4 ABOUT VOLCANOS AND EARTHQUAKES. 



was struck upwards from below with such force as to un* 

 ship and spHt up the main-mast. 



(50.) Evidences of a similar sudden and upward ex- 

 plosive action are of frequent occurrence among the 

 extinct volcanos of Auvergne and the Vivarais, where in 

 many instances the perforation of the granitic beds which 

 form the basis or substratum of the whole country ap- 

 pears to have been effected at a single blow, accom- 

 panied with little evidence of disturbance of the sur- 

 rounding rocks much in the same way as a bullet will 

 pass through a pane of glass without starring or shatter- 

 ing it. In such cases it would seem as if water in a 

 liquid state had suddenly been let in through a fissure 

 upon a most intensely heated and molten mass beneath, 

 producing a violent but local explosion, so instantaneous 

 as to break its way through the overlying rocks, without 

 allowing time for them to bend or crumple, and so dis- 

 place the surrounding masses. 



(51.) The same kind of upward bounding movement 

 took place at Riobamba in Quito in the great earth- 

 quake of February 4, 1797, which was connected with 

 an eruption of the volcano of Tunguragua. That earth- 

 quake extended in its greatest intensity over an oval 

 space of 120 miles from south to north, and 60 from 

 east to west, within which space every town and village 

 was levelled with the ground ; but the total extent of 

 surface shaken was upwards of 500 miles in one direc- 

 tion (from Puna to Popayan), and 400 in the other. 

 Quero, Riobamba, and several other towns, were buried 

 under fallen mountains, and in a very few minutes 



