140 



Prof. J. Prestwich. 



crater of Frumento, which extended in a direction away from the 

 central cone for a distance of 1^ mile. Scrope says that in nearly 

 every lateral eruption of Etna, the production of such a fissure has 

 been observed. Similar instances are not wanting in Vesuvius. In 

 1738 a fissure crossed the whole island of Lancerote; while in the 

 great eruption of Hecla of 1783 the fissure which was then formed 

 was supposed to extend not less than 100 miles in length.* 



Fig. 2. — Diagram plan of a volcano, showing the radial lines of fissures or dykes. 



Scrope further remarks that "the rents thus produced in the frame 

 of a volcanic mountain are sometimes of such a size as to cleave its 

 whole mass in two. This occurred in the volcano of Inachian, one of 

 the Moluccas, in 1646. The crater of the Soufriere of Montserrat, and 

 the volcanic cone of Guadaloupe both appear to have been thus split 

 through. So also the Montagne Pelee of Martinique." Piazzi 

 Smyth states that the cinder beds surroundiug the summit of Tene- 

 riffe are traversed by dykes proceeding in radial lines from the Peak.f 

 Phillips describes some of the dykes and fissures of Vesuvius, and gives 

 a section showing the relative position of the ejected debris and lava 

 beds of this mountain, and of the dykes radiating from the central 

 core. J 



These dykes, like other masses of lava, have generally become 

 fissured in cooling, and the interstices thus formed serve as water 

 channels, not only for the rain which falls upon them, or to small 

 springs high up the mountain as in Teneriffe,§ but, what is more im- 

 portant, they may serve as channels of drainage to the water-bearing 

 scoriaceous and lava beds which they intersect. For it will be under- 

 stood from the preceding account that these latter beds may often 



* " Volcanos," 2nd Edit., 1862, pp. 161—163. 

 f " Teneriffe," p. 80. 



t " Vesuvius," pp. 132 and 191, and PI. VI. 

 " Teneriffe," p. 86. 



