ibus hument.
Hic fontes Natura novos emissit, at illuc
Clausit: et antiquis concussa tremoribus orbis
Fulmina prosiliunt...."
--Lib. xv. 262.

[3]
"Nec, quae sulfureis ardet fornacibus, Aetne
Ignea semper erit; neque enim fuit ignea semper.
Nam, sive est animal tellus, et vivit, habetque
Spiramenta locis flammam exhalantia multis;
Spirandi mutare vias, quotiesque movetur,
Has finire potest, illas aperire cavernas:
Sive leves imis venti cohibentur in antris;
Saxaque cum saxis...."
--_Ibid._, 340.

[4] Strabo, lib. vi.

[5] Tacitus, lib. vi. 16, 20.

[6] _Principles of Geology_, 11th edition, vol. i., ch. 3.

[7] 2 vols., Edin. (1795).

[8] Edin. (1802).

[9] A more extended list of early works will be found in Daubeny's
_Volcanoes_ (1848).

[10] 11th edition (1872).

[11] 4th edition (1888).

[12] "The History of Volcanic Action during the Tertiary Period in the
British Isles," _Trans. Roy. Soc., Edin._ Vol. xxxv, (1888).

CHAPTER II.

FORM, STRUCTURE, AND COMPOSITION OF VOLCANIC MOUNTAINS.

The conical form of a volcanic mountain is so generally recognised, that
many persons who have no intelligent acquaintance with geological
phenomena are in the habit of attributing to all mountains having a
conical form, and especially if accompanied by a truncated apex, a
volcanic origin. Yet this is very far from being the fact, as some
varieties of rock, such as quartzite, not unfrequently assume this
shape. Of such we have an example in the case of Errigal, a quartzite
mountain in Donegal, nearly 3000 feet high, which bears a very near
approach in form to a perfect cone or pyramid, and yet is in no way
connected, as regards its origin or structure, with volcanic phenomena.
Another remarkable instance is that of Schehallion in Scotland, also
composed of quartz-rock; and others may be found amongst the ranges of
Islay and Jura, described by Sir A. Geikie.[1]

Notwithstanding, however, such exceptions, which might be greatly
multiplied, the majority of cone-shaped mountains over the globe have a
volcanic origin.[2] The origin of this form in each case is entirely
distinct. In the case of quartzite mountains, the conical form is due to
atmospheric influences acting on a rock of uniform composition,
traversed by numerous joints and fissures crossing each other at obtuse
angles, along which the rock bre