CHAPTER III. 

 VOLCANIC GEOLOGY. 



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

 lites. — Hornhlendic andcsites. — Trachytes. — Rhyolites. — Basalts. — The order of succession of the 

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

 teaus. — Certain modifications of the order given by Richthofen. — Resolution of the order into two 

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

 montal 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 

 petrographic characters. — Modes of stratification. — They originate from the break up of massive 

 lavas, and a,re chiefly alluvial accumulations. — Motamorphism 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 iEtna 

 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 during 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. Eveiy great group of vol- 



