chap. i. . Calcareous Deposit altered by Heat. 9 



basis to a breccia, including small pieces of black, glossy 

 scoriae. A little above this, where the lime is less abun- 

 dant, and the lava more compact, numerous little balls, 

 composed of spicula of calcareous spar, radiating from 

 common centres, occupy the interstices. In one part of 

 Quail Island, the lime has thus been crystallised by the 

 heat of the superincumbent lava, where it is only 

 thirteen feet in thickness ; nor had the lava been origi- 

 nally thicker, and since reduced by degradation, as could 

 be told from the degree of cellularity of its surface. I 

 have already observed that the sea must have been 

 shallow in which the calcareous deposit was accumulated. 

 In this case, therefore, the carbonic acid gas has been 

 retained under a pressure, insignificant compared with 

 that (a column of water, 1708 feet in height) originally 

 supposed by Sir James Hall to be requisite for this 

 end : but since his experiments, it has been discovered 

 that pressure has less to do with the retention of carbonic 

 acid gas, than the nature of the circumjacent atmo- 

 sphere ; and hence, as is stated to be the case by Mr. 

 Faraday, 1 masses of limestone are sometimes fused and 

 crystallised even in common lime-kilns. Carbonate of 

 lime can be heated to almost any degree, according to 

 Faraday, in an atmosphere of carbonic acid gas, without 

 being decomposed ; and Gay-Lussac found that frag- 

 ments of limestone, placed in a tube and heated to a 

 degree, not sufficient by itself to cause their decomposi- 

 tion, yet immediately evolved their carbonic acid, when 

 a stream of common air or steam was passed over them : 

 Gay-Lussac attributes this to the mechanical displace- 

 ment of the nascent carbonic acid gas. The calcareous 



1 I am much indebted to Mr. E. W. Brayley in having given me 

 the following references to papers on this subject: Faraday, in the 

 ' Edinburgh New Philosophical Journal,' vol. xv. p. 398; Gay-Lussac, 

 in ' Annales de Chem. et Phys.,' torn, lxiii. p. 219, translated in the 

 ' London and Edinburgh Philosophical Magazine,' vol. x. p. 496. 



