MUSEUM OF COMPARATIVE ZOOLOGY. 373 



slope of these dunes is rather steep, sometimes over thirty degrees, 

 and steeper to the seaward than on the landward side. 



In the interior of Salt Key there is a pool of intensely salt water, 

 the tint of which is pinkish or flesh-colored, owing to the accumulation 

 of a little Alga. When agitated by the wind, this pool is hedged all 

 round by foam of the purest white, arising from the frothing of the 

 viscous water. Along the edge the accumulation of this microscopic 

 plant forms large cakes, not unlike decaying meat, and of a very offen- 

 sive odor. The foundation rock of this key is exactly like what Gressly 

 described as the " facies corallien " of the Jurassic formation ; while 

 the deposit in deep water, consisting chiefly of muddy lime particles, 

 answers to his "facies vaseux." 



Double-headed Shot Key is a long, crescent-shaped ridge of rounded 

 knolls, not unlike " roches moutonnees," at intervals interrupted by 

 breaks, so that the whole looks like a dismantled wall, broken down here 

 and there to the water's edge. The whole ridge is composed of the finest 

 oolithes, pretty regularly stratified, but here and there like torrential 

 deposits ; the stratification is more distinctly visible where the rocks have 

 been weathered at the surface into those rugged and furrowed slopes 

 familiarly known as " karren " in Switzerland. It is plain that we have 

 here the same formation as on Salt Key, only older, with more thor- 

 oughly cemented materials. The uniformity of the minute oolithes 

 leaves no doubt that the sand must have been blown up by the wind 

 and accumulated in the form of high dunes before it became consolidated. 

 The general aspect of Double-headed Shot Key is very different from 

 that of Salt Key. The whole surface is barren, — not a tree, hardly a 

 shrub, and the scantiest creeping vegetation. The rock is very hard, 

 ringing under the hammer, and reminds one of the bald summits of the 

 Jura, such as Tete de Rang, near La-Chaux-de-Fond. It is evident that 

 what is beginning on Salt Key has here been not only completed, but is 

 undergoing extensive disintegration in Double-headed Shot Key, both 

 by the action of atmospheric agents over the surface and by the action 

 of tides and winds against the base of the key. 



Among these older oolithic deposits, forming the main range of 

 Orange Key and of Double-headed Shot Key, we recognize formations 

 of more recent date, occupying the cavities of ancient pot-holes, which 

 have gradually been filled with materials identical with those of the 

 older deposits. The pot-holes themselves show nothing very peculiar ; 



