THE ORIGIN OF IGNEOUS ROCKS. 
183 
. an eruption of lava on the surface of the earth cannot he 
supposed to exhaust the reservoir from which it was drawn. 
It must rather be the escape of a comparatively small part 
of its contents. Moreover, the intermediate forms of the 
magma have been erupted previous to that of the extremes, 
and extremes of differentiation will be located where ex¬ 
tremes of temperature in the reservoir have been established, 
and will be drawn off through different vents. 
It would be an awkward task to explain how it might 
happen that in the course of volcanic events all of the rhyo¬ 
lite in one region should be erupted at once and all of the 
basalt at once, and always in a definite order of succession ; 
but in regions of oft-repeated eruptions it is found that these 
rocks alternate with one another to a greater or less extent, 
and that transitional forms exist in limited amounts ; so that 
it is the general law only which remains, namely, that the 
order of eruption for any complete series of igneous rocks is 
from intermediate to more and more extreme varieties. 
Localized magmas. — The geographical distribution of 
groups of rocks that possess different chemical characteris¬ 
tics has a most important bearing on the question of the 
source of igneous magmas and the condition of the interior 
of the earth. 
The preponderance of rocks rich in alkalies throughout 
large areas of the earth’s surface and their scarcity in other 
regions, which has been pointed out in an earlier part of this 
paper, indicates a difference in the general magmas from 
which the whole series of igneous rocks of various regions 
have been derived. The vast extent of these chemically 
diverse areas shows the profound and persistent character of 
the differences. 
This may be illustrated in a general way by considering 
the distribution of two broadly defined groups of rocks, one 
of which we may call the alkali group and the other the 
sub-alkali group. In the first we would include natural 
groups, parts of which are rich in alkalies, especially those 
rocks low in silica; but some of the members of these groups 
