826 THE WONDERS OF GEOLOGY. Lect. VIII. 



doxical fact, by supposing that a deep mass of drift snow 

 was covered by a stream of volcanic sand, which is an 

 extremely bad conductor of heat ; and thus the subsequent 

 liquid lava might have flowed over the whole, without 

 affecting the ice beneath, which at such a height (ten 

 thousand feet above the level of the sea) would endure as 

 long as the snows of Mont Blanc, unless melted by volcanic 

 heat from below. 



13. Organic Remains in Lava. — The siliceous shields, 

 or cases of Infusoria, are often found as a component part 

 of volcanic ash and tuff, both of ancient and modern origin, 

 and were probably derived from the subterranean pools or 

 lakes ; as in the case of the showers of fishes which occa- 

 sionally descend during a volcanic eruption.* Infusorial 

 shields are not uncommon in the volcanic dust that falls on 

 vessels, often hundreds of miles from land. An ancient bed 

 of tuff in Oregon is fall of infusorial remains.f 



In the tuff of Vesuvius, I have seen the impressions of 

 dicotyledonous leaves ; and charred wood is occasionally met 

 with in the scoriae of Herculaneum. 



A curious circumstance occasionally results from the 

 invasion of a grove or forest by a stream of lava. The 

 trunks of the trees, at their base, become enveloped by the 

 molten mass, but the upper part and the branches are set on 

 fire, and burn down to the surface. The trunks surrounded 

 by the lava are only charred, and if, as often happens, this 

 carbonaceous matter is washed away, or otherwise removed, 

 hollow cylindrical tubes, having their sides marked with the 

 imprint of the bark of the trunks, remain in the solid rock. 

 Such moulds are not uncommon in the Isle of Bourbon, in 

 those lava currents that have extended their ravages through 

 the palm-forests. 



* Humboldt's Cosmos, p. 222. A putrid fever prevailed in 1691, 

 in Ibarra, north of Quito, from the quantity of dead fish ejected from 

 the volcano of Jmbaburu. f Dr. Bailey. 



