( 103 ) 



lines, and the enamel into which It is 

 changed in the furnace has the opacity and 

 blacknefs of pitch, and its magnetifm is 

 greater than that of the lava before fufion. 



The beds of tufa, llkewlfe, do not eflen- 

 tially differ from each other. They are an 

 ill-kneaded mixture of lumps of argillaceous 

 earth, more or lefs tindured with yellow 

 oxyde of iron, which earth is eafily pulve- 

 rable. With it are mixed numerous fhoeils, 

 that from the foftnefs of the bafe may be 

 feparated entire, which they fcarcely ever 

 can be in the lava. Notwithftanding, how- 

 ever, the facility with which they may be 

 detached, it is difficult to determine their 

 cryftallization, not merely from their ex- 

 treme minute nefs, for fome of them are two 

 lines In length, but from their being fcarcely 

 ever found fmgJe, almoft every one being a 

 group of aggregated {hoerls. When one, 

 however, is found fmgle and detached, it 

 appears to be an hexagonal prifra terminated 

 by two trihedral pyramids. They are black, 

 fliining in the recent fradures, and are fome- 

 H 4 what 



