J'j'CJ OBSERVATIONS ON BASALT, &C. 



total difappearance of the minute fieldfpar and hornblende of 

 of the bafalt ; inftead of which, the regenerated ftone contains 

 thin laminae of cryftals, which are probably augites. 

 Thcdivifion of J cannot leave this fubjeCt without noticing fome particulars, 

 buiar mafl"M°" ' n wn ' cn tr, e procefs of arrangement defcribed in the early part 

 during its de- of this letter, appears to yield a probable explanation of fome 

 XS fi ttefe£" of the P eculiarities of bafalt - The general diipofilion of bafalt 

 raation of radi- to divide into globular mafTes, in decomposing, is too re- 

 Setin her ° ldS markable a fa6i to have efraped the attention of naturalifrs; 

 though, as far as I am informed, no Satisfactory explication of 

 it has been given. The common effects of decomposition are 

 obvioufly inadequate ; for it is common to fee a large block of 

 amorphus bafalt feparate into numerous balls, after a few 

 months or years expofure to the weather ; and, rapid as the 

 procefs of decomposition has been in the intervening portions, 

 thefe balls refitt its father progrefs with uncommon obstinacy. 

 May not this be attributed to the formation of the radiated 

 fpheroids, whofe occurrence in my experiment I have already 

 mentioned? and may not their greater refiftance of weather 

 iimply arife from their aggregation being more perfect than 

 that of the incoherent molecules which have filled the intervals 

 between them ? Though the radiated ftruclure has difappear- 

 cd to the eye, thefe portions of the ftone retain the fupe- 

 riority of more perfect internal arrangement ; and, if my 

 pigmy experiments could yield fpheroids of two inches dia- 

 meter, there can be no difficulty in fuppofing that the grand 

 operations of nature may produce them of feveral feet. The 

 feparation of the decomposed fragments in concentric coats, 

 feems eafily explained; for I have already pointed out the fa- 

 cility with which the radii of the fpheroids feparated at nearly 

 the "fame distances from their centres, and the form of the 

 fragments which refulted, refembling fragments of bombs. * 

 md thefe may If this idea be not confidered as entirely diverted of plauhbi- 

 fmmztlhebl- ^f» * ma y venture to extend the fame principle, to account 

 (kick columns, for the wonderful regularity of the prifmatic configuration of 



* Even granite has been frequently obferved to affett globular 

 decompofition* and divifion into fragments of concentric coats. 

 This mode of decompofition extends to many fubftances, that Wer- 

 ner has called the formation it feems to indicate, " abgefonderte 

 Jlucke," which has been rendered in Englifh, dijiintt concretions. 



bafaltic 



