ON THE USE OF LIME, MARL, AND SHELL-SAND. 



Composition of Oolitic Limestone from Downside, Brockley Combe, 

 near Bristol. 



99-10 



Composition of Oolitic Limestone, Combe Downs, Bath. 



Water -74 



Silica -85 



Protoxide of iron 1-04 



Alumina '61 



Carbonate of lime 95'74 



soda -71 



Magnesia -23 



Chloride of sodium '05 



99-97 



This stone was used in the restoration of Henry the Seventh's 

 Chapel, Westminster Abbey, but appears readily to decay, and is 

 decidedly inferior to Dundry stone, although it differs but little 

 from the latter in composition. 



b. Hydraulic Limestones. A hydraulic limestone is a stone 

 which, on burning, furnishes lime that sets under water. The 

 physical characters of hydraulic limestones vary exceedingly ; 

 their colour generally is dark blue or brown, more rarely yellow, 

 and, generally speaking, they are hard and compact. This de- 

 scription of limestone is always an impure form in which carbon- 

 ate of lime occurs in nature, and generally, though not always, 

 the most impure limestones produce the best hydraulic lime or 

 cement on burning. The impurities are chiefly magnesia, sand, 

 clay, silicate of lime, and alkaline silicates. Hydraulic lime- 

 stones are not confined to a particular geological formation, but 

 they abound in the lias formation. Lias lime, therefore, is com- 

 monly regarded as a bad agricultural lime, and for this reason 

 seldom used by farmers if another description of lime can be 

 procured at a reasonable price. Some silurian limestones also 

 form good cements after burning. 



In order to convey to the reader an idea of the composition of 

 a good hydraulic limestone, I give the analysis of a limestone 

 which occurs in the neighbourhood of Totnes. The specimen 

 which I analysed was forwarded to me by Mr. Watson, of 

 Dorsly, near Totnes, who informs me that it was taken from 



