CHANGES OF LEVEL IN CORAL-REEF REGIONS. 355 
on an island at a higher level would be proof of an elevation 
of the land. 
When the tide is three feet, beach accumulations of large 
masses seldom exceed eight feet above high tide, and the finer 
fragments and sand may raise the deposit to ten feet; but 
with a tide of six feet twice this height may be attained. 
With the wind and waves combined, or on prominent points | 
where these agents may act froin opposite directions, such 
accumulations may be fifteen to twenty feet in height, and 
occasionally thirty to forty feet. These latter are drift de- 
posits, finely laminated, generally with a sandy texture, and 
commonly without a distinguishable fragment of coral or 
shell; and in most of these particulars they are distinct from 
reef-rocks. 
2. On islands not coral, the existence of sedimentary de- 
posits, or layers of rolled stones, interstratified among the 
layers of igneous or other rocks constituting the halls. 
B. Evidence of subsidence. 
1. The existence of wide and deep channels between an 
island and any of its coral reefs; or in other words, the exist- 
ence of barrier reefs. 
2. The existence of lagoon islands or atolls. 
3. The existence of submerged atolls. 
4. Deep bay-indentations in the coasts of high islands as 
the terminations of valleys. —The kind of evidence here re- 
ferred to has been fully explained on page 273. It may be 
added that the absence of coves, or deep-valley bays, may be 
evidence that no subsidence has taken place, or only one of 
comparatively small amount. 
C. Probable evidence of subsidence now in progress. 
1. An atoll reef without green islets, or with but few small 
spots of verdure.—The accumulation requisite to keep the 
