272 On the Geology of the Key of Sombrero, W. 1. 



inches to 1 foot. Occasionally they are somewhat angular and 

 fragmentary. Some corals are replaced with Calc-Spar. 



This bed is the richest of all in species. The fourth mode of 

 fossilization prevails, but the material of the most common 

 coral, Porites astracoides (?), has everywhere been converted 

 into a soft and friable white carbonate of lime. 



§ 20. History. — The erosion to which the fissures, produced 

 by the last elevation, were subjected during the first part of 

 this period, was probably chiefly due to the prolonged action of 

 running streams, derived from the rains or spray, trickling 

 through those fissures which connected with the sea. It is fur- 

 ther probable that by the plunging- force of the ocean waves, 

 overleaping the barrier in storms and ground-swells, portions of 

 the hard upper crust of Lagoon limestone were torn up, the 

 softer rock underneath exposed, and an opportunity thus 

 afforded for the excavation of basins of a variety of shapes. As 

 the subsidence progressed, these incursions of the sea made 

 havoc more frequently over the central area, tearing up the 

 rock wherever most weakened by fissures. Their transient 

 occurrence is shown by the general angularity of the nodules, 

 and their frequently slight removal out of their original posi- 

 tions. 



The nodules scattered over the surface were gradually swept 

 into the widened fissures and basins, and the presence of Calc- 

 Spar among them appears to be a conclusive proof at the same 

 time of the elevation of the surface above the sea-level, and of 

 the streams trickling down the fissures. Its adherence to the 

 sides of the fissure and nodules shows that it was formed in situ, 

 while its intersection in one locality by a fissure of the fourth 

 system proves that it was not formed at a subsequent period. 

 As many of the veins and the deposits of Calc-Spar reach from 

 the surface of the bed down to the sea-level, where further 

 observation is cut off, it is thus rendered certain that the ele- 

 vation of the surface of the island was at least thirty feet above 

 the sea, though probably much more. 



