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in other cases, because the lava has welled out in dome form without 
_ the production of scoriz. Mount Ararat, for example, is said to 
have no crater; but so late as the year 1840 a fissure opened on 
its side whence a considerable eruption took 
place. 
Though the interior of modern volcanic 
cones can be at the best but very partially 
examined, the study of the sites of long- 
extinct cones laid bare after denudationshows == 
that subsidence of the ground has commonly 
taken place at and round a vent. Evidence 
of subsidence has also been observed at some 
modern volcanoes (ante p. 232). Theoretically 
two causes may be assigned for this structure. Fic. 48.—PLan or Votca- 
In the first place the mere piling up of a Socetsve Cuavuns 
huge mass of material round a given centre 
tends to press down the rock underneath, as some railway embank- 
ments may be observed to have done. ‘This pressure must often 
amount to several hundred tons on the square foot. In the second 
place the expulsion of volcanic material to the surface must leave 
cavities underneath imto which the overlying crust will naturally 
gravitate. These two causes combined, as suggested by Mr. Mallet, 
afford a probable explanation of the saucer-shaped depressions in 
which many ancient and some modern vents appear to lie.* 
The following are the more important types of volcanic cones :?— 
1. Cones of Non-voleanic Materials.—These are due to the 
discharge of steam or other aeriform product through the solid 
crust without the emission of any true ashes or lava. ‘The materials 
ejected from the cavity are wholly, or almost whoily, parts of the 
surrounding rocks through which the volcanic pipe has been drilled. 
Some of the cones surrounding the crater-lakes (maare) of the Eifel 
consist chiefly of fragments of the underlying Devonian slates. 
2. Tuff-Cones, Cinder-Cones.—Successive eruptions of fine 
_ dust and stones, often rendered pasty by mixture with the water so 
- copiously condensed during an eruption, form a cone in which the 
materials are solidified by pressure into tuff. Cones made up only 
of loose cinders, like Monte Nuovo in the Bay of Baie, often arise 
on the flanks or round the roots of a great volcano, as happens to a 
small extent on Vesuvius, and on a larger scale upon Etna. They 
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1 Mallet, Q. J. Geol. Soc. xxxiii. p. 740. See also the account of “‘ Voleanic Necks,” 
in Book IV. Part vii. 
2 Von Seebach (Z. Deutsch. Geol. Ges. xviii. 644) distinguished two volcanic types. 
ist, Bedded Volcanoes (Strato-Vulkane), composed of successive sheets of lavas and tuffs, 
and embracing the great majority of volcanoes. 2nd, Dome Volcanoes, forming hills 
composed of homogeneous protrusions of lava, with little or no accompanying fragmentary 
discharges, without craters or chimneys, or at least with only minor examples of these 
volcanic features. He believed that the same volcano might at different periods in its 
history belong to one or other of these types—the determining eause being the nature 
of the erupted lava, which, in the case of the dome voleanoes, is less fusible and more 
viscid than in that of the bedded volcanoes. 
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