CHAPTEE III. 



VOLCANIC GEOLOGY. 



A region of extinct volcanism.— Initial epochs.— Tufas.— The most ancient eruptive rocks.— Prox)y- 

 lites.- Hornblendic andesites.- Trachytes.— Ehyolites.- Basalts.— The order of succession of tho 

 eruptions.- Eichthofen's generalization sustained by the succession presented by the Iligli Pla- 

 teaus. —Certain modifications of the order given by EichtLofen.— Kesolutiou of tho order into two 

 semi-series.- Fnignicntal volcanic rocks.— Their great extent and mass.- Two classes of frag- 

 moutal deposits.— Tufas.— Considerations as to their origin and mode of accumulation.- They 

 are the detritus of more ancient lavas.— Their age.— Volcanic conglomerates.— Their texture and 

 petrogr.aphic characters.- Modes of siratilicatiou.— They originate from the break up of massive 

 lavas, and are chiefly alluvial accumulatious.— Metaniorphism of the clastic volcanic strata. 



The District of the High Plateaus is a region of extinct volcanism. 

 The magnitude of the eruptions which have taken place there is small com- 

 pared with what we know of some other regions, but it is great when com- 

 pared with what we may see in most of the volcanic districts of Europe. 

 It is smaller, I presume, than that of Iceland, but greater than that of J^tna 

 or Central France. It is not the magnitude, however, which is so very 

 striking or suggestive, but the variety of the phenomena and the great 

 stretch of geological time through which their history ranges. The oldest 

 eruptions go back to the middle Eocene; the latest cannot be as old as the 

 Christian era. It is hard to believe that they are as old as the conquest of 

 Mexico by Cortez. Between the opening and cessation of that activity 

 (if, indeed, it has even yet ceased forever) the eruptions have been inter- 

 mittent. There have been long periods of repose, but daring the pauses 

 the subterranean forces were only gathering strength and material for fresh 

 outbreaks. 



The highest interest in the region lies in the remarkable variety of the 

 phenomena presented It lacks but little of being a complete category of 

 volcanology, and what it lacks it compensates by presenting something new. 

 Nearly every form of eruption is exhibited. Every great group of vol- 



