4^3 



I know that Sir Janifs Hall has acquired mofi; defervcdly, great 

 credit for his fkilful management of his Laboratory, in fufing bafalt 

 v/ithout impairing its (tony appearance ; to difiFer from him, there- 

 fore upon a bafaltic fubjed, will, I fear, be deemed prefumptuous; 

 but it muft be obferved, that Sir James did not analyfe this foffil ; 

 he (as Mr. Playfair tells us) pronounced upon infpedion alone ; nor 

 has he pofTeffed the fame advantage that I enjoy, of viewing it in 

 its natural (late and arrangement. 



From thefe laft, ftrong arguments may be drawn to prove it to 

 be ba/alf and not fch'iftus, as Sir 'James thinks ; the ftrata of this 

 flone and of the contiguous bafalt are exadly Cmilar, and of equal 

 thicknefs (about 14 inches each ;) they are fometimes difpofed in 

 alternate flrata, fomedmes in accumulations of each fpecies fepa- 

 rately. 



The furfaces of the P ata of filicious, and of unqueftioned bafalt, 

 equally exhibit rude paves, and when quarried into, equally pro- 

 duce maffive prifms ; thofe in the filicious bafalt break into fmaller 

 prifms, moftly irregular pentagons of every fize down to great 

 minutenefs. 



Mr. Playfair fays, " The fpecimens he faw had nothing of a 

 " fparry or cryflallized ftrufturc." If by this he means they were 

 not of prifmatic form, I refer him to the cabinet of his friend Dr. 

 Hope, where he will find many of the fpecimens of this filicioui 

 bafalt, adlual prifms, as they might all have been but for the diffi- 

 culty of conveying large fpecimens fo far. 



Prifmatic conftruftion is utterly incompatible with the fiffile nature 

 of fchiftus, but it feems to be a property effential to bafalt ; I never 

 found this foffil in its original ftratum (and it is always ftratified) 

 that it was not difpofed in prifms, and although the great prifms 

 of the columnar bafalt do not break into fmaller, yet there are other 

 varieties which have a fubordinate principle of conllruction, the 

 large prifms breaking into fmaller, as in the bafalt dykes, and alfo 



(3 P 2 ) in 



