Structures in Obsidian, Perlite, and Leucite. By F. Rutley. 183 



let us suppose that the underlying cryptocrystaUine layer repre- 

 sents a mass of granite or other rock matter, once in a state of 

 fusion, and which upon sohdification shrank nearly 10 per cent, of 

 its original bulk, as Bischof has shown that trachytic rocks do in 

 passing from a vitreous to a crystalhne state ; let us also imagine 

 that the little lacunae of molten vitreous matter enveloped within 

 this cryptocrystaUine mass represent reservoirs of molten matter 

 of considerable size, and that the glass rods extruded through the 

 cortical portion of the spherulitic crust are dykes, which have been 

 forced through pre-existing fissures by the contractile force of 

 matter passing from the molten to the solid state ; and we have here 

 a somewhat correct rendering, I believe, of the way in wliich many 

 great geological phenomena have taken place. Nature makes no 

 difierence between the most colossal and the most minute portions 

 of her work. The contraction which takes place upon the solidifica- 

 tion of a cubic inch of rock from the molten state is but a fraction, 

 which if multiplied, will give the contraction of cubic miles of 

 similar rock solidifying under like circumstances. The cubic inch 

 is but the cubic mile in miniature, and we may even hold within 

 our hands a small fragment of stone whose minute structure 

 represents more truly than books can tell, or than diagrams can 

 depict, the nature of some of those changes which are ever going 

 on beneath our feet ; of which, indeed, we should know but little, 

 save for volcanic eruptions and those hardened weather-beaten 

 wrecks of once deep-seated molten masses of rock, which are now 

 exposed by denudation and within the reach of our hammers. 



