Ch. 5XX.] OF VOLCANIC ROCKS. 521 



The test of age by superposition is strictly applicable to all stratified 

 volcanic tuffs, according to the rules already explained in the case of 

 other sedimentary deposits. (See p. 97.) 



Test of age by organic remains. — "We have seen how, in the vicinity 

 of active volcanos, scoriae, pumice, fine sand, and fragments of rock are 

 thrown up into the air, and then showered down upon the land, or into 

 neighboring lakes or seas. In the tuffs so formed shells, corals, or any 

 other durable organic bodies which may happen to be strewed over the 

 bottom of a lake or sea will be imbedded, and thus continue as permanent 

 memorials of the geological period when the volcanic eruption occurred. 

 Tufaceous strata thus formed in the neighborhood of Vesuvius, Etna, Strom- 

 boli, and other volcanos now active in islands or near the sea, may give 

 information of the relative age of these tuffs at some remote future period 

 when the fires of these mountains are extinguished. By evidence of this 

 kind we can establish a coincidence in age between volcanic rocks, and 

 the different primary, secondary, and tertiary fossiliferous strata. 



The tuffs alluded to may not always be marine, but may include, in 

 some places, freshwater shells ; in others, the bones of terrestrial quad- 

 rupeds. The diversity of organic remains in formations of this nature is 

 perfectly intelligible, if we reflect on the wide dispersion of ejected matter 

 during late eruptions, such as that of the volcano of Coseguina, in the 

 province of Nicaragua, January 19, 1835. Hot cinders and fine scoriae 

 were then cast up to a vast height, and covered the ground as they fell 

 to the depth of more than 10 feet, and for a distance of 8 leagues from 

 the crater in a southerly direction. Birds, cattle, and wild animals were 

 scorched to death in great numbers, and buried in ashes. Some volcanic 

 dust fell at Chiapa, upwards of 1200 miles, not to leeward of the volcano, 

 as might have been anticipated, but to windward, a striking proof of a 

 counter current in the upper region of the atmosphere ; and some on Ja- 

 maica, about 700 miles distant to the northeast. In the sea, also, at the 

 distance of 1100 miles from the point of eruption, Captain Eden of the 

 Conway sailed 40 miles through floating pumice, among which were some 

 pieces of considerable size.* 



Test of age hy mineral composition. — As sediment of homogeneous 

 composition, when discharged from the mouth of a large river, is often 

 deposited simultaneously over a wide space, so a particular kind of lava, 

 flowing from a crater during one eruption, may spread over an extensive 

 area; as in Iceland in 1783, when the melted matter, pouring from 

 Skaptar Jokul, flowed in streams in opposite directions, and caused a 

 continuous mass, the extreme points of which were 90 miles distant from 

 each other. This enormous current of lava varied in thickness from 100 

 feet to 600 feet, and in breadth from that of a narrow river gorge to 15 

 miles.f Now, if such a mass should afterwards be divided into separate 



* Caldcleugli, Phil. Trans. 1836, p. 27. 

 f See Principles, Index, "Skaptar Jokul." 



