| OCR Text |
Show CHAPTER III. VOLCANIC GEOLOGY. A region of extinct volcanism.â€"Initial epochs.â€"Tufas.â€"The most ancient eruptive rocks.â€"Propy-lites.â€"Hornblendic andesites.â€"Trachytes.â€"Ehyolites.â€"Basalts.â€"The order of succession of the eruptions.â€"Richthofen's generalization sustained by the succession presented by the High Plateaus.â€"Certain modifications of the order given by Richthofen.â€"Resolution of the order into two semi-series.â€"Fragmental volcanic rocks.â€"Their great extent and mass.â€"Two classes of frag-mental deposits.â€"Tufas.â€"Considerations as to their origin and mode of accumulation.â€"They are the detritus of more ancient lavas.â€"Their age.â€"Volcanic conglomerates.â€"Their texture and petrographic characters.â€"Modes of stratification.â€"They originate from the break up of massive lavas, and a,re chiefly alluvial accumulations.â€"Metamorphism 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 compared with what we know of some other regions, but it is great when compared 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 JEtna 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 intermittent. 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. Every great group of vol- 55 |
