384 L. V. Pirssoii — Note on some Volcanic Bocks, etc. 



comparatively slight variations of temperature. Such increase 

 of pressure may be sufficient to condition the crystallization 

 of compounds which later under lower pressures would be 

 incapable of existing, the mobility of the magma being in- 

 creased by the presence of water vapors as shown by the 

 author cited and thereby conditions more favorable for the 

 production of the compound. 



Feldspar is scattered freely through the glass in small micro- 

 lites of sanidine in flat tables of excessive thinness and twinned 

 according to the Carlsbad law. They are outlined by the base 

 (001), clinopinacoid (010) and positive 

 hemidome (201) and appear as in the 

 annexed figure. They are sometimes 

 solitary and sometimes grouped into 

 somewhat spherical aggregates and thus 

 exhibit the only tendency toward spheru- 

 litic structure observed in the rock. 



Gldbulites are present composed largely 

 of minute specks of iron ore as is shown 

 by the colorless zones surrounding them, 

 from which they have extracted the ferruginous coloring mat- 

 ter of the glass. 



Tuffs are represented by several specimens. They are soft, 

 light brown in color and enclose fragments of volcanic rock. 

 In thin section they are seen to be composed of fragments of 

 glass which are generally very fresh, mingled with broken 

 crystals of sanidine and augite. 



In conclusion it is of interest to note that in thus establish- 

 ing, as is evinced by the material investigated, the recent vol- 

 canic nature of this "solitary island, there is one more added to 

 that line of mid-Atlantic volcanoes, which sweeping southward 

 through the Azores, Cape Yerde Islands, Ascension, St. He- 

 lena, Tristam da Cunha, and Gough's Island, terminates its 

 volcanic fires on Bouvet Island on the confines of the Antarctic 

 Ocean. 



Laboratory of Mineralogy and Petrography, 

 Sheffield Scientific School. New Haven, Jan., 1893. 



