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The feldspar-like granules are generally compact, colored por- 

 tions of converted shells, having a crystalline form, and there are 

 aggregates of ferruginous and aluminous phosphates, arising from 

 the same kind of action on ferruginous matter, which, in the form 

 of a tine clay, or volcanic ash, has been brought within the 

 sphere of the action of the acid phosphates. The cavities some- 

 times present minute crystalline facets of phosphate of lime 

 crystals, while the capillary channels and pores, which give the 

 trachyte-like character, are really the passages through which the 

 carbonic acid and other gases escaped, during the transformation 

 of the organic matter, precisely as they occur in basalt and trap, 

 where igneous action has been supposed to have been influential. 



This rock is covered more or less by Atlantic guano rock, 

 presenting the variety which consists of compact, light-colored 

 phosphate of lime, containing about twenty parts in one hundred 

 of carbonate of lime, and in some parts is a consolidated shell- 

 bank ; the recent shells and coral fragments being visible. 

 Where, through time and favorable exposure, the bone remains 

 have thoroughly decomposed the shells, hand specimens would 

 be mistaken for the flesh-colored, massive phosphate of lime of 

 New Jersey. These more or less well-cemented and altered 

 rocks are also connected with still more recent deposits, retaining 

 even the odorous animal remains of oily acids ; and the whole 

 formation, above that of the trachytic form of rock, contains the 

 remains of infusoria. 



Thus a small island of the Atlantic, lying about eighteen 

 degrees north of the equator, presents us with an epitomized 

 succession of rock strata, formed from materials which, once 

 endowed with life, have served to nourish other living systems, 

 and then given rise to chemical changes, resulting in the produc- 

 tion of various mineral solids which remain. 



The trachyte-like rock forming the basis rock of this island, 

 theoretically, may have received its geological and chemical 

 characters in ocean water. A subsidence of the land, after its 

 surface had been deeply covered with organic remains, would 

 allow of that aqueous action of decomposition and cementation 

 which we notice, and the subsequent desiccation would explain 

 the natural divisions by rents. The formation of silicates of iron, 

 manganese, and alumina from phosphates of lime, is a mineral- 



