Modern Igneous Rocks and the Primary Formations. 125 
centre, where the rocks are commonly igneous. Volcanic sands 
are blown by the winds to a greater distance from the centre of 
eruption than the lavas flow, or if submarine, are diffused farther 
by the waters. 'The centre, when exposed to view by subsequent 
convulsions and rents of the mountain, is sometimes pure solid 
basalt, with no trace of stratification, or division into lava beds. 
I was particularly struck with this fact in the island of Tahiti, 
which is a type of many others in the ocean. ‘The island has 
been so altered by convulsions and denudation, that no trace re- 
mains of an ancient crater. It is but a mass of sharp ridges and 
mountain peaks, the central about eight thousand feet high, and 
I never suspected its true nature till months afterwards, the mod- 
ern and ancient igneous formations of the Sandwich Islands were 
examined. For six or eight miles towards the interior, the island 
consists of alternating basaltic conglomerates and tufas, dipping 
outward towards the shores at a small angle; beyond this, the 
basaltic layers are of great thickness—one hundred to two hun- 
dred feet being not. uncommon ; and the central peaks are solid 
to their summits without a trace of stratification—one solid mass 
of semicolumnar basalt—apparently the cooled interior of the vol- 
canic mountain. I would refer to my forthcoming reports for a 
particular account of this interesting island. 
In the Andes the same is every where exemplified. Ascending 
them, the traveller passes over conglomerates and pseudo-porphy- 
ries and allied rocks, till he nears the summit, where stand at in- 
tervals lofty mountain turrets of basalt, and rude crests of por-— 
phyry, acknowledged centres of the ancient volcanoes of this 
immense chain. Occasional dykes and subordinate crests are met 
with on the ascent, but the most magnificent views of moun- 
tain architecture are seen about the loftier portions of this range. 
How is it now with granitic regions?. When granite, gneiss, 
and the schists are associated, does not the grand central mass 
consist of granite or gneiss, and do not the schists occupy the 
more distant or outer portions of these regions? ‘The exceptions 
to this prove the point we have established, that the schists may 
be either primitive or derivative rocks. But the general fact is 
too apparent not to have been noticed and described in all geolo- 
gical treatises, even the earliest. Argillite is commonly exterior 
to mica slate, and the talcose rocks and serpentine generally out- 
side of the syenites, if they occur together. We should not 
