AGASSIZ: THE FLORIDA ELEVATED ItEEF. 35 



by a coralline bottom, on which are found scattered specimens of Msean- 

 drina, coarse fragments of Astrseans, Echini, and corallines such as 

 Udotea aud Halimeda mainly. The bottom is of a similar character 

 half-way between Washerwoman Buoy and Boca Chica. In the middle 

 of the main channel, all along the reef, the same coralline sand is usually 

 found thickly packed with Udotea and Halimeda. 



At Sand Key we found the reef rock elevated fully two feet above 

 high-water mark. The parts of the former elevated reef form a series 

 of pinnacles, on the sea face of which corals are now growing. The 

 central part of the key is covered by a mass of fragments of corals 

 of all sizes, finely ground with sand, and thrown up to a height of 

 about eight feet. Much of the elevated reef is often uncovered by 

 hurricanes, as was the case during the hurricane of September, 1894, 

 when the sea washed over the whole island. The elevated reef of which 

 Sand Key forms a part was evidently not raised to as great a height as 

 the ancient reef farther north. At Sand Key it is not difficult to dis- 

 tinguish between the elevated reef and the living reef, the line of 

 demarcation is quite sharp. 



At Sombrero Key we could not separate the elevated reef from its 

 coating of recent corals, especially since a good deal of coral had evi- 

 dently been killed by exposure to the air. But off Sombrero Light we 

 found old elevated reef rock honeycombed and pitted, and surmounted 

 by growing heads of coral, just as we had observed them at the Hen 

 aud Chickens. 



An examination of the Chart (Plate XVI.) shows that the shores of 

 the islands which stand opposite great gaps in the reef are all covered 

 with coral sand, as, for instance, in the great stretch between American 

 Shoal and Alligator Beef, where the shores are exposed to the sweep 

 of the sea, which throws up at times great masses of coral and coralline 

 sand, covering the elevated reef. This undoubtedly extended in a more 

 or less continuous stretch all the way along the former shore line of the 

 southern face of Florida. It has, however, at some points, been little by 

 little covered by the sand thrown up from the stretches exposed to the 

 sea, and at others the wearing action of the sea has broken through 

 the reef and formed the shallow passages which now separate adjoiniug 

 islands. 



East of Alligator Rejjf the main line of keys is far more sheltered, a 

 nearly continuous reef stretching from that point as far as Fowey 

 Rocks, there being only an occasional gap in the broad reef. This 

 stretch of the reef is in striking contrast to the narrow and iuskrnificant 



