GENETIC RELA TI ON SHIPS AMONG IGNEO US ROCKS. 8 4 1 



accumulative. Hence, if the process is one of synthesis or com- 

 mingling, the mixture should be the more complete the longer 

 the process has been in operation. On the other hand, if the 

 process is one of differentiation the separation should be the more 

 perfect as time goes on. The various bodies of rock occurring 

 in a large volcanic region have been erupted at widely different 

 times, and while belonging to a connected period of volcanic 

 activity may often represent the lapse of ages. Their genetic 

 relationship has been the result of some active principle coex- 

 tensive with this vast time, and persistent or intermittent ; the 

 effect in either case must be accumulative. 



It is found in all regions carefully investigated that there is a 

 sequence in the eruption of different varieties of rocks which is 

 most characteristic. From the nature of the causes leading to 

 the extrusion of volcanic lavas, the irregularities of the conduits 

 through which they reach the surface and the probable diversity 

 in the physical conditions obtaining in different regions, it is to 

 be expected that the course of events will not be the same in all 

 cases, or constant in any one instance. Hence the sequence of 

 rocks will not be uniform for all regions, nor will it necessarily be 

 simple in any case. The sequence discovered by von Richthofen, 1 

 when expressed in general terms, is of very wide application, and 

 is to the effect that the earliest eruptions are of rocks having an 

 average or intermediate composition, and that subsequent erup- 

 tions bring to the surface magmas of more and more diverse com- 

 position ; the last eruptions producing the most diverse forms. 

 The transition from a magma of intermediate composition to 

 those of extremely divergent composition, is clearly the result 

 of a process of differentiation. "This correspondence between 

 the petrographical and the geological succession," as Brogger 2 

 remarks, "appears to prove conclusively a genetic connection 

 between successive eruptions." The same conviction has been 

 expressed by Geikie, Teall and others. Evidences of the mixing 



1 F. von Richthofen : The Natural System of Volcanic Rocks, 4to. San Fran- 

 cisco, 1868. 



2 Loc. cit. p. 83. 



