[3] "Nec, quæ sulfureis ardet fornacibus, Ætne 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 breaks up and falls away, so that the sides are always covered by angular shingle forming slopes corresponding to the angle of friction of the rock in question. In the case of a volcanic mountain, however, the same form is due either to accumulation of fragmental material piled around the cup-shaped hollow, or crater, which is usually placed at the apex of the cone, and owing to which it is bluntly terminated, or else to the welling up from beneath of viscous matter in the manner presently to be described.