GEOGNOSY AND GEOLOGY. O27 
senting the Iceland voleanic region. In the south of Iceland is the high 
cone of Hecla, and the snow and ice-covered volcano of Eyafiél. Of this vol- 
cano as of the island of Westmann, in front of it, a view is presented in pi. 44, 
fig. 18. They lie on the southern exit of a wide valley, which is continued 
between trachytic masses. Northwardly this valley runs towards a group of 
voleanoes constituted by Krabla, Leirhnukur, and others, while the elevated 
Oriifejdkul to the east rears its proud head towards the sky. 
A phenomenon, sometimes called an aerial volcano, is not unfrequently 
found to accompany earthquakes. This is constituted by a small cone of 
eruption, consisting of accumulated mud masses, often impregnated with 
saline waters. From the mouth is emitted gaseous matter, generally 
hydrogen, which is alternately inflamed and extinguished. The air 
voleanoes of Turbaco in Columbia (pf 50, fig. 3), are very conspicuous in 
this respect. 
Leopold von Buch makes a distinction between volcanic centres and vol- 
canic lmes. The first kind consist of a central volcano surrounded hy several] 
smaller ones, which are pretty equally distributed in every direction. Mount 
Ktna, in Sicily, is a central volcano, with its smaller cones of eruption 
arranged about its base. In the view of Etna on pil. 45, fig. 6, a, indicates 
Montagnuola; 6, Torre del Filosofo; c, the highest point of the mountain ; 
d, Lepra; e, Finocchio; f, Capra; g, the cone of 1811; h, the Cima del 
Asino; i, Musara; k, Zoccolara; J, Rocca de Calanna. On the volcanic 
chart of the same region (fig. 9), 1 indicates the volcanic formation, 2 the 
newer pliocene, and 38 the latter formation combined with the former. Le. 7 
represents the Campi Phlegrei; a, Montenuovo; b, Monte Barbaro; ¢, Sol- 
fatara ; d, Lake Lucrine; e, Lake Averno; f, the city of Pozzuoli; and g, the 
tongue of land Baja. 
Linear volcanic series are seen in high development on the elevated crest 
of the Cordilleras de los Andes, extending over a line of many hundred 
miles, with individulal cones succeeding each other at greater or less 
intervals. 
Various hypotheses with regard to voleanoes have been propounded by 
the earlier geologists, few of which are now considered to exhibit any,show 
of probability. One of the least objectionable of modern theories is that of 
Housmann, who considers lavas to be nothing else than products of 
oxydation of bodies previously unoxydized, which exist at the confines 
between the molten nucleus of the earth andthe hard crust. That the 
interior of the earth must consist of denser masses than the exterior, is 
sufficiently evinced by the fact, that the mean density of the entire earth 
amounts to 4.70 (5.50 according to Cavendish), and that of the outer crust 
to but 3:00. When these unoxydized bodies, which consist principally of 
potassium, sodium, aluminum, silicon, iron, &c., come into contact with 
water, this is decomposed, and oxydes are formed with the evolution of 
great quantities of heat and hydrogen; the latter, mixed with oxygen of the 
air, produces the explosions. The sea-water penetrating at a great depth, 
appears to be the principal source of the water required; a fact well 
illustrated, by the situation of volcanoes within a moderate distance of 
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