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GENETIC RELATIONSHIPS AMONG IGNEOUS ROCKS.
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accumulative. Hence, if the process is one of synthesis or comingling, 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 occuring 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 coëxistensive 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 eruptions bring to the surface magmas of more and more diverse composition; 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 Brögger[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 Francisco, 1868.
  2. ↑ Loc. cit. p. 83.
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