356 



EXCAVATION OF ROCKS BY RUNNING WATER. [Ch. XV. 



ascribed instability to the earth's surface, and represented the 

 continents which we inhabit as the theatre of incessant chano* 



e 



and movement, his antagonists, who regarded them as un- 

 alterable, assailed him in a similar manner with accusations 

 founded on religious prejudices.* We might appeal to the 

 excavating power of the Anio as corroborative of one of the 

 most controverted parts of the Huttonian theory ; and if the 

 days of omens had not gone by, the geologists who now wor- 

 ship Yesta might regard the late catastrophe as portentous. 

 We may, at least, recommend the modern votaries of the 



goddess to 



lose no time in 



making 



a pilgrimage to her 





shrine, for the next flood may not respect the temple. 



Excavation of rocks by running water. — The rapidity with 

 which even the smallest streams hollow out deep channels in 

 soft and destructible soils is remarkably exemplified in vol- 

 canic countries, where the sand and half-consolidated tuffs 

 oppose but a slight resistance to the torrents which descend 

 the mountain side. After the heavy rains which followed the 

 eruption of Vesuvius in 1824, the water flowing from the Atrio 

 del Cavallo cut, in three days, a new chasm through strata of 

 tuff and ejected volcanic matter, to the depth of twenty-five 

 feet. I found the old mule-road, in 1828, intersected by this 

 new ravine. 



But deep chasms may be gradually eroded through the 

 hardest rock, by running water, charged with foreign matter. 

 Good illustrations of this phenomenon may be seen in many 

 valleys in Central France where the channels of rivers have 

 been barred up by solid currents of lava, through which the 



streams have re-excavated a passage, often of great width 



and from twenty to seventy feet in depth. In these cases 

 there are decisive proofs that neither the sea, nor any de- 

 nuding wave or extraordinary body of water, has passed over 



the spot since the melted lava was consolidated. 



Every 



hypothesis of the intervention of sudden and violent agency 

 is entirely excluded, because the cones of loose scorise, out of 

 which the lavas flowed, are oftentimes at no great elevation 

 above the rivers, and have remained undisturbed during the 





* Illustr. of Hutt. Theory, § 3, p. 147 



