814 



THE WONDERS OF GEOLOGY. Lect. VIII. 



or by the expansion of elastic vapours. A dome or cone 

 is thus produced, with a central opening, around which 

 the uplifted strata are concentrically arranged ; being 

 covered to a greater or less extent by the materials poured 

 out by subsequent eruptions. These are termed by M. Yon 

 Buch, Craters of Elevation, The structure of such vol- 

 canic mountains will be readily understood by referring to 

 the sections of the Wren's Nest {ante, p. 771), and of C rich 

 Hill, near Matlock {ante, p. 685), both of which are 

 examples of originally horizontal strata elevated into a 

 dome by a protrusion of volcanic matter. If in either of 

 these instances the upheaving force had been sufficient to 

 propel the trap through the middle of the dome, a crater of 

 elevation would have been formed, through which the 

 igneous matter would have escaped. It is by a movement 

 of this kind, as we have already had occasion to explain, 

 that valleys of elevation have been produced {ante, p. 779). 

 Mr. Scrope's observations on the structure of Vesuvius 

 will serve to explain the formation of craters of eruption: — 



" Vesuvius is an exceedingly regular mountain on a small scale. All 

 the visible lavas, and the greater part of the conglomerates, are basaltic ; 

 and, owing to the great fluidity of lavas of this mineral character, they 

 have, when produced from the common vent, taken their course in spread- 

 ing sheets down the outer slope of the mountain ; while the scoriae and 

 fragmentary substances, projected at the same time into the air, were 

 spread pretty evenly over them ; so that the result of successive erup- 

 tions of this kind, has been the formation of a regularly conical moun- 

 tain, with a gradually diminishing slope on all sides, from the central 

 heights to the plain around ; exhibiting in the ravines that furrow its 

 sides, as well as in the abrupt sections afforded by the walls of the 

 great crater, its composition of repeatedly alternating beds of basalt 

 and basaltic conglomerates, more or less irregular in thickness, but 

 dipping uniformly on all sides away from the vent, with an inclination 

 corresponding exactly to the external slopes of the mountain. 



" The eruptions of Vesuvius seem very rarely to have taken place 

 from any other than the central vent ; a few small cones immediately 

 above Torre del Greco, thrown up in 1794, and the cone on which the 

 Camaldoli della Torre is built, are the only indications of explosions 



