172 FLGETZ ROCKS. 3. Its ftrudture becomes porphyritic when it con tains imbedded cryftals; as is the cafe with cer tain varieties *that contain cryftals of quartz, others that contain boracite ; and, laftly, in fome that include cryftals of arragon. In fomes places, as in Salzburg, Italy, and on the banks of the Wolga, mafles of fulphur, feveral hundred weight, are found imbedded in gypfum ; and in certain cafes the gypfum is mixed with limeftone, Pe- trifadlions rarely occur in gypfum. 4, Rock-falt appears to be peculiar to floetz-gyp- fum, or at leaft very nearly allied to it. It very rarely contains any metalliferous mixture. 5/ Itfeldom reaches to a great height, but it forms rocky and cliffy acclivities, which are particularly diftinguilhed by their white colour. It is alfo re markable for the number of funnel-fhaped hol lows it contains. Thefe are formed by the waffl ing away of fubjacent, and finking of fupcr- incumbent ftrata. 6. There are two principal and well-afcertained formations of gypfum, and feveral others, whofe geognoftic relations have not hitherto been well afcertained. 1. Firft Floetz-Gypfum. Erfter Floetz-Gyps.—-Werner. \ 1. Thisformationreftsonthe firft floetz-limeftone, and is covered by the variegated or fecond fand- ftonc
