BTINSEN'S VIEWS. 273 



views expressed in the more important contributions to the literature on the 

 subject. 



Bunsen's views. Bunsen, after a visit to Iceland, where he laboriously 

 studied the volcanic phenomena displayed on a grand scale, conceived 

 the idea of two distinct bodies of lava, one acid and the other basic, the 

 former of which he designated as the normal trachytic, the other as the 

 normal pyroxenic magma. He was disposed to regard all volcanic products 

 intermediate in composition between these types as admixtures in varying 

 proportions derived from two distinct foci of eruption, the relative propor- 

 tions of each depending in great part upon the intensity of eruptive 

 energy. He sought to apply his views to all other volcanic regions, citing 

 as an identical mode of occurrence the table-land of Armenia. 1 The grand 

 division of volcanic products into acid and basic lavas has been received 

 by most vulcanologists, but his theories to account for the very varied con- 

 stitution of volcanic rocks has not obtained the same general acceptance. 

 In this chapter the writer adopts the views of Bunsen as regards two great 

 groups of lavas, but differs with him as to the origin of the varied transi- 

 tion products of eruption. 



The writer has used the expression feldspathic magma in preference to 

 trachytic magma, as the former is a mineralogical term contrasting sharply 

 with the expression pyroxemc magma. This is rendered all the more neces- 

 sary since the word trachytic now possesses a different signification from 

 what it did at the time when it was first employed by the German scientist. 

 Typical trachytes are somewhat rare and confined to restricted areas, since 

 many of the rocks formerly considered as trachyte have been found to be 

 characterized by plagioclastic feldspars, and hence more properly come under 

 the head of andesite. This is the case with the feldspathic rocks of Ice- 

 land, which Bunsen investigated and upon which he bases his conclusions. 



Durocher's Theories. Durocher, 3 after studying the composition and petro- 

 graphical characters of a large number of crystalline rocks, endeavored by 

 ingenious and somewhat complex theories to establish universal laws to 

 account for the variations observed in crystalline rocks of all ages and 



1 Ueber die Proeesse AVI vulkanischen Gesteinsbildung Islands. Poggendorf s Anualen, 1851, 

 Band 83, pp. 197-272. 



3 Essai do petrologie compare. Ann. d. mines, Paris, 5th ser., 1857, Tome xi, pp. 217-259. 

 MON XX 18 



