OBSER.VATI0NS ON BASALT, &C, 



problems. It will appear, that they flrikingly illuitrate the ana- 

 logy which exifts between the aqueous and igneous formations, 

 and (how that precifely the fame order and kind of arrangement 

 is followed, in the generation of ftony malles from water as 

 from fire; for, the change of ftructure, which I have obferved 

 to be the molt inexplicable part of the procefs by which glafs 

 paffes into ftone, is almoft exactly imitated in the formation of 

 Calcareous fta- calcareous ftalactites. Succeffive depofi lions of calcareous ear- 

 laftites : at firft bonate, form a ftalaclite, which at firft is fibrous. A continuance 

 Irregularly fpa- °^ tne P roce ^ s caufes the fibrous ftructure to difappear, and the 

 thofe, then fpar* ftalaclite becomes irregularly fpal hole. The irregularities then 

 vanifh, and it becomes perfect calcareous fpar,divifible into large 

 rhomboids, with the form peculiar to that mineral; and all the 

 gradations may be found in the fame fpecimen. Nor is this 

 charge confined to a few folitary fpecimens; for a confutable 

 Coaft near Sun- extent of eoaft. near Sunderland, is formed of a lime-fione eom- 

 derland fo form- pofed of radiated fpheroids, from half an inch to three inches 

 diameter, imperfectly united. When one of thefe fpheroids 

 attains fomething more than the ufual magnitude, it becomes 

 compact in the heart; and it is not unufual to difcover por- 

 tions of the rock, in winch the radii have entirely dilappeared, 

 and the whole mafs has become compact. It is probable that 

 the entire formation of oolithi and pifolithi is owing to the fame 

 caufe; and that they are prevented from ever arriving at great 

 fize, by the union of their furfaces, and their fubfequent con- 

 folidation into compact limeftone, into which they are conti- 

 / nuallv found to graduate. 



P fil - of Hitherto, I have felecled ir- fiances from fubfiances which 



doubtful origin, have an undiluted claim to an aqtixeoius origin. I (hall now, on 



fimilarly arrang- ^ authority of Dolomieu, waftance a fimilar arrangement, in a 



ed in its ftruc- J ..-,.., .. n 



ture. fubftance refpectmg the ong;n v; which theonfis are not agreed. 



A fpecies of petrofilex is found in the Val de Nido, in Corfica, 



which contains radiated peirofiliceous glands, from half a line 



to an inch in diameter. Thefe glandr only differ from the bans 



bv their radiated ftructure, and their colour; and appear to 



indicate very clearly, that the rock was fubjedied to a fpecies of 



arrangement which, if it had been completed, would have 



changed its nature, and probably would have rendered it por- 



phyritic; for Dolomieu obferves, that the centre of the glands 



was often occupied by a fmall cryftal of feldfpar.* The extra- 



* Polomieu. Journal de Phyfique, 1794, page 260. 



5 ordinary 



