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



does not readily slake spontaneously, like good mountain-lime, 

 and often requires to be mixed at once with water in order to 

 become reduced to powder. 



Silurian limestones, as well as limestones belonging to the 

 Caradoc sandstone, occur in Glamorganshire and Pembrokeshire. 

 An analysis of a Silurian limestone has been given already ; I 

 will now place on record the analysis of a limestone which 

 occurs in Pembrokeshire, and which by geologists would be 

 termed Caradoc sandstone : 



Composition of Caradoc Sandstone, Pembrokeshire. 



Water of combination 



Oxides of iron and alumina 



Carbonate of lime 



strontia 



magnesia 



Sand and some insoluble silicious matter 



100-17 



It is worthy of remark that stones called in common life sand- 

 stones, may, like the specimen of Caradoc sandstone, contain a 

 high percentage of carbonate of lime, and be well adapted for 

 burning into quicklime. This is actually the case with the 

 Caradoc sandstone ; it produces indeed in burning a valuable 

 lime, since it slakes readily and yields what is technically called 

 a tolerably fat lime. 



It will be seen that the analysis shows about 11 per cent, of 

 sand and insoluble silicious matter, which is more than the pro- 

 portion of insoluble matters in the preceding Lias limestones. 

 Notwithstanding this larger amount of insoluble silicious matter 

 in the Caradoc sandstone, it yields a better lime than the Lias 

 limestone, which circumstance is explained by the fact that the 

 insoluble matter in the latter occurs principally as clay, whilst 

 in the former it exists chiefly as sand. 



Apart from the practical interest that attaches to this analysis, 

 it is interesting in a mineralogical point of view, inasmuch as it 

 exhibits a considerable quantity of carbonate of strontia, a con- 

 stituent which so far as I know has not been noticed before in 

 Caradoc sandstone. 



In the preceding analyses the amount of carbonate of lime 

 and that of the ordinary impurities, such as magnesia, oxide of 

 iron, sand and clay, which accompany the former in agricul- 

 tural limestones, has been determined ; but with one exception no 

 notice has been taken of the small proportions of phosphoric 

 and sulphuric acid, which it is well known occur occasionally 

 in limestones. 



As phosphoric acid is a valuable fertilising constituent, it 



