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



The colour of mountain limestone is various : some beds are 

 light-grey, others yellow, blue, dark-grey, almost black. The 

 dark-coloured specimens are often very dense, crystalline, hard, 

 and not unlike basalt. Many beds are full of shells, others 

 almost destitute of fossil remains. In short, the aspect and phy- 

 sical characters of mountain limestones, no less than their com- 

 position, vary considerably, as will be seen by inspecting the 

 subjoined analyses. In the first place, I shall notice the com- 

 position of an excellent light-coloured, compact mountain lime- 

 stone from South Wales, which in the following tabulated results is 

 grouped together with a limestone from Dartbridge, near Totnes, 

 on account of the similarity in composition which both exhibit. 



Composition of Mountain Limestone from South Wales, No. I. ; and 

 Limestone from Dartbridge Quarry, Totnes, No. II. 



No. I. No. II. 



Water -286 '49 



Oxides of iron and alumina . . . . "390 



Carbonate of lime 96-350 



magnesia 2'289 



Insoluble silicious matter '858 



93 



95-57 



85 



2-05 



100-173 99-85 



In both stones the percentage of carbonate of lime is high ; both 

 are compact, crystalline, and give, on burning, good lime. No. 1, 

 especially, is an excellent limestone, for the quicklime produced 

 from it swells up very much on slaking, and hardly contains 

 any silicious impurities. 



It is often the case that two quarries, not far removed from 

 each other, yield limestones very different in appearance and in 

 value. By comparing the analysis of the limestones from Dart- 

 bridge Quarry, situated in a slate district, with that of Haberton- 

 ford Quarry, and mentioned under Hydraulic limestones, the 

 reader will at once be struck with the great variations which the 

 stones from these two quarries exhibit. Differences equally 

 great will be observed on comparing the Dartbridge limestone 

 with the following analysis of a limestone from Petehole 

 Quarry, situated in a heavy clay district in the neighbourhood of 

 Totnes : 



Composition of Limestone from Petehole Quarry, near Totnes. 

 Organic matter (graphite), and a little water of combination 



Oxides of iron and alumina '. Jt: '$ 



Carbonate of lime 



Lime in a state of silicate , 



Sulphate of lime , , 



Magnesia in a state of silicate 



Insoluble silicious matter , 



100-00 



