CHAP. I. 



Calcareous Deposit altered by Heat. 



the two remaining species are unknown, and have been 

 described by Mr. Sowerby. Until the shells of this 

 Archipelago and of the neighbouring coasts are better 

 known, it would be rash to assert that even these two 

 latter shells are extinct. The number of species which 

 certainly belong to existing kinds, although few in 

 number, are sufficient to show that the deposit belongs 

 to a late tertiary period. From its mineralogical char- 

 acter, from the number and size of the embedded 

 fragments, and from the abundance of Patellae, and 

 other littoral shells, it is evident that the whole was 

 accumulated in a shallow sea, near an ancient coast-line. 

 Effects produced by the flowing of the superincum- 

 bent basaltic lava over the calcareous deposit. — These 

 effects are very curious. The calcareous matter is altered 

 to the depth of about a foot beneath the line of junction ; 

 and a most perfect gradation can be traced, from loosely 

 aggregated, small, particles of shells, corallines, and 

 Nulliporas, into a rock, in which not a trace of mechani- 

 cal origin can be discovered, even with a microscope. 

 Where the metamorphic change has been greatest, two 

 varieties occur. The first is a hard, compact, white, 

 fine grained rock, striped with a few parallel lines of 

 black volcanic particles, and resembling a sandstone, 

 but which, upon close examination, is seen to be crystal- 

 lised throughout, with the cleavages so perfect that they 

 can be readily measured by the reflecting goniometer. 

 In specimens, where the change has been less complete, 

 when moistened and examined under a strong lens, the 

 most interesting gradation can be traced, some of the 

 rounded particles retaining their proper forms, and others 

 insensibly melting into the granulo-crystalline paste. 

 The weathered surface of this stone, as is so frequently 

 the case with ordinary limestones, assumes a brick-red 

 colour. 



