ALLUVIUM. KECENT FORMATIONS. 



779 



tricts the stems and leaves of the frailest plants, exposed to the action of the waters, 

 become incrusted with silex, and may be seen in various stages of petrification, slightly 

 covered with the deposit, or completely permeated by it, and hardened into stone. 



Bituminous matters, in the form of naphtha, the most limpid of the bitumens ; of petro 

 leum, or rock oil, a thicker liquid ; of mineral pitch, which shows consolidation ; and of 

 asphalte, in which the substance is so far solidified as to become brittle, are plentifully 

 exuded from the surface cf the earth in various countries, in the Birman Empire, around 

 the Caspian Sea, in Palestine, Greece, Italy, and the United States, either in an insulated 

 state, or impregnating springs and pools. The various bitumens are supposed to be pro 

 duced from vegetable matter buried in the earth, by the processes by which vegetable 

 substances are ultimately converted into coal, the force of internal heat driving the pro 

 duct to the surface, which it reaches through chinks and fissures in the strata. The great 

 abundance of bituminous matter now in course of production, and the manner in which 

 it becomes incorporated with layers of soil, are highly interesting points to the geologist, 

 because they enable him to explain the bituminous character of many of the older rocks 

 in which no appearance of vegetable structure can be detected. 



6. Volcanic lava ; obsidian ; pumice ; mud, scoriae, and ashes, with gaseous pro 

 ducts. 



The points of volcanic eruption at present in action, or of which we have any record 



of action during the historic pe 

 riod, are few in number when com 

 pared with those conical masses 

 whose crateriform summits and 

 composition proclaim their igne 

 ous origin and former office. The 

 annexed chart represents one of 

 these ancient causes of geological 

 change, an extinct volcano in the 

 isle of Palma, one of the Canaries, 

 a group of islands all of which 

 are of volcanic formation. It is 

 impossible to define the period 

 that has elapsed since the era of 

 repose commenced in relation to 

 these ancient furnaces ; but some 

 may be supposed to have been 

 active during the age of man ; 

 while in the case of Etna we have 

 an example of one of unimpaired 

 energy, whose fires were burning 

 long before he appeared upon the 

 earth, or during the tertiary 

 epoch. There are a few volcanic 

 vents which have been constantly 

 erupting since their phenomena 

 have been chronicled, always con 

 taining lava in a state of ebul 

 lition, and emitting gaseous exha- 



Chart of the Isle of Palma, showing the Crater. lationS. StTOmboli, in the Lipari 



Isles, is one of this class, of which we have accounts which go back two thousand years; 



