1 8 St. J ago. 



PART I. 



effects of a volcanic focus, bursting through deep-seated 

 beds of different niineralogical composition. The great 

 abundance of free silex in the trachytes of some coun- 

 tries (as described by Beudant in Hungary, and by 

 P. Scrope in the Panza Islands), perhaps solves the en- 

 quiry with respect to deep-seated beds of quartz ; and 

 we probably here see it answered, where the volcanic 

 action has invaded subjacent masses of limestone. One 

 is naturally led to conjecture in what state the now 

 earthy carbonate of lime existed, when ejected with the 

 intensely heated lava : from the extreme cellular ity of 

 the scorias on Eed Hill, the pressure cannot have been 

 great, and as most volcanic eruptions are accompanied 

 by the emission of large quantities of steam and other 

 gases, we here have the most favourable conditions, 

 according to the views at present entertained by 

 chemists, for the expulsion of the carbonic acid. 1 Has 

 the slow re-absorption of this gas, it may be asked, 

 given to the lime in the cells of the lava, that peculiar 

 fibrous structure, like that of an efflorescing salt ? 

 Finally, I may remark on the great contrast in appear- 

 ance between this earthy lime, which must have been 

 heated in a free atmosphere of steam and other gases, 

 with the white, crystalline, calcareous spar, produced by 

 a single thin sheet of lava (as at Quail Island) rolling 

 over similar earthy lime and the debris of organic 

 remains, at the bottom of a shallow sea. 



Signal Post Hill. — This hill has already been 



1 Whilst deep beneath the surface, the carbonate of lime was, I 

 presume, in a fluid state. Hutton, it is known, thought that all 

 amygdaloids were produced by drops of molten limestone floating 

 in the trap, like oil in water : this no doubt is erroneous, but if the 

 matter forming the summit of Eed Hill had been cooled under the 

 pressure of a moderately deep sea, or within the walls of a dike, 

 we should, in all probability, have had a trap rock associated with 

 large masses of compact, crystalline, calcareous spar, which, accord- 

 ing to the views entertained by many geologists, would have been 

 wrongly attributed to subsequent infiltration. 



