PHYSICAL AND LITERARY. 29 



in<y premirpd, the difficulty is eafily fur- 

 mounted. Upon the expoilng of made 

 to the air, the clay in its compofiiion, 

 moiflened from time to time by the raia 

 and dews, gradually moulders away ; and 

 in confequence of this, the cohefion of 

 the whole mafs is at lalt deilroyed. Oi| 

 the other hand, when this marie is calci- 

 ned, the increafed cohefion of itji clay 

 locks in the calcarious particles, and, de- 

 nying admifEon to the water, fupports 

 the mafs againft the aclion of this fluid *, 

 Limeftone, on the contrary, and other 

 fubftances confiRing of calcaneus earth 

 or calcarious earth and fand compacled 

 into a folid mafs, undergo no change 

 from being expofed to the air ; becaula 

 none of their conftituent parts have their 



cohefion 



* Stone-marles, upon being expofed to the air, divide 

 fird into maffes of a confiierable halk. Thefe divi/idns 

 run lor the cioft part horizontally or perpeiidtcuiarly 

 with relpeiSl to the narnral fituation of the marie, U^oa 

 examining the divided farfaces, 1 find tlietn covered with 

 a thin coat of very fine chiy, with li:rle or no niix'ur* 

 of calcarious earth. Kencc it is evident, why the rcfo» 

 'lution begins at the parts where this clay Ii§s. 



