382 L. Y. Pirsson — Note on some Volcanic Pod's 



The augite as a phenocryst is rare but presents nothing re- 

 markable. The feldspar phenocrysts are of labradorite as 

 indicated by their optical properties and by their sp. g. 2-708 

 taken with a heavy solution and Westphal balance. There is 

 the usual albite and pericline twinning. The groundmass con- 

 sists of fine feldspar laths in character near andesite, the small 

 olivines referred to and between these globulitic material, 

 augite microlites, iron ore, glass, etc. This interstitial material 

 is of a dark grayish brown color in which the colorless feld- 

 spar laths lie, looking like slits in the section. 



A determination of silica in this rock gave 49-55 per cent 

 and its sp. gr. is 2 643, the lowness of which is no doubt due to 

 the glass of the groundmass. From these and the mineral com- 

 position it is evidently to be referred to basalt. 



Trachyte. 



This is represented by trachytic tuffs and obsidian. The 

 latter is macroscopically, a black, pitchy looking glass filled with 

 pores which by their extension in one direction indicate flow 

 movements in the viscous fluid. Rarely small feldspar pheno- 

 crysts are to be seen. Before the blowpipe this glass melts 

 readily, puffing up into a grayish pumice which then melts to 

 glass with the utmost difficulty, a small amount of water 

 present increasing the mobility and lowering the melting point 

 as noted previously by other observers on acid glasses. The 

 chemical analysis of this glass gave the following composition : 



Sp.gr. =2-210 SiO„ 61-22 



TiO" 2 __ -42 



Al o 3 18-01 



Fe^O s 1-32 



Feb 4-51 



MnO tr. 



MgO -44 



CaO 1-88 



Na 2 1 6-49 



K 2 593 



H 2 -46 



Total 100-68 



From the moderate amount of silica, low lime and mag- 

 nesia and high alkalies, it is evident that this glass is to be 

 referred to trachyte, perhaps of the phonolithoid type. 



In thin section it appears as a fresh unaltered glass of a deep 

 brown color scattered through which are a few grains of iron 

 ore, apatite crystals, an occasional sanidine or olivine pheno- 



