BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 26, pp. 255-286 JUNE 29, 1915 



OBSIDIAN FEOM HRAFNTINNUHEYGaUR, ICELAND: ITS 

 LITHOPHYS.'?] AND SURFACE MARKINGS ' 



]SY FRED. E. WRIGHT 

 (Preaenlfil in abstract before the Society Deceinljer SO. 1000) 



CONTENTS 



Page 



Historical review 255 



The Hrafntinnuliryggur ob.sidian 258 



General description 258 



Chemical characteristics 259 



Spherulites and lithophysse 262 



Secondary minerals and etching phenomena produced by hot circulat- 

 ing solutions 275 



The moldavites 280 



Summary 285 



Historical Review 



To the student of rocks the forms and relations of crystals and of 

 cr\^stal aggregates precipitated from a cooling magma are in large meas- 

 ure the expression of the physical conditions under which the magma 

 solidified. This was thoroughly appreciated by the' pioneers in petrology, 

 who observed that as the physical conditions of freezing of such magmas 

 varied, so also did the resulting products of crystallization, with respect 

 both to the kinds of crystals formed (mineral composition) and to the 

 habits and relations between the crystals (rock texture). Of the differ- 

 ent kinds of crystals thus studied none has received more attention tliaii 

 tlie sphoi'ulites; and yet oiii- knowledge of them is still iiuomplete, espe- 

 cially of the hollow spherulites or lithophysse, the best examples of which 

 have been roniid in llic obsidian of Yellowstone National Pai'k. These 

 were studied many years ngo by Tddings^ in detail and with special rtWcr- 

 ence to their mode of formation. At that time petrologists wi'ic mil 



' Mamiscripi i-ccclvcd li.\ tlif Secret a r.\' i>r llif Society .\\iyi\ l;0, ini.'i. 

 - T'. S. f!e(>li>ulc;il .Siirvi'.v, Seventh .\nn. lieport, ISS.'i. pp. L'."i:!-i!0;"i. 



{ 255 ) 



