On the Geology of the Key of Sombrero, W. I. 2fil 



coral-bed, from their fragility and that of the corals, and from 

 the absence of fragments, it may be inferred that the reef grew 

 in comparatively quiet water. As the heavy ground-swells 

 have considerable force even to the limiting depth of coral-life, 

 sufficient to have frequently stirred up the sand and killed and 

 broken fragile organisms, the quietness of the locality could not 

 have been due simply to great depth. We are forced to believe 

 then at the outset, that some barrier encircled the reef, probably 

 of much greater age, — a constant accompaniment of the lower 

 beds, varying at different periods in height relative to the 

 bottom of the lagoon. Indeed such a barrier must necessarily 

 have been formed on the outer edge of the oscillating area, on 

 the first occasion that the superincumbent sea was sufficiently 

 shallow to support coral life, creating an atoll when it reached 

 the sea level. This elevation of its summit was at least suffi- 

 ciently great to break the force of the ground-swells and storm- 

 waves, but may have occurred only along its outline from E.S.E. 

 around by North to W.N.W., whence alone comes the violence 

 which at the same time battered the sea-face and threw up 

 debris. However, the universal distribution of the reef shows a 

 considerable communication between the sea and the lagoon 

 waters, either over the barrier or through its intersecting passages. 



The thickening of the reef towards the N. and E. seems to 

 denote the proximity of the barrier-reef at those points, since 

 the coral formation must have flourished in a higher degree and 

 for a longer time in the shallower waters near the shore of the 

 lagoon and the passages of the barrier. 



From the general level of the surface of the reef it may be 

 inferred that the communication of the lagoon-waters with the 

 sea subsequently decreased greatly, either on account of the 

 rising of the barrier above the sea-level, the partial closino- of 

 its passages; or perhaps the increasing depth, and that coral-life 

 ceased over the central area, with the exception of many scat- 

 tered clumps and the frequent super-position of a more delicate 

 species. 



APRIL, 1S66. 20 Ann. Lyo. Nat. Hist., Vol, VIII. 



