On the Geology of the Key of Sombrero, W. I. 273 



In the progress of the subsidence, the central basin became 

 again submerged by the infiltration of sea-water, and deposits 

 of impure calcareous mud and of green clay were again suc- 

 cessively formed and mingled with the pebbles. The per-oxi- 

 dation of the iron in many masses of conglomerate has probably 

 been produced at later periods, by the percolation of aerated 

 waters through intersecting fissures. In these turbid and im- 

 pure waters of course no corals, but only a few marine shells, 

 could grow. As the lagoon deepened, the incursions of the sea 

 had a continually diminishing influence upon the bottom ; the 

 nodules formed and swept into the veins decreased in size and 

 lost their angular shapes ; and finally only the debris, ever 

 forming, of the foreign rock was introduced into the conglo- 

 merate deposits. 



It happened occasionally in the latter part of the subsidence, 

 in some narrow and deep gulley of the bottom, whose limits 

 inclosed a pool of quiet water rarely disturbed by the agitation 

 of the surface, that the materials in suspension were deposited 

 film after film, so that the upper portion of the sediment assumed 

 a laminated instead of the amorphous character of the earlier 

 and coarser portions below. The very great excess of the 

 detritus in this period, over that at the beginning of B, may be 

 attributed to the longer resistance to external erosion and per- 

 foration of the higher and bulkier barrier formed during the last 

 subsidence. 



The barrier was at last broken through, the fresh ocean 

 waters again filled the lagoon, and new coral deposits were 

 formed. The characteristics of the bed show that freer commu- 

 nication with the sea now existed than on any previous occa- 

 sion, probably because the number and depth of the fissures 

 had led to the production of more numerous and broader inlets. 

 The comparatively dwarfed individual development of the 

 corals, notwithstanding the abundance of nuclei, was not so 

 favorable for their growth as that afforded without the barrier. 

 The principal reasons may be the absence of surf to supply 



