ON THE USE OF LIME, MARL, AND SHELL-SAND. 9 



thus showing that it is not merely the presence of certain impu- 

 rities, but also the state of combination in which they occur, 

 which renders one limestone better adapted for the making of 

 cement than another. 



c. Agricultural Limestones. For agricultural purposes all 

 limestones are applicable which contain above 60 per cent, of 

 lime and are free from impurities, the presence of which in lime- 

 stones, often much richer in lime, has the effect of yielding a 

 lime, after burning, that sets under water. 



The comparative value of different limestones varies exceed- 

 ingly, since some kinds produce, on burning, products that slake 

 with difficulty, and are, in comparison with others, poor in lime ; 

 whilst some when burned in the kiln readily slake and furnish 

 a voluminous minutely-subdivided powder rich in lime. These 

 differences are due to natural variations in the composition of 

 limestones from different geological formations, as well as to 

 variations in the composition of limestones of the different beds 

 of one and the same formation. Generally speaking, the higher 

 the percentage of carbonate of lime, and the smaller the amount 

 of foreign matters, which a limestone naturally contains, the 

 better it is adapted for burning. However, the physical cha- 

 racter, no less than the chemical composition, of limestones, has 

 to be regarded in estimating their fitness for burning. The hard, 

 compact, crystalline kinds generally produce a better lime other 

 circumstances being equal than softer, more friable, and lighter 

 specimens. There are, indeed, some limestones, rich in car- 

 bonate of lime, which, on account of their slight cohesive power 

 and porosity, cannot be economically employed for the making 

 of quicklime. But, on the whole, the composition of limestones 

 affords excellent indications as to their fitness for burning into 

 quicklime. 1 shall, therefore, consider in detail the composition 

 of several specimens of limestones from various localities in the 

 West of England. 



The limestones which I had an opportunity of examining 

 geologically are distinguished as Carboniferous, or mountain lime- 

 stone ; Lias limestone ; Silurian limestone ; Caradoc sandstone ; 

 Oolitic limestone. 



Mountain limestone occurs in great perfection in South Wales 

 and in Devonshire. In the last-mentioned county it often forms 

 isolated hills of very limited extent, and as the soils in the neigh- 

 bourhood are peculiarly destitute of lime, these mountain lime- 

 stone hills are considered a great boon to the agricultural 

 community. 



Mountain limestone also is found in Somersetshire, where it 

 and lias lime are much employed for agricultural purposes. 



