6lO THE JOURNAL OF GEOLOGY. 



magmas became more and more silicious, and the volume of the 

 lava erupted smaller. But this change in composition was not 

 uninterrupted, for there are evidences of the alternate eruption 

 of basic magmas as well. Dikes of more silicious rocks are trav- 

 ersed by later dikes of basic rocks. This has taken place both 

 within and outside of the core. Some of these basic rocks are 

 uncommonly low in silica for rocks of this region. They are all 

 found at some distance from the core, with one exception, which 

 is an intrusion within the core. They are lamprophyric in the 

 sense used by Professor Rosenbusch, and approach more or less 

 closely typical camptonites, monchiquites, kersantites, and min- 

 ettes. They are connected with the basalts of the district by min- 

 eralogical and structural transitions. 



These exceptionally basic rocks are the chemical complements 

 of the acidic ones in the core and appear to be among the latest 

 extrusions. While they agree with one another in having a low 

 percentage of silica, they differ in the relative abundance of mag- 

 nesia, lime and iron oxide on the one hand, and of alumina, soda 

 and potash on the other. 



As already pointed out by the writer in another place, the 

 variability in composition of all of the volcanic rocks in this vol- 

 cano illustrates one mode of differentiation of a magma at a par- 

 ticular center of eruption. A comparison of the chemical and 

 mineral composition of the rocks of this district furnishes addi- 

 tinal evidence of the fact that magmas which are chemically 

 similar will crystallize into different groups of minerals according 

 to the conditions through which they pass. Thus chemically 

 similar magmas may form basalt under one set of conditions, and 

 gabbro under others ; the first composed of plagioclase, augite, 

 olivine, magnetite and sometimes hypersthene ; the second con- 

 sisting of plagioclase, augite, hypersthene and biotite, besides some 

 magnetite, orthoclase and quartz, with or without hornblende. 

 Minerals, then, which are primarily functions of the chemical 

 composition of rocks are also functions of the physical conditions 

 affecting crystallization. Some of the conditions under which 

 the molten magmas solidified within the dikes and core of the 



