274 



LYELL'S ELEMENTS OF GEOLOGY. 



Tests of Relative Age of Volcanic Rocks. 



they fell to the depth of more than ten feet, and for a distance 

 of eight leagues from the crater in a southerly direction. Birds, 

 cattle, and wild animals were scorched to death in great num- 

 bers, and buried in these ashes. Some volcanic dust fell at 

 Chiapa, upwards of 1200 miles to windward of the volcano, a 

 striking proof of a counter-current in the upper region of the 

 atmosphere, and some on Jamaica, about 700 miles distant to 

 the north-east. 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 w^ere some pieces 

 of considerable size.* 



Test of age by mineral composition. — As sediment of homo- 

 geneous composition, \vhen 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 erup- 

 tion, 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 fragments by denudation, we might still 

 perhaps identify the detached portions by their similarity in 

 mineral composition. Nevertheless, this test will not always 

 avail the geologist, for, although there is usually a prevailing 

 character in lava emitted during the same eruption, and even in 

 the successive currents flowing from the same volcano, still, in 

 many cases, the different parts even of one lava-stream, or, as 

 before stated, of one continuous mass of trap, vary so much in 

 mineral composition and texture, as to render these characters 

 of minor importance when compared to their value in the chro- 

 nology of the fossiliferous rocks. 



It will, however, be seen in the description which follows, of 

 the European trap rocks of different ages, that they had often a 

 peculiar lithological character, resembling the differences before 

 remarked as existing between the modern lavas of Vesuvius, 

 Etna, and Chili. (See p. 98.) 



It has been remarked that in Auvergne, the Eifel, and other 

 countries where trachyte and basalt are both present, the trachytic 

 rocks are for the most part older than the basaltic. These rocks do, 

 indeed, sometimes alternate partially, as in the volcano of Mount 



* Caldcleugh, Phil. Trans. 1836, p. 27., and Official Documents of Nicaragua, 

 t See Principles of Geology, Index, " Skaptar Jokul." 



