78 E. 



phosphate, said to be very rich in phosphoric acid, but of which the supply 

 is naturally somewhat limited. Several of these caves are from 60 to 70 feet 

 deep. The limestone on the top and making the flanks of the central hill 

 of the island is usually of a distinct color, in places a deep blue ; while that 

 making the lower flat is more commonly white in color. Both varieties of 

 limestone, wherever exposed, give ofl'a ringing, metallic sound when struck, 

 and are quite brittle ; but when burned both give a white powder. Holes 

 and shafts, sunk on the upper flat, show the limestone, under good cover, to 

 exist as a pure white stone, often amorphous carbonate of lime. 



The topography of the upper terrace suggests a lagoon character, inclosed 

 by a coralline limestone wall on all sides, the interior shut off by it from all 

 further encroachments of the sea, which, as it slowly dried up, gradually filled 

 with phosphate (guano) and became elevated enough to support a meager 

 growth of cactus, stunted palms and a few small trees, while extracting from 

 the stagnant sea water the various ingredients it carries in solution — potash, 

 soda, iron salts, etc. 



The irregular growth and spread of the coral limestone connecting the 

 surrounding main reef wall is well displayed wherever the phosphatic mate- 

 rial has been removed, showing a mass of pock-marked limestone, filled with 

 roughly cylindrical holes and trenches. 



In places the phosphate is so intimately mixed with the limestone as to 

 give the latter a conglomeratic appearance, especially well seen near the 

 border of the main surroundiug reef, where attrition was greatest. 



Subsequent elevation of the island has exposed the original sea slope of 

 the reef until a later phosphate deposit took place at or near sea level, rep- 

 resented now by the lower terrace, which is from 150 to 900 feet wide ; and 

 as the greatest elevation of the island has been towards its southeastern side, it 

 is there that we find the lower terrace the widest. Unlike the upper flat, this 

 terrace could never have been an enclosed lagoon, but was continually ex- 

 posed to the sea wash until it, too, was gradually lifted to its present position, 

 10 to 70 feet above the water. Erosion was therefore more complete here, 

 and leaching of the phosphate by rains and winds Avas carried on with less 

 interruption than upon the enclosed top of the island ; therefore it is in the 

 lower-flat (gray) phosphate that we find the least percentage of sesquioxide of 

 iron and alumina, because here the sea water carrying them in solution, un- 

 like that Qf the upper enclosed lagoon, escaped quickly and thus diminished 

 the amount of their precipitation. 



Climate and Vegetation. — Navassa, like other islands near the equator, is 

 not exposed to continued rain storms and has no protracted rainy season. 

 Moreover, the absence of all fresh water has still further favored the preser- 

 vation of the phospate deposits, and accounts for the absence of soil and the 

 stunted growth of timber, as well as for the presentation of the phosphate 



