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



of the points of the curve, on account of the little knowledge 

 we possess, concerning either the absolute or relative annual 

 increase of deposits of corals, shells, and sand, and concerning 

 the vertical range of species, in a lagoon whose communication 

 with the sea varied so widely. 



For our approximate purposes we will assume that the rate 

 of deposition was the same in all materials except the coral, and 

 lay off the abscissae (the measures of time) in direct proportion 

 to the thickness of each bed, with a deduction of four-fifths* 

 from that of every coral band. 



As to the conjugate axis we have a few observations to guide 

 us, but a determination of the vertical distribution of the fossils 

 in each bed might have produced more definite results. The 

 deposits seem to have varied according to the subsidence of the 

 bottom through portions of one or more marine zones, more 

 or less influenced by the character of the barrier, on account 

 of which the organisms and deposits of a lower zone may 

 sometimes have begun at a higher level through the greater 

 tranquillity of the lagoon. The only one of the division lines 

 which can be defined is the well known lower limit (about 100 

 feet) of the Coralline zone. The more open or closed condition 

 of the barrier is represented by the oscillations of a dotted line 

 below and above the sea-level in correspondence, but not in 

 parallelism, with the curve of oscillations below. 



Thus then this little rocky islet stands out in the open ocean, 

 a solitary pillar, like those of the Temple of Serapis, marking 

 the old convulsive throbs and prolonged oscillations of the 

 deep-sea bottom. 



* Dana's "Manual of Geology," page 591. 



