. Bulletin of the Geological Society of America. Geology; Geology -- United States. 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 i
. Bulletin of the Geological Society of America. Geology; Geology -- United States. 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 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 Please note
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