BASALTIC GLASS. 



159 



rich in olivine. Under the microscope the rock appears wliolly nornial, 

 showing- tlie usual niicrolitic g-rounchnass of plagioclase needles and small 

 augite grains, while the porphyritic olivines are undecomposed. The 

 analysis shows, however, that it is rather siliceous for a basalt, in spite 

 of the great quantity of olivine. Under III is given the composition of an 

 ordinary basalt from Knoxville. 



' A small amount of iron in the ferric stato was not determined, because unnecessary for tlio purpose fur wliich tlie 

 analysis was originally made. 



Atomic ratio of I, H- : Si : R"' : R"=0. 047 : 5. 027 : 0. 506 : 0. 474. 

 Atomic ratio of 11, H- : Si : ft" : R"=0. 032 : 3. 825 : 0. 937 : 0. 887. 

 Atomic ratio of III, H- : .«i : S>-i : IV ='). 118 : :!. 444 : 0. 6(19 : 1. rCG. 



The difference in composition between the glass and the nearly holo- 

 crystalline basalts is extremely similar to that pointed out between the crys- 

 talline and glassy forms of andesite. There is nearly three times as much 

 alkali in the glass as in the olivinitic basalt from Burns Valley, and onl}' 

 one-fifth as much lime and magnesia. 



It is a curious fact that if 10 per cent, of lime were added to this 

 obsidian it would closely approximate in composition to some kinds of 

 window-glass. 



Analogous occurrences. — Tlie assoclatiou of Comparatively acid glasses with 

 neutral or basic lavas is not unknown. In the trach\-te of Mt. Amiata, in 

 Tuscany, Professor vom Rath found small grains of a substance which had 



