Ch. XXX.] RELATIVE AGE OF VOLCANIC ROCKS. (357 



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 Jamaica, about 'ZOO 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 by 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 hind 

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

 extensive area; as in Iceland in 1783, when the melted matter, pour- 

 ing from Skaptar J okul, 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 after- 

 wards 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 much in mineral composition and texture. 



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 Mont Dor, in Auvergne ; and we have 

 seen that in Madeira trachytic rocks may overlie an older basaltic se- 

 ries (p. 653) ; but the great mass of trachyte occupies more generally 

 perhaps an inferior position, and is cut through and overflowed by 

 basalt. It can by no means be inferred that trachyte predominated at 

 one period of the earth's history and basalt at another, for we know 

 that trachytic lavas have been formed at many successive periods, and 

 are still emitted from many active craters ; but it seems that in each 

 region, where a long series of eruptions have occurred, the more fel- 

 spathic lavas have been first emitted, and the escape of the more 

 augitic kinds has followed. The hypothesis suggested by Mr. Scrope 

 may, perhaps, afford a solution of this problem. The minerals, he 

 observes, which abound in basalt are of greater specific gravity than 

 those composing the felspathic lavas ; thus, for example, hornblende, 

 augite, and olivine are each more than three times the weight of 



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

 f See Principles, Index, " Skaptar Jokul." 

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