Page:A Treatise on Geology, volume 1.djvu/352

 336 we comprehend, in the former case, the formation of a long bank of coral, by the successive submersion of a line of mountain summits; and the filling of a sea with many small annular reefs, by the sinking of island rocks: in some cases, the circular form of lagoon islands may, however, be as well understood by supposing them to have grown on a submarine volcanic crater. A decisive proof of the truth of Mr. Darwin's views, in the particular instance, would be the discovery of solid coral rock at a much greater depth than that which is stated to limit the existence of the lamelliferous polypi.

The general results of and views arising from Mr. Darwin's investigations include the following very important points:—

1 . That linear spaces of great extent in the equatorial regions are undergoing movements of an astonishing uniformity, and that the bands of elevation and subsidence alternate.

2. From an extended examination, the points of volcanic eruption all fall on the areas of elevation. The importance of this law is evident, as affording some means of speculating, wherever volcanic rocks occur, on the changes of level even during ancient geological periods.

3. Certain coral formations acting as monuments over subsided land, the geographical distribution of organic beings is elucidated by the discovery of former centres, whence the germs could be disseminated.

4. Some degree of light might thus be thrown on the question, whether certain groups of living beings peculiar to small spots are the remnants of a former large population, or a new one springing into existence. (From Geol. Proceedings, 1837; and notes taken during the reading of Mr. Darwin's paper to the Geol. Society.)

Shell Beds.—What the circumstances are, which favour in a special degree the accumulation of shells on the bed of the sea, may be partly conjectured; but