THE OBSERVED ORDER OF SUCCESSION OF ERUPTIONS. 



67 



properties of the rocks. This inquiry immediately presents itself the instant 

 we settle upon the conviction that eruptions have an assignable order of 

 occurrence, and the mind at once springs to the conclusion that there ought 

 to be such an association. If there be an order of eruption, there must be 

 a cause for it, and for that cause we look to the properties of the rocks 

 themselves. But at first glance no such correlation appears. If we arrange 

 them in a series expressing the great groups in the order of their chemical 

 constitution, and place in juxtaposition an arrangement according to the 

 order of eruption, we fail to find at first a clear correlation. Taking Richt- 

 hofen's five orders, we have the following comparison : 



Arrangement by chemical constitution. 



1. Rliyolite. 



2. Trachyte. 



3. Propylite. 



4. Andesite. 



5. Basalt. 



Arrangement by order of eruption. 



1. Propylite. 



2. Andesite. 



3. Trachyte. 



4. Rhyolite. 



5. Basalt. 



With chemical constitution go the other properties, mineral constitu- 

 ents and specific gravity. No relation here presents itself to the order of 

 eruption. Yet I think that upon closer inspection a systematic correlation 

 may be made to appear by an examination of the sub-groups instead of the 

 great groups, and the correlation of the sub-groups will reflect itself in the 

 great groups. Taking the more important sub-groups, those which are most 

 persistent in their characters, of most frequent occurrence, and of the larg- 

 est volume, the following succession of eruptions presents itself in the 

 High Plateaus :* 



1. Hornblendic propylite. 



2. Hornblendic andesite. 



3. Hornblendic and augitic trachytes (less acid trachytes). 



4. Augitic andesite (Richthofen). 



5. Sanidin trachyte (more acid trachytes). 



6. Liparite. 



7. Dolerite. 



8. Rhyolite (proper). 



9. Basalt (proper). 



* For classification and exact meaning of terms here employed see nest chapter. 



