TEST OF AGE OF VOLCANIC ROCKS. 
523 
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 cov¬ 
ered the ground as they fell to the depth of more than ten 
feet, for a distance of eight leagues from the crater, in a south¬ 
erly direction. Birds, cattle, and wild animals were scorched 
to death in great numbers, and buried in ashes. Some vol¬ 
canic dust fell at Chiapa, upward of 1200 miles, not to lee¬ 
ward 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 700 
miles distant to the north-east. In the sea, also, at the dis¬ 
tance 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 ho¬ 
mogeneous 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; 
thus in Iceland, in 1783, 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 sepa¬ 
rate fragments by denudation, we might still, perhaps, iden¬ 
tify 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 vol¬ 
cano, 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 in Madeira trachytic rocks overlie an old¬ 
er basaltic series ; but the trachyte' occupies more generally 
an inferior position, and is cut through and overflowed by 
* Caldcleugli, Phil. Trans., 1836, p. 27. 
t See Principles,‘‘Skaptar Joknl.” 
