THE GREAT FAULT TROUGHS OF THE ANTILLES 97 



3,206 fathoms (5,858 m.). The general form of this depression, 

 in so far as it has been revealed by soundings, and its alignment 

 with the fault trough of southern Haiti, together with the presence 

 of other depressions of unknown shape nearer Jamaica, suggest 

 that there has been more or less trough faulting all along the arc 

 from Ocoa Bay to Swan Island. 



Swan Island is built of limestone, mostly Pleistocene or Recent 

 in age. A specimen from the higher part of the island consists 

 chiefly of the coral, Orhicella cavernosa (Linn.). The particular 

 form present in the specimen now lives in those waters and has 

 not been found in deposits older than Pleistocene. In places the 

 rock is a loosely cemented, coarse, calcareous sandstone. Pebbles 

 and bowlders of hard limestone, containing terrigenous material, 

 are abundant along the beaches. A specimen, showing well- 

 defined, closely spaced bedding-planes and made up largely of 

 Glohigerina with amorphous silica filling the cavities, contained 

 flakes of muscovite one millimeter in diameter and subangular 

 quartz grains. Another specimen stained with ferric oxide, 

 contained about 1.5 per cent AI2O3 as well as some quartz,^ The 

 land mass from which these terrigenous materials were derived must 

 have been close at hand, but it has entirely disappeared. 



It has not been possible to obtain seismologic data for Swan 

 Island extending back over a long period of time, but during the 

 summer of 1920 two shocks were felt.^ On July 11 at 10:40 a.m. 

 a distinct rumbling was heard, and this was immediately followed 

 by a pronounced shock, apparently vertical, lasting five seconds. 

 On August 18 at 6:04 p.m. a rather violent earthquake of 

 approximately fifteen seconds' duration was experienced. The 

 motion was reported as vertical and similar in character to that of 

 the earthquake of July 11. No unusual sea conditions were 

 noticed. Tools were dislodged in the engine-room and shop of the 



^ A set of rock specimens was procured from Swan Island through the courtesy of 

 George S. Davis, General Manager of the Radio Telegraph Department of the United 

 Fruit Company. The writer is also indebted to him for his kindness in furnishing 

 seismologic data for Swan Island. The specimen of coral was identified by Dr. T. W. 

 Vaughan, and the Glohigerina by Dr. Jos. A. Cushman and Dr. Vaughan. 



2 These earthquakes were observed and reported by George H. Rogers, Chief 

 Operator of the Swan Island Radio Station. 



