SYNTHETIC CHAEACTEE OP BASALT. J 23 



explain away without resorting to doubtful postulates and conjectures. 

 Iron oxide forms at least 10 to 12 per cent, of normal basalt, and, while it 

 is found abundantly in almost all foliated rocks, it cannot be admitted that 

 it forms so large a percentage of their average constitution. With regard 

 to lime, however, which forms about 8 or 9 per cent, of the basalts, the 

 percentage is apparently in harmony with what we know of the constitu- 

 tion of the foliated rocks. With regard to the remaining important com- 

 ponents — magnesia, soda, and potash — the same relative correspondence is 

 found ; but whether the correspondence be exact or not, we have not the 

 data for determining. 



Relative order of abundance of the oxides constituting basalts and the foliated 



rocks. 



Basalts. Foliated rocks. 



SiUca. Silica. "} r Silica, 



Alumina. Alumiua, 



Iron oxide. Lime. 



Lime. Magnesia. » 



Magnesia. Iron oxide. ) 



Soda. Soda. 



Potash. Potash. j 



5- or 



Alumiua. 



Lime. 



Iron oxide. 



Magnesia. 



Soda. 



Potash. 



With the single exception of iron oxide, therefore, the basalts, as 

 nearly as we have the means of ascertaining, have a constitution repre- 

 senting approximately the average composition and proportions of the 

 foliated rocks. There is no other known' volcanic rock which approaches 

 that relation so nearly; all others contain too much silica and alkali and 

 too little lime. But so long as the iron oxide remains an outstanding 

 anomaly we cannot be justified in pronouncing the basalts to be the exact 

 synthetic type. It remains to be added that the basalts alone fail to show 

 that agreement in chemical constitution with any known and abundant 

 metamorphic rock which we find in all other volcanic groups In truth, its 

 whole range of characters is indicative of an origin among magmas which 

 have never passed through the reactions and mechanical processes which 

 prepared and arranged the materials of the sedimentary strata. Lastly, 

 the basalts are among the most abundant of eruptive rocks, and if we 

 reckon with them the more ancient dolerites or diabases, they have always 

 been abundant in all ages as far back as our knowledge extends. 



