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



In some narrow veins brownish masses of Calc-sparhave been 

 formed among and around the pebbles. 



§ 18. History. — The remarkable fineness of the last deposits 

 of this bed suggests that during this period the bottom was 

 depressed to the greatest depth yet reached, and that the com- 

 munication with the sea was less than ever before. The 

 absence of any fragments intermediate in size between the per- 

 fect shells and the minute pieces, as well as the occasional pre- 

 servation of the lustre and markings of the latter, suggest that 

 these have not been formed by attrition, but perhaps by those 

 fish and crustaceans which feed upon molluscs. As the degree 

 of concentration of the lagoon waters, in this and former peri- 

 ods, may have experienced considerable variation, we may 

 deduce from the general abundance of Bulla a peculiar hardi- 

 hood in that species of supporting such a variation without 

 extinction. Again, as the formation of the Lagoon limestone 

 must have been very slow, the depth (about half an inch) occu- 

 pied by an individual Bulla must represent a far longer period 

 than that of its life. 



In several localities soft masses of calcareous mud, a few 

 inches in diameter, and permeated with fossils perfectly pre- 

 served, occur in the Lagoon limestone. All the surrounding 

 rock must have been equally soft during its formation, and 

 when consolidation subsequently took place these small masses 

 were protected therefrom by being accidentally inclosed in a 

 rigid shell. This seems to be an exception to the generally 

 accepted theory that in all cases only a trifling depth of a sedi- 

 mentary limestone is at any moment unconsolidated during the 

 process of formation. 



In the latter part of this period a temporary interruption to 

 the deposition of material occurred, probably due to the same 

 cause as those in the former period. 



By the elevation which next ensued, the bottom of the lagoon 

 was uplifted, as will be shown, to a point at least thirty feet 

 above the sea-level. The violence and tension naturally accom- 



