PACIFIC EXPLORATION: J. P, IDDINGS 
413 
more mobile with the Lower Carboniferous and especially during the 
Jurassic and Cretaceous. During this very long time, the eastern 
half of the continent, a land about 1800 miles east and west and 2200 
miles north and south, nearly all went down more and more beneath 
the level of the sea to a maximum depth of about four miles and an 
average depth of between one and two and a half miles. Further, the 
entire area of the Oceanides also subsided, and possibly to an equally 
great depth; while this was taking place the bottom was apparently 
folded and built up by volcanic material into many more or less parallel 
ridges, a series of arcs extending over an area of about 3500 miles east 
and west and the same distance north and south. Finally, we may add 
that the entire western half of the Pacific bottom appears to be as mobile 
as any of the continents of the northern hemisphere, with the difference 
that the sum of the continental movements is upward, while that of the 
ocean bottoms is downward. This paper will be published at greater 
length and with illustrations in the American Journal of Science. 
^ Suess, Natural Science, 2, 180 (1893). 
2 Park, Geology of New Zealand, 1910. 
' Siissmilch, Geology of New South Wales, 1911. 
THE PETROLOGY OF SOME SOUTH PACIFIC ISLANDS 
AND ITS SIGNIFICANCE 
By Joseph P. Iddings 
BRINKLOW. MARYLAND 
Read before the Academy. April 17. 1916. Received. May 12. 1916 
Petrology as a more comprehensive term than petrography embraces 
all the phenomena and material characters of rocks, as well as the 
theories regarding their origin and the relations between the rocks of 
the earth and the problems of geodynamics. Knowledge of the com- 
position, mode of occurrence, and distribution of igneous rocks should 
contribute materially to the elucidation of those problems in geology 
which are concerned with the constitution and behavior of the outer 
and inner portions of the earth. 
For many years some geologists and petrologists have been con- 
vinced that the various kinds of igneous rocks in different parts of the 
world, both volcanic outflows and intruded bodies, are so intimately 
related to one another within each region that they must have been 
derived from some parent lava, or rock magma, by processes of physico- 
chemical differentiation; and further, that in different regions of the 
earth the series of igneous rocks in each possess chemical and mineral 
