2G2 On the Geology of the Key of Somhrero y W. I. 



These remnants were next overlaid with coarse sand from 

 the shore of the lagoon and the barrier-inlets, and thereby killed. 

 The greater quietness and depth of the water favored the abun- 

 dant growth of a few species of fragile shells, whose unbroken 

 condition proves that they grew where they now lie. 



This abundance then suddenly ceased and sand predominated 

 more and more in the deposits, most likely on account of the 

 increasing depth. The increasing compactness of the rock 

 towards the surface of the bed may be attributed to the gradual 

 closing of the barrier-inlets, the decreasing quantity of coarse 

 sand introduced through them, the less agitation of the waters 

 of the lagoon from without, and the finer comminution of the 

 mud formed upon its shores. 



This subsidence was terminated by a sudden elevation of the 

 central area again into shallow water, accompanied with a ten- 

 sion which perhaps produced the vertical fissures in the top of 

 the bed. The superficial networks of short fissures may, how- 

 ever, be due to contraction, if the surface was slightly elevated 

 above the sea-level and exposed to the sun and atmospheric 

 agents. 



Bed B. 



§ 12. Character. — This bed, more or less deeply eroded (espe- 

 cially towards the north), forms the greater part of the surface 

 of the northern level (Fig. 1). Along the S.W. side of the Key 

 its thickness varies from k\ to 7 feet, averaging 5 feet, but 

 increases to 7 or 8 towards the south, to 9£ towards the north 

 (wherever the original upper limit remains), and to from 7 to 9 

 feet towards the east. Before denudation its thickness was pro- 

 bably very great on the north end, but at present it thins out 

 northward. 



It is usually composed of two or three bands, though very 

 often nearly uniform. In the northern level the lowest band 

 or the whole bed exclusively consists of Madrepore limestone. 

 As we go southward we find the coral reef to end abruptly, or 



