SUMMARY 285 



conceivable that all of the outer zone of intense compression and part of 

 the inter zone of dilatation have been etched away, and only the central 

 core of the original fragment is left. It does, however, prove that the 

 present surface markings of the moldavites are not original surface 

 markings produced on the fragment during its flight through space. 

 This conclusion is in agreement with the inferences which have been 

 drawn by Professor Merrill from the etching phenomena. 



It may be noted that in highly siliceous glasses the birefringence de- 

 veloped for a given load is less than that developed under similar condi- 

 tions in ordinary, less silieeous glasses, wliich have niueli higher euefh- 

 cients of expansion. 



Summary 



'IM 



he oljsidian at H rafntinniilirvggur, near Myvatn, Iceland, is of special 

 interest to the geologist because of the un\isual op])ortnnity it offers for 

 the study of the effects resulting From the |)hysico-cheniical conditions of 

 cooling, in the present paper the formation especially of spherulitic, 

 lithophysal, and pumieeous structures is discussed: certain remarkable 

 surface markings resembling the pits and grooves on moldavites are also 

 described briefly. They were produced by the etching effect of hot vol- 

 canic emanations on fragments of obsidian glass. 



The evidence given above indicates that in the formation of the litho- 

 i:>hysae gases were active. These volatile components, wliich were released 

 from the magma dining tlie crystallization of the radial spherulites, at- 

 tacked part of the material of the spherulites; new ciystal compounds, as 

 tridvmite and fayalite, were formed which bespeak conditions of forma- 

 tion different from those undei- which the original spherulites were crys- 

 tallized. The pressure of the liberated volatile components aided mate- 

 rially in the original formation and subsecpient enlargement of the litho- 

 phvsal cavities. The general hydrostatic tension (external pull) result- 

 ing from shrinkage of the central part of the cooling magma jirobably 

 aided in this development. l)ut the inclosed gas pressing against the walls 

 of the cavity was also an important factor. 



Volatile components set free during the crystallization of a sjiherulite 

 may either escape along minute cracks and spaces in the sphei ulite to its 

 margin and there form a bubble in the viscous magma ot the viscosity 

 of the magma may be such that the internal gas pressure forces asunder 

 the spherulite. In the first case the presence of the gas bubble adjacent 

 to the spherulite hinders the further growth of the spherulite at that 

 point, with the result that the spherulites with adjacent bubble cavities 



