[ 5 2 3 [Book VI. 



CHAP. X. 



CALCAREOUS EARTHS. 



Chalk. LimeJ}ene.*Marlle. Marie. Calcareous Spar. Iceland 

 Crjtftab, P etr if actions. ^-P arts of Animals found in Marie, &V. 

 Gyffoms. Great Varieties. Alabafter* Fibrous Stone. -^Mineral 

 Glafs. Selcnite. Gypfeous Spar. Plaifter of Paris ; how pre- 

 pared. Fufeble or Derbyjhire Spar. Spars, how formed. Beauti- 

 ful Appearances in different Caverns. Metallic C onibi nations ivitk 

 calcareous Earth. 



CALCAREOUS earth is fometimes found in 

 the form of powder, but more frequently in that 

 of a concrete fubftance called chalk, which differs with 

 refpeft to the finenefs of its particles and firmnefs btf 

 texture. 



i. Chalk confifts of calcareous earth or lime, united 

 with, carbonic acid, and an union of the fame princi- 

 ples allb conftitutes lirr.eftone and marble. Thefe 

 fubftances only differ, from common chalk in their de- 

 gree of purity, or in the manner of their aggregation, 

 admitting of more or lefs polifh. The different co- 

 loured veins in marble are produced by the admixture 

 jaf other iubftances, (moft commonly iron) unequally 

 diftributed through the mafs. 



a. Strata of marie alfo contain calcareous earth, more 

 or lefs. blended with a confjderable proportion of clajr 

 and fand. 



3, Calcareous earth is often found projecting into 

 the incerftices and crevices of rocks in a cryftallized 

 ftate, and is then s called calcareous fpar. It is more 

 or lefs tranfparent, and fliivers into flat fragments of a 

 rhomboidal figure. One variety is called Iceland cryf- 



tals. 



