Chap. 10.] Iceland Cryjlals. 53 



tals. They engaged the attention of Sir Ifaac Newton, 

 by their remarkable quality of refracting the rays of 

 light without feparating them into colours, fo as to 

 make a line drawn on paper appear double, when 

 viewed through them. The Englifh lead-mines are 

 full of fpars ; their fhape is in general a firm column, 

 terminated at each end with a pyramid. Of thefe 

 calcareous fpars there are many varieties. 



4. Calcareous earth appears in the form of ani- 

 mal and vegetable fubftances, petrified into Hone by 

 being expofed to petrifying waters. Thefe fill up the 

 pores of the fubftance with calcareous earth, and 

 incruft them. Hence we may conclude, that this earth 

 is foluble in water, and is depofited in certain circum- 

 ftances. The quantity of earth, however, contained 

 in the water is very fmall, and therefore the petrifac- 

 fadlions are formed flowly. Thofe organic bodies, 

 which refift putrefa<5Hon mod, are frequently found 

 petrified, fuch as bones, fliells, and the harder kinds of 

 wood} on the contrary, the foft parts of animals, 

 which are very fubjecl: to putrify, are fcarcely ever 

 found petrified. Mr. Kirwan remarks, that petrifac- 

 tions are moil commonly found in ftrata of marie, 

 chalk,- lime-ftone, or clay; feldom in fand-ftone, ftill 

 more rarely in gypfum, but never in gneifs, granite, 

 bafaltes^or fhoerl; they fometimes occur among pyrites . 

 and ores of iron, copper, and filver, and almoft always 

 confift of the fpecies of earth, fcone, or other mineral, 

 which immediately furrounds them. Thofe of fhells 

 are generally found neareft the furface of the earth, 

 thofe of fifh deeper, and thofe of wood deepeft. A 

 very remarkable circumftance is, that petrifactions are 

 found in climates where their originals 'could not have 

 exifted. From the gradual and infenfiJDle concretion 

 E 3 of 



