CHEMICAL COMPOSITION OF LAVAS. 265 



inchisive, to the j)yn>xeiiic niajiina. Analyses numbered 2, 4, 7, 8, and 9 

 give the composition of typical rocks from ditl'erent natui-al groups and of 

 the most extensive bodies of rlivolite, hornblende-mica-andesite, j)yroxene- 

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

 about G per cent more silica than the one standing- next Ijelow it in the 

 series. 



All the vast accunudation of lavas may be regai'ded either as belong- 

 ing- to, or as variations from, these main types, or else as transition ])roducts 

 between two closely related natural groups. 



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

 feldsj)athic magma are best developed, solfataric action has so decomi)osed 

 them that it becomes a matter of nuich ditHculty 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 

 hornV)lende-nuca-andesite, represented by the rock northeast of Hoosac 

 Alountain, carr}-ing-, according to analysis, G7'83 per cent of silica. The 

 tine rhyolite from Finto Peak, free from feiTO-magnesian silicates and rich 

 in \\ell developed orthoclase, is typical as regards chemical composition of 

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

 {dacement. 



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

 per cent of silica less than does the hornblende-mica-andesite, 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 ovei-h-ing 

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

 tal)le. 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 ex])lains 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 

 suflicient to account for both the |)redominance of biotite and the absence 

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



