CHANGES OF LEVEL 393 
Beach accumulations of large masses seldom exceed ezght feet 
above high tide, and the finer fragments and sand may raise the de- 
posit to ten feet. But with the wind and waves combined, or on pro- 
minent points where these agents may act from opposite directions, 
such accumulations may be thirty to forty feet in height. ‘These are 
drift deposits, 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, (pp. 45, 64.) 
2. Sedimentary deposits, or layers of rolled stones interstratified among 
the igneous layers. 
3. Compactness of the igneous rocks. ‘The great uncertainty of this 
evidence has been shown in the preceding chapter. 
B. Evidences of Subsidence. 
1. The existence of nide and deep channels betveen an island and any 
of tts coral reefs ; or in other words, the existence of barrier reefs. 
2. Lagoon islands or atolls. 
3. Submerged atolls. 
A. Deep bay-indentations in coasts as the terminations of valleys.—In 
the remarks upon the valleys of the Pacific islands, it has been shown 
that they were formed in general by the waters of the land, unaided 
by the sea; that the sea tends only to level off the coast, or give it an 
even outline. When, therefore, we find the several valleys continued 
on beneath the sea, and their enclosing ridges standing out in long 
narrow points, there is reason to suspect that the island has subsided 
after the formation of its valleys. For such an island as Tahiti could 
not subside even a few scores of feet without changing the even outline 
into one of deep coves or bays, the ridges projecting out to sea on 
every side, like the spread legs of a spider. The absence of such 
coves, on the contrary, is evidence that any subsidence which has 
taken place, has been comparatively small in amount. 
5. Seashore alluvial flats or deposits. 
6. The lava surface of a volcanic island, sloping without interruption 
beneath the water, stead of terminating in a shore cliff of a hundred 
feet or so. 
C. Probable Evidence of Subsidence now in Progress. 
1. An atoll reef without green islets, or with but fer small spots of ver- 
dure.—TVhe accumulation requisite to keep the reef at the surface- 
level, during a slow subsidence, renders it impossible for the reef to 
rise above the waves unless the subsidence is extremely slow. 
99 
