BUNSEN'S VIEWS. 278 



views expressed in tlu^ more iinportant coutributioiis to the literatuic on the 

 subject. 



Bunsen's Views— liniisen, after a visit to Iceland, where he hiborioush- 

 studied the voh-anic pliciiomena disphiyed on a grand scah', conceived 

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

 former of which he desig-uated as the normal trachytic, the other as the 

 iioiMiial pvnixenic magma. He was disposed to regard all volcanic ])ro<hicts 

 intermediate in composition between these types as admixtnres in varving 

 2)roporti()ns derived from two distinct foci of ernption, the relative propor- 

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

 energy. He songht to apply his views to all other volcanic reg-it^ns, citing 

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

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

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

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

 In this cliai)ter the writei- adopts the ^^ews of Bunsen as regards two great 

 grou})s of lavas, but differs \vith him as to the origin of the varied transi- 

 tion products of eruption. 



The writer has used the expression feldspatliic magma in preference to 

 trachvtic magma, as the former is a mineralogical tenn contrasting sharply 

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

 sary since the word trachytic now jjossesses a different signification from 

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

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

 many of tlie rocks formerlv considered as trachyte have been found to be 

 characterized 1)V plagiochistic feldspars, and hence more properly come luider 

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

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



Durocher's Theories.— Duroclier," after studviug the composition and petro- 

 grajjhical characters of a lai'ge number of crvstalline rocks, endeavored by 

 iugenit)us and somewhat complex theories to establish universal laws to 

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



' IJeber die Processe (\vv vulkauiscben OesteinsbiUluu-; Islands. Poggendorrs Anualen, 1851, 

 Band 83, pp. 197-272. 



-Essai de p6troloj;ir romparee. Aun. d. mines, Paris, otb ser., 1857, Tome xi, pp. 217-259. 

 HON XX 18 



