62 2 /. E. SPURR 



as he called them. He regarded ail lavas between these 

 extreme types as mixtures of the two in varying proportions. 

 It will be noted that Bunsen's explanation 1 was essentially a 

 theory of mixing rather than of differentiation. 



Durocher 2 accepted Bunsen's idea of the existence of an 

 acid and a basic magma, and admitted the possibility of a 

 minoflino: of the two in certain cases to produce intermediate 



DO 1 



types, but did not follow this idea to the extent of Bunsen. 

 Durocher proposed the hypothesis, which was not substantiated 

 by much study of volcanic action in the field, that certain lavas 

 may be produced by segregation, or differentiation, i. e., by the 

 breaking up of a magma into several different parts, as a result 

 of chemical activity. Durocher's ideas of the origin of igneous 

 rocks, therefore, include the idea of segregation, or differentia- 

 tion, together with that of mixing. 



Roth 3 later also held that a magma may segregate during 

 crystallization into lavas of different mineralogical composition, 

 although his ideas of the processes of differentiation were not 

 specifically identical with those of Durocher, who had consid- 

 ered these processes analogous with those by which metals are 

 segregated in metallurgical operations. 



Von Waltershausen, 4 after studying the lavas of Sicily and 

 Iceland, came to the conclusion that lavas were derived from a 

 zone of molten material between the earth's crust and its solid 

 interior, and that the material arranged itself according to the 

 laws of gravitation, so that the most siliceous lava, which is also 

 the lightest, was nearest the surface ; the most basic at the bot- 

 torn, and the intermediate lavas in the zones between. His own 

 observations in the field seemed to point out that the lavas were 



I Ueber die Processe der vulkanischen Gesteinsbildung Islands, Poggendorf's 

 Annalen, 1851, Band 83, pp. 197-272. 



2 Essai de Petrologie comparee, Ann. d. Mines, Paris, 5th series, 1857, Tome XI, 

 pp. 217-259. 



3 Tabellarische Uebersicht der Geistens-Analysen, mit kritischen Erlauterungen, 

 Berlin, 1861. 



4 Ueber die vulkanischen Gesteine in Sicilien und Island und ihre submarine 

 Umbildung, Gottingen, 1853. 



