354 J. D. Dana on the Volcanoes of the Moon. 
ly, we may believe, as at the present period. Here then we see 
reason for what has been considered a most improbable supposi- 
tion, the existence of an immense area covered in most parts by 
shallow seas and so fitted for marine life. 
If we follow the progress of the land, we find that with each 
great epoch there has been a retiring of the sea. In the coal de- 
posits we have an abundant land vegetation. Subsequently; the 
progress on the whole was giving increased extent and height to 
the land and diminishing the area of the waters. Instead there- 
fore of a bodily liftmg of the continents to produce the apparent 
elevation, it may actually have been a retreating of the waters 
through the sinking of the ocean’s bottom. The process how- 
ever has not been a continuous one: for during each epoch,—the 
Silurian and the more recent,—there have been subsidences as 
well as seeming and actual elevations, and various oscillations of 
the continental surface, from subaérial to submarine and the re- 
verse. When contraction had once taken place over the conti- 
nents as well as under the ocean, there may have afterwards been 
expansions again through the return of heat from some cause. 
And thus various irregularities have taken place, such as the rocks 
indicate. In the tertiary period and since, the apparent rise of 
the land has been still to some extent in progress. _ And. is. there 
any evidence that this could have arisen from a sinking of the 
ocean’s bed? The evidence is undoubted. For Mr. Darwin has 
shown satisfactorily, (and farther observations to the same end, 
and to many interesting conclusions, will be presented in the wri 
ter’s geological report on the Pacific), that a subsidence of some 
thousands of feet has taken place since the corals commenced 
* their growth. Every coral island is a register of this subsidence.* 
And why should not the ocean’s bottom subside, as well as the 
land? What has given the continental portions of our globe 
their elevation, as compared with other parts, if not the unequal 
contraction of the whole? Can we safely affirm—in words of 
high authority—“ that the stability of the sea and the mobility of 
the land are demonstrated truths in geology,”+ when mobile land 
forms also the bed of the ocean, and its changes must affect the 
-* See Silliman’s Journal, xlv, 131, 1843. x 
~ t Leonard Horner, Esq., Anniversary Address before the Geological Society of 
London, January, 1846; Q rly Jour. of the Geol. Soc., No. 6, p: 199. . 
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