THE ORIGIN OF IGNEOUS ROCKS. 
127 
equivalents. Such rocks may result from the separation of 
minerals from a magma having a very different composition 
from that of the rock in question. The absence of volcanic 
representatives of the typical peridotites and their rarity as 
dikes may be due to this.” 
The paper closes with a notice of Vogt’s hypothesis re¬ 
garding the deposits of iron ore, which has been previously 
mentioned. 
In the same number of the Quarterly Journal of the 
Geological Society that contains the article just noticed is 
the anniversary address of the president of the society, Sir 
Archibald Geikie. The address, together with that delivered 
at the previous anniversary, constitutes one of the most 
important contributions to the history of volcanic action, 
although it is confined to the area of the British Isles. 
Without entering into a discussion of the origin of the 
volcanic rocks of the region, Geikie calls attention to the 
nature and composition of the lavas and to the sequence of 
their eruption, and makes it evident that their source is to 
be sought in changes taking place in* the general magma 
from which they have originated. 
Since many of the facts brought out in this comprehensive, 
but necessarily incomplete history have a direct bearing on 
the paper prepared by the present writer, allusions to them 
have been inserted in the body of the paper. 
Having reviewed in considerable detail the most important 
contributions to the development of a theory of the origin 
of volcanic rocks, we may proceed to the statement of the 
geological, petrographical, and chemical data which appear 
to the writer to maintain the thesis of this paper. But be¬ 
fore doing so the writer wishes to express his indebtedness 
to Mr. Arnold Hague, geologist in charge of the Yellowstone 
Park Division of the U. S. Geological Survey, for the free¬ 
dom with which he has been permitted to use any informa¬ 
tion or material considered essential for the purpose just 
mentioned. This embraces geological observations as well 
as numerous chemical analyses, which have been prepared 
for the report on the Yellowstone National Park. 
