94 
IDDINGS. 
the dikes, and not thin offshoots from the dikes into the 
gneiss. 
This view of the origin of trap dikes being admitted, he 
continues: “ We may further admit in the case of a great 
body of plutonic rock being impelled by repeated move¬ 
ments into the axis of a mountain chain, that its more liquid 
constituent parts might drain into deep and unseen abysses, 
afterwards, perhaps, to be brought to the surface under the 
form either of injected masses of greenstone and augite- 
porphyry or of basaltic eruptions.” 
Darwin has thus formulated two hypotheses—one based 
on a partial crystallization and separation by settling on ac¬ 
count of differences of specific gravity between the crystals 
and the mother-liquor, and the other based on the partial 
crystallization of a magma and the oozing or squeezing out 
of uncrystallized portions. Both consider the different varie¬ 
ties of eruptive rocks to have originated by the separation of 
some primitive homogeneous mass. 
In 1849 Dana, in his report as geologist on the Wilkes 
Exploring Expedition, * discusses the mineral constitution 
of the basaltic islands of the Pacific. He observes that the 
volcanic rocks of these islands vary “ between basalt and 
clinkstone or porphyry, the former passing into the latter as 
feldspar becomes the predominant and finally the constituent 
mineral.” The rocks grade from glassy to “a variety of 
crystalline texture like syenite.” In seeking “ the origin of 
these different rocks and the peculiar conditions and rela¬ 
tions under which they occur,” he calls attention to the fact 
“ of the occurrence of feldspathic varieties at the center of 
the mountains, while the exterior and circumferential por¬ 
tions consist of basaltic rocks,” and asks, “ What has sepa¬ 
rated the feldspar, iron, and augite that constitute the basaltic 
rocks and left nearely pure feldspar alone at the center” ? 
The difference in the specific gravity and fusibility of 
*Dana (J. D.) United States Exploring Expedition during the years 
1838-1842, under the command of Charles Wilkes, U. S. N. 4°. Phila¬ 
delphia, 1849. Yol. 10, Geology, p. 372 et seq. 
