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liincls— lavas with a petrofillceous bafe, and 

 lavas with a horn-ftone bafe. The former 

 hive a fmooth, and, fometimes, conchoidal 

 fradure, a fine and compact texture, and 

 contain a confiderable quantity of flicerls 

 and rhomboidal feltfpars. The glafs pro- 

 duced from them in the furnace is white, 

 with a complete fufion of the above-men- 

 tioned cryftalhzations. 



The lavas with a horn-flone bafe are 

 foft ; all emit an argillaceous fmell, have 

 an earthy texture, and, for the moft part, 

 abound in feltfpars, cryftallized fhoerls, and 

 black hexagonal micas. One of thefe lavas 

 bears the mod evident marks of having 

 flowed. It is full of fmall pores and vacui- 

 ties, not only on the furface but in the in- 

 ternal part. Thefc vacuities are of different 

 fizes, from thofe that can only be difcovered 

 with the microfcope, to thofe half an inch 

 in breadth. Many of them are oval, and 

 the longer diameter is almofl: always placed 

 in the fame diredion. This obfervation is of 

 confiderable importance ; firft, becaufe it 

 Q^ 2 fhovv'S 



