On the Geology of the Key of So?nhrero, W. I. 253 



from and generally overhanging a ledge at the level of the sea, 

 continually washed by its rise and fall. The face of the cliff, 

 together with the greater part of the surface of the Key, is wea- 

 thered to a dark, slaty-blue color; the former being girdled (in 

 the southern and higher level) by two distinct lines of strati- 

 fication, as if where least denuded there were three beds. The 

 vertical veins of guano, generally colored reddish-brown, some- 

 times intersect the face of the cliff; but it is only by landing 

 upon the Key that they can be plainly distinguished, together 

 with the signs of denudation, — the fractured edges of the beds, 

 and general ruggedness of the surface, which pertain to its cha- 

 racter as an elevated reef. 



§. 3. — Ground-swells coming from N.N.W. to W.N.W. are fre- 

 quent during the winter season, especially from November to 

 March inclusive, and are often very heavy. 



There are two facts which prove that the agitation of the 

 water reaches to the bottom in such swells. 



First, That fish-pots, which are sunk on the bottom on the 

 leeward side of the Key, in thirteen fathoms water, have been 

 repeatedly carried away and lost on the sudden occurrence of a 

 swell. Secondly, That the lighters, which have been occa- 

 sionally swamped at their moorings by violent swells, are always 

 broken up and cannot be recovered. Another proof of the 

 friction upon the bottom may be the greenish tint which the 

 sea assumes during a ground-swell over the Sombrero bank, by 

 which it is said that the outline of the bank is quite perceptible 

 to an observer on a passing vessel, though not from the Key. 



§ 4. Earthquakes. — During the last five years several slight 

 shocks, rarely exceeding a tremor, have been felt on the Key 

 and in its vicinity. As usual in this region their local character 

 has been conspicuous, some having been confined to two islands 

 or only to one, even in the same bank. This peculiarity, and 

 the series of oscillations which seem to have affected the Som- 

 brero bank alone, may be illustrated by the following table of 

 the shocks which have been observed on the Key and at St. 



