112 



Reply. That 

 the defcriptions 

 of lavas by Spal- 

 lanzani and 

 others, are in- 

 correct as to the 

 appearance and 

 the matter itfelf j 

 and that the 

 aqueous origin 

 of' bifalt is de- 

 due", ble from fi- 

 milarityof ftruc- 

 ture in other 

 foilils l'o formed. 



P. Theprefence 

 of pyrites urged 

 as the proof of 

 igneous produc- 

 tion; 



but thefe are 

 very abundant 

 among matters 

 never fubje&ed 

 to fire. 



P. Infulated 

 maffes of indu- 

 rated fandftone 

 in whin, urged 

 as a proof of ig- 

 neous fufion. 



ROCKS OF THE TR A PP-FORM ATlON, 



ralization, every maxim of found philofophy requires that the 

 bataltie ftructure, in all cafes, Ihould be afcribed to it."" The 

 accounts given by Spallanzani and other travellers of columnar 

 lava, are mod evidently incorrect ; the appearances they have 

 defcribed are accidental rents, fuch as we obferve in fandftone, 

 limeftone, and even in bafalt. Many of the prifmatic lavas 

 defcribed by Spallanzani, are either bafalt, greenftone, or por- 

 phyry. That bafalt columns have been formed by chryftalliza- 

 tion from a ftate of folution in water, is rendered probable from 

 the analogy of their ftructure with that of other foffils which 

 have been certainly fo formed. Bafalt we know occurs in the 

 fame mafs in tabular and columnar diitincTt concretions, corres- 

 ponding in this particular with the lamellar and columnar dif- 

 tin<5t concretions of truly chryftalline foffils*. 



At fe&ion 66 we are informed, " That a mark of fufion, or 

 at leaft of the operation of heat, which whinftone poffefies in 

 common with many other foffils, is its being penetrated with 

 pyrites, a fubftance, as has been already remarked, that is of 

 all others moft exclufively the production of fire." 



Mr. Playfair probably did not know that the great beds of 

 bituminated wood which are found covered with fand, clay, &c. 

 in the alluvial hills, are very frequently much impregnated 

 with pyrites ; and that lately Mr. Hatchett had alfo demon- 

 ftrated its formation in the humid way. 



He continues, " Another mark of fufion, more diftinclive of 

 whin, is, that both in veins and maifes it fometimes includes 

 pieces of fandftone, or of the other contiguous ftrata, com- 

 pletely infulated, and having the appearance of fragments of 

 rock floating in a fluid fufficiently denfe and ponderous to fuf- 

 tain their weight. Though thefe fragments have been too re- 

 fractory to be reduced into fufion themfelves, they have not 

 remained intirely unchanged ; but are, in general, extremely 

 indurated, in comparifon of the rock from which they appear 



cient proof of the exiftence of quantities of inflammable matter, 

 equal to the fupport of all the volcanoes that have, or can exift oa 

 our globe.' 



* When bafalt, having a columnar ftructure, occurs in veins, we 

 find that thefe columns are perpendicular to the walls of the vein» 

 This phenomena I have frequently obferved, and confider it as fnp- 

 porting in fome degree the opinion mentioned above. 



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