken during the movement. The only cause of such 

 cooling appears to be the vaporizing of subterranean waters flowed over by 

 the hot lava-stream. 



All lavas crust over readily, and then are slow in further consolidation, 

 owing to the rock being a very poor conductor of heat. 



The texture depends on rate of cooling, — the most rapid rate producing 

 glass, — glassy crusts in the case of basaltic lavas, and massive glass in 

 trachytic regions. Ordinary cooling ending in an indistinct or fine crystal- 

 line texture ; and from this, there may be all grades in the same mass or 

 thick stream, up to a true granite-like structure, as shown by Judd (1874), 

 Hague and Iddings (1885), and as indicated by the author in 1849. Judd 

 establishes, through facts from the Western Isles of Scotland, that in a single 

 area a volcanic rock may vary in texture from a glassy lava to a rock of 

 granitoid structure, both among basaltic and feldspathic lavas. 



250-256. 



Kgs. 250-253, Feathery forms of pyroxene and feldspar; Figs. 254-256, Microlites — all of Mount Loa lavas. 



E. S. Dana, '88. 



Through some method of change, perhaps an alternation of melting and 

 cooling, the fine basalt of Mount Loa and Kilauea often has the pyroxene 

 and feldspar in feathery tufts, like common forms of frost on windows. 

 (Figs. 250-256.) "The feldspar needles lie parallel with the pyroxene 



