Kf 



NH 



THE 



JOURNAL OF GEOLOGY 



JULY— AUGUST, igoo 



IGNEOUS ROCK-SERIES AND MIXED IGNEOUS 



ROCKS 



«,# I. IGNEOUS ROCK-SERIES 



By an igneous rock-series we may understand an assemblage 

 of rock-types, differing perhaps widely but still with a certain 

 community of characters, associated in the same district and 

 belonging to the same suite of eruptions, and further, holding a 

 similar position in the scheme of igneous rocks belonging to 

 that suite. Adopting the differentiation hypothesis, we may 

 conceive them as derivatives of the same order from one com- 

 mon source, resulting from differentiation along similar lines and 

 to the same degree. The fundamental characteristics of such a 

 series, having regard to chemical composition, are of two kinds: 

 (i) those belonging to the individual rock-types and shared by 

 all the types included in the series [e. g., each member is rich in 

 some particular constituent, as compared with average igneous 

 rocks of like silica-percentage) ; and (2) those belonging to the 

 assemblage of types as a whole, depending upon variations in 

 the composition of the members as compared with one another 

 {e. g., a particular constituent may in one rock-series fall off 

 steadily with increasing silica-percentage, in another series it 

 may rise to a maximum and then decline) . These character- 

 istics, and especially those which fall under the second head, 

 Vol. VIII, No. 5. 389 



£ 



