J 1Q OBSEKV ATjONi ON" BASALT, &C. 



ate always Separated from the large ones, and the divifions of 



the large ones are very irregular. * 



Thefededuc- Alter the Statement ol' my opinion, that perfect Similarity of 



tions do not iiructure may exiSt in the produces of aqueous and igneous 



fuppole other J ' ^ ° 



the aqueous or formation, it will hardly be neceflary to conclude thele obler- 



igneous forma- vations will) remarking, that I lhould not confider the eftab- 



tion of bafaltes ° 



tobe eftabli/hed.' im n)ent ol thele peculiar modes or arrangement as the 



tiighteft demonltration of the igneous origin of bafalt. It 



appears to me, that the truth of my deductions is entirely 



independent of either theory, and that, if ever the period 



fliouid arrive when the origin of bafalt fhall be determined 



by irrefragable demonstration, the inferences I have drawn 



may be accommodated with equal facility to either mode of 



agency. f 



The 



* The observations alluded to were made during the courfe of 

 laft fummer (1803), on the very numerous bafalt veins, or, as 

 they are there called, Whin Dykes, which traverfe the red fand- 

 ftone and red i'andftone breccia, which forms the greateft part of 

 the coaft of" the Firth of Clyde, between Greenock and the Largs. 



T Mr. Keir, in his paper on the crystallizations formed in glafs, 

 Suggefts the probability of bafaltic pillars being formed by the cryS- 

 tallization of vitreous lavas. See PhiloSophical Tfanfaclions for 

 1776, Vol.LXVI. page 530. 



Dolomieu was of opinion, that the prifmatic form was peculiar 

 to lavas which had flowed into the Sea; and he attributed it to the 

 fhrinking of the mafs : his description of the appearances exhibited 

 by what he calls the prifmatic lavas at the foot of Etna, merits 

 quotation. 



*' In the lavas of Etna, the form and dimenfions of the columns 

 vary as much as the manner in which they are grouped ; hexaedral 

 and pentaedral priSms are moft abundant ; then the tetraedral, the 

 triedral, heptaedral, and o&aedral. The lead I have feen are only 

 four inches diameter; others are more than three feet; they are 

 commonly of a lingle fhoot, which is fometimes 60 feet high ; 

 others are divided by articulations, which are from one to fix feet 

 aSunder. 



" 1 have more than once obferved a large column divide into 

 feveral fmaller in its upper part. The columns are generally larger 

 near the top than the bottom of the ftream of lava, becaufe they 

 Subdivide ; and they are always leaft in that part of the ftream of 

 Java which firft entered the water, the refrigeration being more 



prompt, 



