TOPOGRAPHICAL FEATURES. 431 
The Indian Ocean is another vast oceanic area; and it comports 
exactly with these views that over northern Hindostan, rise the lofty 
Himalayas. 
We may conclude, therefore, that the subsidence of the oceanic 
areas has produced, to a great extent, the mountains of the continents. 
Until after the carboniferous era, we have httle evidence of high moun- 
tains in America; for the Appalachians are of more recent date ;* and 
the Rocky Mountains and Andes, although they had commenced to 
expand before, did not reach their present elevation till the tertiary 
period or later. We also infer that the oceans and continents of the 
globe have never changed places. ‘The cause was at work from the 
beginning which resulted finally in producing the present oceanic 
depressions. During earlier times of igneous action, when the de- 
pression was still shallow, the oceanic area may have been largely 
covered with dry land. But as it deepened, these lands decreased in 
extent from the subsidence in general progress; the continents were 
more and more uncovered as the ocean cavity enlarged to receive the 
waters; and after various changes, the existing condition has resulted, 
in which nearly three-fourths of the globet still present a surface of 
water. There is some evidence that no continent has occupied the 
present position of the Pacific, within any of the more recent geologi- 
cal epochs, in the absence of all native quadrupeds from its islands, 
and even from New Zealand. It hardly requires remark that minor 
changes of elevation that have taken place in both the oceans 
and continents, or along their borders, do not conflict with these 
general conclusions. ‘The term continent should properly include all 
that surface of land which, whether submerged or not, is actually 
raised far above the great oceanic depressions. *'The outlines of con- 
tinents may be greatly varied by slight changes of level; but not so 
the extent of these more elevated portions of our sphere. 
In these operations we see a sufficient cause for those oscillations 
in the water level which the character of the rocks indicate. The 
changes of level in progress would be gradual or abrupt according 
as the tension produced a progressive yielding, or met with resistance 
which gave way only after an accumulation of force; moreover, 
during the earliest periods, variations in the places of igneous action 
would vary much the water level. ‘There is hence an abundant cause 
for changes of level on the globe, without appealing to an incompre- 
* American Journal of Science, ii. ser., ili, page 95, 181. 
T Prof. S. P. Rigaud, Trans. Cambridge Philosophical Society, vi. 289, 1837. 
