J. D. Dana — Origin of Coral Reefs and Islands. 101 



northeastern Hawaii and 250 of southwestern, and within 80 miles 

 of northeastern Oahu; and a sounding but 125 fathoms less 

 than 3,000 was obtained by the Challenger within 40 miles of 

 eastern Hawaii (or half its diameter). To the westward, along the 

 north side of the chain, the deep-sea area appears to be two or 

 three times more distant, according to the Challenger results; 

 the condition on the south side is uncertain. It would seem 

 from the great depth near Hawaii, that the region of this great 

 island, although it is now actively volcanic and has little growing 

 coral about it, had undergone more subsidence than the coral 

 reef end of the chain, and that its height and steepness of sub- 

 marine slopes are due to the fact that its outflows of lava have 

 kept ahead of the subsidence, and also built up nearly 14,000 

 feet above the sea. 



This height is large, but the mean pitch of the sides of the 

 volcanic mountains of the island is between 5° and 7° 45', and 

 hence it is only the height which successive outflows should 

 have produced over a vent at the sea-level ; and it may be that 

 the accumulation above tide level has been made since the sup- 

 posed subsidence ceased. The depth of 2,875 fathoms found 

 by the Challenger 40 miles east of Hawaii shows a mean sub- 

 marine slope to that point of 4° 30', as if here also was a slope 

 made by flowing lava. But more soundings are needed to 

 prove that the slope is a gradual one. 



14. The facts reviewed show the uncertainty of evidence as to 

 little or no subsidence, or as to recent elevation, from (1) nar- 

 row reefs, or from (2) the volcanic character of islands, and leave 

 untouched the evidence of actual subsidence from the features 

 of barrier and atoll reefs and from deeply indented coasts. 



15. After the above considerations, it is clear that the theory 

 of subsidence meets well the facts as to the varying extent of reef 

 among reef-bordered high islands. According to it, (1) steepness 

 of submarine slope may characterize the side of a barrier reef 

 (as well as of an atoll) fronting east or west, north, or south, 

 as is true of high islands ; but it is least likely to occur 

 in the direction of the trend of the island or group, or that of 

 current drift. (2) Fringing reefs, or no reefs, may characterize 

 one side, that of bold bluffs, and wide barriers the opposite. 

 (3) The barrier reef may be made on the submarine slopes of the 

 land, or on a broad plateau or low-land area between ranges 

 of elevations, one or both of which have disappeared in the 

 subsidence. (4) By continued subsidence, the side having a 

 fringing reef or no reef, may, later in the subsidence, be that of 

 a very broad barrier reef, because of the form of the surface 

 of the subsiding land ; and vice versa. 



The third of these propositions is well illustrated by the 

 facts from the Maldives, as reported by Darwin. On account 

 of its importance I add an illustration from the Feejees. 



