el) a! besos. 
~ Parr Sxcr.i.§ 2] VOLCANIC ACTION. 209 
so considerable that the mountain might then again have been claimed 
as an extinct volcano.’ Thus, in the 131 years between 1500 and 
1631, so completely had eruptions ceased that the crater had once 
more become choked with copsewood. A few pools and springs of 
very salt and hot water remained as memorials of the former condition 
of the mountain. But this period of quiescence closed with the 
eruption of 1631,—the most powerful of all the known explosions of 
Vesuvius, except the great one of 79. In the island of Ischia, Mont’ 
Epomeo was last in eruption in the year 1302, its previous outburst 
haying taken place, it is believed, about 17 centuries before that date. 
From the craters of the Hitel, Auvergne, the Vivarais, and central 
Italy, though many of them look as if they had only recently been 
formed, no eruption has been known to come during the times of 
human history or tradition. In the west of North America, from 
Arizona to Oregon, numerous stupendous volcanic cones occur, but 
even from the most perfect and fresh of them nothing but steam 
and hot vapours have yet been known to proceed. But the existence 
there of hot springs and geysers testifies to the continued existence 
of one phase of volcanic action. 
In short, no real distinction can be drawn between dormant and 
extinct voleanoes. Volcanic action is apt to show itself again and 
again, even at vast intervals within the same regions and over the 
same sites. The dormant or waning condition of a voleano, when only 
steam and various gases and sublimates are given off, is sometimes 
called the Solfatara phase, from the well-known dormant erater of 
that name near Naples. 
Sites of Voleanie Action.—Volcanoes may break through any 
geological formation. In Auvergne, in the Miocene period, they 
burst through the granitic and gneissose plateau of central France. 
In Lower Old Red Sandstone times they pierced contorted Silurian 
rocks in central Scotland. In late Tertiary and post-Tertiary ages they 
found their way through recent soft marine strata, and formed the huge 
piles of Etna Somma and Vesuvius; while in North America, during 
the same cycle of geological time, they flooded with lava and tuff 
many of the river courses, valleys, and lakes of Nevada, Utah, 
Wyoming, Idaho and adjacent territories. On the banks of the 
Rhine, at Bonn and elsewhere, they have penetrated some of the 
older alluvia of that river. In many instances, also, newer voleanoes 
have appeared on the sites of older ones. In Scotland the Carboni- 
ferous voleanoes have risen on the ruins of those of the Old Red 
Sandstone, those of the Permian period have broken out among the 
earlier Carboniferous eruptions, while the Miocene lavas have been 
injected into all these older volcanic masses. The newer puys of 
Auvergne were sometimes erupted through much older and already 
greatly denuded basalt-streams. Somma and Vesuvius have risen 
out of the great Neapolitan plain of older marine tuff, while in central 
Italy newer cones have been thrown up upon the great Roman plain 
of more ancient volcanic debris. The vast Snake River lava-fields of 
P 
