6 St. J ago. paet i. 



the sea, measured from the upper line of junction with 

 the superincumbent basaltic lava, is about sixty . feet ; 

 and its thickness, although varying much from the 

 inequalities of the underlying formation, may be esti- 

 mated at about twenty feet. It consists of quite white 

 calcareous matter, partly composed of organic debris, 

 and partly of a substance which may be aptly compared 

 in appearance with mortar. Fragments of rock and 

 pebbles are scattered throughout this bed, often forming, 

 especially in the lower part, a conglomerate. Many 

 of the fragments of rock are whitewashed with a thin 

 coating of calcareous matter. At Quail Island, the 

 calcareous deposit is replaced in its lowest part by a 

 soft, brown, earthy tuff, full of Turritella3 ; this is 

 covered by a bed of pebbles, passing into sandstone, and 

 mixed with fragments of echini, claws of crabs, and 

 shells ; the oyster shells still adhering to the rock on 

 which they grew. Numerous white balls appearing like 

 pisolitic concretions, from the size of a walnut to that of 

 an apple, are embedded in this deposit ; they usually 

 have a small pebble in their centres. Although so like 

 concretions, a close examination convinced me that they 

 were Nulliporas, retaining their proper forms, but with 

 their surfaces slightly abraded : these bodies (plants as 

 they are now generally considered to be) exhibit under a 

 microscope of ordinary power, no traces of organisation 

 in their internal structure. Mr. George R. Sowerby 

 has been so good as to examine the shells which I col- 

 lected : there are fourteen species in a sufficiently perfect 

 condition for their characters to be made out with some 

 degree of certainty, and four which can be referred only 

 to their genera. Of the fourteen shells, of which a list 

 is given in the Appendix, eleven are recent species ; one, 

 though undescribed, is perhaps identical with a species 

 which I found living in the harbour of Porto Praya ; 



