268 



K. E. W IJKillT^ — ()1?SIDIAN FROM ICELAND 



volatile roiupoiients luiviiig been I'eleased during the initial cr'yslalli/.ation 



of the spherulites. This is }3roved by several facts: 



(d) The black obsidian glass can be seen at several points to have 



flowed into a lithophysal cavity 

 whose walls had collapsed 

 slightly. The inflowing obsid- 

 ian Avas so stiff that it extended 

 as a tongue of glass into the cav- 

 ity (see center of figure 2 and 

 tigures 4 and o). These tongues 

 are of different shapes; the}' ex- 

 hil)it in cross-section the outline 

 of the cracks through which they 

 entered and are fluted longitu- 

 dinally with straight grooves 

 and lines which were impressed 

 on them by tlie irregular out- 

 line of the crack shown in figure 

 5. They resemble the product 

 obtained by the outflow, undei' 

 great compression, of a^v vis- 



PiGURE i.^Radiai Lithui>hii-sa, in pari cous or plastic material, as iron. 



recrystaiiized butter, or checse, thr(vigli an 



Lithophysal cavity has collapsed slightly and irregular orifice. The ,bsidian 



IS pierced by tongue of black obsidian. Speci- ^ moiLiian 



men 8S433. Magnification. 15 X. tongues on entering the cavities 



l)rokc down and crushed the delicate 

 lithophysal crystals extending from the 

 Avails. This proves that the major pari 

 of the recrystallization within the lith- 

 ophyssc took ])lace at a relatively higli 

 temperature, while the obsidian wa.- 

 still capable of stiff, viscous flow. 



(/;) The tridymite crystals have the 

 form of hexagonal plates. These plates 

 show file irregular birefracting areas 

 cliaracteristic of tridjauite. The tem- 

 perature of formation was accordingly 

 above the inversion temperature, 120° Kh;ri;i: r>.sii<nriii pnied Tonfmc nf 

 C. Whether or not it was above 870°, '!'";'" '^^'^"'y »'««« projectimj i„i., 



' Uthophjjsal Cavity 



the inversion temperature of quartz- spccinmi ss4:i;:. xMaguiiicaiiou, -u x. 



