CHEMICAL COMPOSITION OF LAVAS. 265 



inclusive, to the pyroxenic magma. Analyses numbered 2, 4, 7, 8, and 9 

 give the composition of typical rocks from different natural groups and of 

 the most extensive bodies of rhyolite, hornblende-mica-andesite, pyroxene- 

 andesite, acidic basalt, and normal basalt. Each of these five rocks carries 

 about 6 per cent more silica than the one standing next below it in the 

 series. 



All the vast accumulation of lavas may be regarded either as belong- 

 ing to, or as variations from, these main types, or else as transition products 

 between two closely related natural groups. 



Along the Hoosac fault, where the most basic unaltered rocks of the 

 feldspathic magma are best developed, solfataric action has so decomposed 

 them that it becomes a matter of much difficulty to determine even approx- 

 imately their original basicity, as they all show more or less evidence of 

 infiltration of siliceous material. The oldest lavas occurring in any exten- 

 sive body and still preserved in a fresh condition consist almost wholly of 

 hornblende-mica-andesite, represented by the rock northeast of Hoosac 

 Mountain, carrying, according to analysis, 6 7 '83 per cent of silica. The 

 fine rhyolite from Pinto Peak, free from ferro-magnesian silicates and rich 

 in well developed orthoclase, is typical as regards chemical composition of 

 the acidic end of the feldspathic magma along the same great line of dis- 

 placement. 



It will be noticed that the dacite from northeast of South Hill carries '8 

 per cent of silica less than does the hornblende-mica-audesite, whereas on 

 theoretical grounds it would be expected to show an amount somewhat in 

 excess, owing to the presence of quartz secretions. The rock was selected 

 on account of its well recognized geological relations with an overlying 

 rhyolite body, an analysis of which, for comparison, will be found in the 

 table. Normal dacite of the Great Basin usually carries about 70 per cent 

 of silica, whereas this rock stands as an intermediate variety between it 

 and the andesite. A study of the chemical analysis explains the mineral 

 composition. The large amount of iron and magnesia in excess of that 

 found in the rhyolite and the falling away in the percentage of potash are 

 sufficient to account for both the predominance of biotite and the absence 

 of sanidine. The plagioclastic nature of the prevailing feldspar assigns the 



