276 GEOLOGY OF THE EUEEKA DISTRICT. 



and western America, studying the development of volcanic rocks. He 

 devoted special attention to the laws governing the mode of occurrence of 

 the different natural groups into which he divided all igneous rocks, and 

 the relations of these groups to each other, being more interested in the 

 geological problems than in the precise chemical composition of the 

 extruded products. As a result of his observations in the field, he was 

 impressed by the great similarity in the nature of lavas in widely sepa- 

 rated regions and the uniformity in the order of their succession. He found, 

 however, that this succession was by no means as simple as suggested by 

 Waltershausen, nearly every volcanic region which he visited presenting 

 abrupt, but similar, alternations from acid to basic rocks, at first sight not 

 readily explained. Richthofen's final conclusions were published in an 

 admirable and remarkably suggestive memoir presented to the California 

 Academy of Sciences, 1 in which he gives what he considers to be the 

 natural law of the sequence of massive eruptions applicable to all centers 

 of volcanic energy. As his conclusions were based largely on observations 

 made in California and the western edge of the Great Basin, they are of 

 more than ordinary interest for purposes of comparison with results since 

 obtained by the investigations at Eureka. 



The natural order of succession of massive eruptions as laid down 

 by Richthofen is as follows: 2 



1. Propylite. 



2. Andesite. 



3. Trachyte. 



4. Ehyolite. 



5. Basalt. 



This law of succession as enunciated by Richthofen is far more com- 

 plex than the simple regular order suggested by Waltershausen, as it 

 supposes the breaking out, first of all, of intermediate lavas represented by 

 propylites and andesites, followed by others of varying composition, 

 but more acidic than the latter and belonging to the order trachytes. The 

 trachytes were succeeded by a still more acid series of lavas, and then the 

 closing of eruptive energy by an abrupt change from the most acidic of all 



1 Natural system of volcanic rocks. Memoirs of the California Academy of Sciences, 1867, vol. I, p. 36. 

 a Op. cit., p. 29. 



