( S4 ) 



agree. This lava contains a multitude of 

 zeolitiG globules, from half a line to an inch 

 in diameter. On breaking them, a Vacuity 

 is found within them, thus forming geodes 

 of a cryftallization more ot lefs perfects 

 Wherever the zeolitic fiibftance ha^ been too 

 confined in the cavity of the lava, the cryf- 

 tallization is extremely imperfect, in con- 

 fequence of the prifms being half-formed 

 and confufedly intermingled ; but where that 

 fubftance had a larger fpace to develop it- 

 felf ia, the prifms are no longer fo indif- 

 tin£t ; many of them, at leaft, are found to 

 incline to a tetrahedral figure ; and where 

 the cavities of the lava have afforded a ftill 

 greater fpace to the zeolitic matter, it has 

 cryftallized in tetrahedral prifms, of a dif- 

 tin(fi conformation. Every prifm has there- 

 fore four faces diftindly feparated. In fome 

 few places thefe prifms are terminated by a 

 tetrahedral pyramid. Many of them are 

 of a milky whitenefs, and thefe are femi- 

 tranfparentj but others have a tranfparency 

 almoft equal to that of quartzofe cryftals, 



A fmgle 



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