- 
208 DYNAMICAL GEOLOGY. _ [Boox III. — 
Naples has found about 100 species of marine shells of living species 
in the old tuffs of Vesuvius. Marine shells have been picked up 
within the crater of Monte Nuovo, and have been frequently observed 
in the old or marine tuff of that district. Showers of ash, or sheets 
of voleanic mud often preserve land-shells, insects, and vegetation 
living on the area at the time. The older tuffs of Vesuvius have 
yielded many remains of the shrubs and trees which at successive 
periods have clothed the flanks of the mountain. Fragments of 
coniferous wood which once grew on the tuff-cones of Carboniferous | 
age in central Scotland are abundant in the “necks” of that region.1 
§ 2. Volcanic Action. 
Volcanic action may be either constant or periodic. Stromboli, in 
the Mediterranean, so far as we know, has been uninterruptedly 
emitting hot stones and steam, from a basin of molten lava, since the 
earliest period of history. Among the Moluccas the volcano Sioa, 
and in the Friendly Islands that of Tofua, have never ceased to be 
in eruption since their first discovery. The lofty cone of Sangay, — 
among the Andes of Quito, is always giving off hot vapours; 
Cotopaxi, too, is ever constantly active. But, though examples of 
unceasing action may thus be cited from widely different quarters of 
the globe, they are nevertheless exceptional. The general rule is 
that a volcano breaks out from time to time with varying vigour, and 
after longer or shorter intervals of quiescence. 
Active, Dormant, and Extinct Phases.—lIt is usual to class 
volcanoes as active, dormant, and extinct. ‘This arrangement, however, — 
often presents considerable difficulty in its application. An active 
voleano cannot of course be mistaken, for even when not in eruption 
it shows by its discharge of steam and hot vapours that it might 
break out into activity at any moment. But in many cases it is — 
impossible to decide whether a voleano should be called extinct or 
only dormant. ‘The volcanoes of Silurian age in Wales, of Carboni- 
ferous age in Ireland, of Permian age in the Harz, of Miocene age 
in the Hebrides, of younger ‘Tertiary age in the western States and 
Territories of North America, are certainly all extinct. But the 
Miocene voleanoes of Iceland are still represented there by Skaptar- 
Jokull, Hecla, and their neighbours. Somma, in the first century of 
the Christian era, would have been naturally regarded as an extinct 
volcano, Its fires had never been known to have been kindled: 
its vast crater was a wilderness of wild vines and brushwood, 
haunted, no doubt by wolf and wild boar. Yet in a few days, 
in the autumn of the year 79, the half of the crater walls was 
blown out by a terrific series of explosions, the present Vesuvius was 
then formed within the limits of the earlier crater, and since that 
time voleanic action has been intermittently exhibited up to the 
present day. Some of the intervals of quietude, however, have been 
' Tian. Koy. Soe, Edin, xxix. p. 470; postea, Book IV, section vii. § i. 4, 

