42 Messrs Lydl and Murchison on the 



Maurice, from 400 to 500 feet below the old alluvium and tu- 

 faceous breccia. At Monton, the depth of the valley below the 

 tufaceous breccia is probably still greater ; but here the high 

 dip of the tertiary strata, and the proximity of basaltic dikes in 

 the hill of Vayres, leave us in doubt how far volcanic disturb- 

 ance may have co-operated with aqueous action. Now, we may 

 demand. Is it reasonable to expect, that, if the present lakes of 

 Aidat and Chambon were filled up, and their lacustrine deposites 

 subsequently removed to as great an extent as those of Perrier 

 and Monton, or as nearly annihilated as that of St Maurice, — 

 or, if the valleys of the Couze de Chambon and Vayres had 

 sunk below their present depth from 100 to 500 feet, — can it 

 be expected that future geologists would stiD be able to trace 

 the former outline of these lakes, or point out the position of 

 their barriers .'' But it should be remembered, that there are 

 other causes besides lava-streams, which frequently dam up the 

 course of rivers in regions within the range of volcanic agency. 

 The last great earthquakes in Sicily and Calabria, in 1783, 

 gave rise to land-falls, wherein huge masses, more than a mile 

 in length, bordering the coast or rivers, were suddenly precipi- 

 tated into the adjoining sea or valley. In such cases, the bar- 

 rier may last till the lake is filled up ; but it will afterwards be 

 more destructible, and will more readily admit of being entirely 

 obliterated, than a lava-current. Two large lakes were thus 

 occasioned in Calabria, the course of two rivers having been 

 obstructed. In the case of Mont Perrier we are aware that 

 several geologists consider a barrier wholly unnecessary. They 

 endeavour to account for the phenomena in the same manner as 

 Signor Lippi explained the successive alluviums, which certainly 

 constitute a large portion of the covering under which Hercula- 

 neum is buried. The matter, it is said, washed down from a 

 neighbouring volcano, is so copious, that rivers cannot remove 

 what the floods carry down. To such an hypothesis, founded at 

 least on plausible grounds, we reply, that so enormous a thick- 

 ness of transported materials, severally distinct in their charac- 

 ter, as are seen at Mont Perrier, could never have been lodged 

 in so narrow a valley, if there had not been a permanent stop- 

 page; for the alternating beds of rounded pebbles could not 

 have been brought down by a river or flood, without the hollow- 



