122 ANIMALS DESTROYED SUDDENLY. 
lation of certain strata, accompanied by the 
sudden destruction, not only of testacea, but also 
of the higher classes of the then existing inhabi- 
tants of the seas. We have analogous instances 
of sudden destruction operating locally at the 
present time, in the case of fishes that perish from 
an excessive admixture of mud with the water of 
the sea, during extraordinary tempests ; and also 
from the sudden imparting of heat, and noxious 
gases, to water in immediate contact with the 
site of submarine volcanoes. A sudden irrup- 
tion of salt water into lakes or estuaries, pre- 
viously occupied by fresh water, or the sudden 
occupation of a portion of the sea, by an 
immense body of freshwater from a bursting 
lake, or unusual land flood, is often fatal to large 
numbers of the inhabitants of the waters thus 
respectively interchanged.* 
The greater number of fossil fishes present no 
appearance of having perished by mechanical 
violence ; they seem rather to have been des- 
troyed by some noxious qualities imparted to 
the waters in which they moved ; either by 
sudden change of temperature,! or an admix- 
* See account of tlie effects of an irruption of the sea into the 
freshwater of the lake of Lowestoffe, on the coast of Suffolk. 
Edinburgh Philosophical Journal, No. 25, p. 372. 
t M. Agassiz has observed that a sudden depression to the 
amount of 15" of the temperature of the water in the river Glat, 
which falls into the lake of Zurich, caused the immediate death 
of thousands of Barbel. 
