252 J.J. Harris Teall—Cheviot Andesites and Porphyrites. 
Kerguelen Island. The smaller stratified islands, however, would 
usually be short-lived, being destroyed by erosion. Volcanic and 
coral islands, on the contrary, are constantly growing and making 
good the loss by erosion. Submarine volcanoes suffer no erosion, 
until their summits reach the surface of the water; and their growth 
is mainly vertical, since the water must ordinarily prevent the lava 
from flowing far from the outlet or crater. Consequently, if a con- 
tinent, the stratified summits of which are high and the volcanoes 
low, is submerged; the former will be soon swept away by erosion, 
and the lavas ejected by the latter will be piled up, monument-like, 
until they reach the surface, when, although erosion checks the 
upward growth, its ravages are constantly made good by fresh out- 
flows of lava. 
In the opinion of the writer, these considerations materially 
diminish the surprise which one feels on first observing that the 
oceanic islands are mainly volcanoes and coral-reefs. For in no 
other class of islands do we find those elements of growth which 
enable them to keep pace with the increasing subsidence and to 
make good the encroachments of the sea. An active volcano can- 
not be permanently submerged, and the same is true of a coral- 
island, provided the subsidence goes on slowly enough. In short, 
nearly all the larger oceanic islands do embrace considerable masses 
of the older stratified formations; and the fact that the smaller ones 
do not as a rule is satisfactorily explained by a comparison with the 
highest points of existing continents, and a due consideration of the 
facts that small stratified islands would necessarily be short-lived, 
and if submerged only one hundred feet would be for ever lost as 
land, and that the volcanic and coral islands cannot usually be either 
submerged or worn away, possessing a power of growth which 
makes them eternal. 
I].—NorrEs oN THE CHEVIOT ANDESITES AND PORPHYRITES. 
By J. J. Harris Teatt, M.A., F.G.S. 
(Continued from p. 152.) 
[PLATE VI.] 
7 He Coquet.—Blindburn.—A dark-grey compact rock with an 
uregular fracture. A few small scattered felspars may be 
recognized. Red streaks and blotches occur. Under the microscope 
two generations of felspar may be recognized; the larger porphyritic 
crystals abounding in inclusions of the ground-mass of the rock, 
while the smaller and later crystals are comparatively free from 
inclusions. Both kinds are somewhat altered, small portions here 
and there showing aggregate polarization, while the main mass of the 
crystals gives definite extinctions. The pyroxene has almost entirely 
disappeared. Sections of characteristic form may however be dis- 
tinctly recognized. These are now principally occupied by an 
aggregate of quartz granules or a mixture of quartz and chalcedony ; 
but here and there fibres of a brilliant and peculiar green mineral 
occur scattered throughout the quartzose aggregate, and these doubt- 
