When the tide is three feet, beach accumulations of large 

 masses seldom exceed ei^ht 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 from opposite directions, such accumulations 

 may be fifteen to t7venty feet in height, and occasionally thirty 

 \o forty feet. These latter are drift deposits, finely laminated, 

 generally with a sandy texture, and commonly without a dis- 

 tinguishable 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 deposits, 

 or layers of rolled stones, intersiratified among the layers of 

 igneous or other rocks constituting the hills. 



B. Evidence of subsidence. 



1. The existence of 7i'ide and deep channels between an Island 

 and any of its coral reefs; or in other words, the existence of 

 harrier reefs. 



2. The existence of lagoon islands or atolls. 



3. The existence of submerged atolls. 



4. Deep bay-ijidentations in the coasts of high islands as the 

 terminations of valleys. — In the course of remarks upon the 

 valleys of the Pacific Islands, presented by the author in his 

 Geological Report, it is shown that they were in general formed 

 by the waters of the land, unaided by the sea ; that the sea 

 tends only to fill up bays, level off the coast, and so give it 

 an even outline. When, therefore, the several valleys of an 

 island continue down beneath the sea, and their inclosing ridges 

 stand out in long narrow points, with abrupt sides, 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. 

 On the contrary, the absence of such coves, or deep-valley 

 bays, may be evidence that no subsidence has taken place, or 

 only one of comparatively small amount. 



T 



