Volcanos. 1 23 



bility to the mass, in the direction of their longest axes. The 

 crystals of all lavas, indeed, are supposed hy our author, when 

 in motion, rather to slide or slip past one another by means of the 

 intervention of a small quantity of fluid between their flat sur- 

 faces, than roll over one another, as is probably the case with 

 the globular particles of perfect fluids. This would naturally 

 result from their peculiar kind of fluidity, and also explains the 

 extreme difficulty with which lavas in motion are induced to 

 swerve from the direction they have once taken. The smallest 

 obstacle is sufficient to check their progress for some time, and 

 even to consolidate the lava to some distance back from the ob- 

 stacle. These solidified parts, when again broken up by the in- 

 creased impetus of the lava behind, occasion the brecciated 

 character of some lavas, where angular fragments are enveloped 

 in a paste of the same material. The zoned and ribboned struct- 

 ure of pearl-stones is similarly accounted for. This sluggishness 

 of lava currents occasions great accumulations of the substance 

 on those points where its motion was checked and diverted, as 

 in the angles of water courses, &c. ; and examples are given from 

 the Vivarais, where huge patches of columnar basalt occupy the 

 concave elbows of the gorges of the granite mountains, the con- 

 necting strips being shallow, or having altogether disappeared. 

 The curious procedure of lava, when it meets with a perpendi- 

 cular obstacle, such as a wall, which it cascades over without 

 touching it, is then noticed and explained ; as also the arched 

 gutters and caverns often formed from its subsidence ; and its ef- 

 fect on grass, trees, and fragments of other rocks ; on marshy 

 ground, and when it enters the sea or any body of water. Its 

 progress below the water is shown to be similar to that on dry 

 land, though slower, with the same degree of fluidity. The 

 water is heated and discoloured by it, and fish often killed in 

 numbers. The fossils of Monte Bolca are attributed by our au- 

 thor to such a catastrophe, since the beds in which they occur 

 are topped by basaltand volcanic calcareous conglomerate." 



The display of electrical phenomena during volcanic erup- 

 tions is often very brilliant ; Mr. Scrope remarks that this 

 was the fact with the eruption of October, 1822. "From 

 every part of the immense cloud of ashes which hung sus- 

 pended over the mountain, flashes of forked lightning darted 

 continually. They proceeded in greatest numbers from the 

 edges of the cloud. They did not consist as in the case of 

 a thunderstorm, of a single zigzag streak of light; but a 

 great many corruscations of this kind appeared suddenly to 

 dart in many directions from a central point, forming a 



