392 PACIFIC OCEAN. 
have passed over the several Hawaiian Islands since the fires ceased 
and wear begun, is fully substantiated. We also learn how com- 
pletely the features of an island may be obliterated by this simple 
process, and even a cluster of peaks like Orohena, Pitohiti and Aorai 
of Tahiti, be derived from a simple volcanic dome or cone. Mount 
Loa, alone, contains within itself the material from which an island 
like Tahiti might be modelled, that should have near twice its height 
and four times the geographical extent. 
IV. CHANGES OF LEVEL IN THE PACIFIC OCEAN. 
We have learned from recent investigations that the continents 
have undergone extensive changes of level, wide seas with only here 
and there an island occupying their place at some former eras. Even 
as late as the tertiary period, it is believed that a large part of the con- 
tinents emerged from the waters. It is an inquiry of interest, there- 
fore, whether the oceans and the land beneath have not participated 
in such changes; or whether, while all other lands were in constant 
change of level, the Pacific islands alone have stood firm, unmoved 
and immovable. ‘This question is not to be answered by a guess or 
assertion. ‘Those who may be unacquainted with geological reason- 
ing, will note that it is as much an assumption to affirm that changes 
have not taken place, as that they have; and the analogies suggested 
will sustain us in saying that the former is the more unreasonable, 
the more “ absurd” affirmation of the two. 
Evidences of change of level are to be looked for in the height or 
condition of the coral reef formations or deposits; in the character of 
the igneous rocks; and in the features of the surface. The points 
of evidence are as follows :— 
A. Evidences of Elevation. 
1. Patches of coral reef, or deposits of shells and sand from the reefs, 
above the level where they are at present forming. 
The coral reef rock has been shown occasionally to increase by 
growth of coral, to a height of four to six inches above low tide level 
when the tide is but three feet, and to twice this height with a tide of 
six feet. It may, therefore, be stated as a general fact that the height 
to which coral may grow above ordinary low tide, is about one-sixth 
the height of the tide, though it seldom attains this height. 
