158 J. D. Dana on Changes of Level in the Pacific Ocean. 
fragment of coral or shell; and in most of these particulars they 
are distinct from reef-rocks. (pp. 369, 370, vol. x1. 
2. Sedimentary deposits, or layers of roiled stones intersirati- 3 
fied among the igneous layers. 
Compactness of the igneous rocks.—The great uncertainty 
of this kind of evidence has been shown in another, place. 
B. Evidences of Subsidence. 
1. The existence of wide and deep channels between an island 
and any of iis coral reefs; or in other words, the existence of 
barrier reefs. 
. Lagoon Islands or Aiolls. 
3. Submerged atolls 
4. Deep bay-indentations in coasts as the terminations of val- 
leys.—In the remarks upon the valleys of the Pacific Islands, 1t 
has been shown that they were in general formed by the waters 
of the land, unaided by the sea; that the sea tends only to level 
¢ the coast, or give it an even ontline. When therefore, we 
nd the several valleys continued on beneath the sea, and their 
enclosing ridges standing out in long narrow points, there is rea 
son to suspect that the island has subsided after the formation of 
its valleys. For such an island as Tahiti could not subside eve 
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 fiats or deposits. , 
6. The lava surface of « volcanic island, sloping without ™~ 
terruption beneath the water, instead of terminating in 4 shore 
cliff of a hundred feet or so. 
C. Probable evidence of subsidence now in progress. il 
‘l. An atoll reef without green islets, or with but few gic 
spots of verdure.—The accumulation requisite to keep the tee 
at the surface-level, during a slow subsidence, renders it. Jmpossl- 
ble for the reef to rise above the waves unless the subsidence 
extremely slow. 
From the above review of evidences of change of level, it apr 
that where there are no barrier reefs, and only fringing 
at times, other causes, corals may not have begun to grow till 
