OBSERVATIONS ON BASALT, &C 177 



The immenfe magnitude of fome bafaltic columns, the ex- Other rocks aifo 

 treme regularity of their prifmatic configuration, and the j a u ^ nar ^ r ^." 

 peculiar ftructure of their articulations, have directed the at- 

 tention of naturalifts to them, much more than to any of the 

 oiher rocks which affect the columnar form. Yet many of 

 thefe are fufficiently remarkable to deferve more particular 

 notice than has generally been paid to them ; and they afford 

 moft illuftrative proof, that this configuration is not confined 

 to either the aqueous or igneous formation ; for fome lavas, 

 univerfally allowed to be fuch, are prifmatic* Columns of 



porphyry 



prompt, and its effefts more marked. Sometimes the columns are 

 placed perpendicularly fide by fide, and form vertical walls, which 

 are fometimes more than 100 feet high, and a league long; fome- 

 times they are heaped obliquely, horizontally, and in all pofitions. 

 Some, without being divided in their length, are larger at one end 

 than the other; and then they are arranged like wood piled up, 

 with all the fmall ends at one fide; fometimes they are formed into- 

 pyramidal bundles, by parting from a common center ; and, finally, 

 there are fome which, by their reunion, form large balls. Thefe 

 radii of lava, which are rather pyramidal than prifmatic, refemble 

 thofe of the globular pyrites, filiated from the center to the cir- 

 cumference, which are found in the chalk of Champagne. 



" On the fliore of la Trezza, near the mole, there is a very 

 curious group of little articulated prifms, which ifTue from a com- 

 mon center, and form fafciculi Angularly twifted. The articula- 

 tions are marked, but the fpecies of vertebrae do not feparate. In 

 the heart of the mountain on which ftands the Caftello di Jaci, 

 there are large balls, from two to four feet in diameter, refembling 

 in form the large pyrites in the chalk of Champagne. Thefe balls 

 of lava are formed of pyramidal columns, united by their points 

 in a common center." Catalogue des Laves de VEtna, page 453. 



The divifion of the upper part of bafaltic columns into feveral 

 fmaller ones, has atfo been obferved in the bafaltic columns of 

 Fairhead, by Dr. Richardfon. Sec Nicholfon's Journal, 4to. 

 Vol. V. page 321. 



* Almoft all the prifms at the foot of Etna, defcribcd by Dolo- 

 mieu, are of dubious origin ; moft of them are probably bafalt. 

 The columns of the Vincentine are of the fame fubltance, and fo 

 are the prifmatic lavas of Auvergne, and of the Vivarais. The 

 bed of lava at la Scala, near Portici, is divided by^vertical fi flu res, 

 which give it the afpecl: of irregular columns. At Aquapendente, 

 in a quarry of undoubted lava, near the road, are fome much more 



Vox.. X.— March, 1803, N perfect 



