300 Dr Davy on the Cause of Induration 



On the Cause of Induration of some Siliceous Sandstones. By 

 John Davy, M D., F.R.S., London and Edinburgh, Inspec- 

 tor-General of Army Hospitals. Communicated by the 

 Author. 



There is a remarkable contrast between the sandstones of 

 the neighbourhood of Edinburgh and Glasgow and those of 

 " Scotland," — a hilly district so called in Barbadoes. Whilst 

 many indications denote that they belong to an analogous 

 formation, their character, as to induration, is widely differ- 

 ent. The siliceous sandstones of the neighbourhood of Edin- 

 burgh and Glasgow, owing to their firmness, and the mode- 

 rate degree of cohesion of their particles, are, as it is well 

 known, excellent building stones ; but most of those of the 

 district of this island mentioned, are unfit for such a purpose, 

 from the looseness of their texture, some of them actually 

 falling to pieces when immersed in water. When chemically 

 examined, however, no well-marked diifference is discovered 

 in their composition. In a crumbly siliceous sandstone, the 

 strata of which are nearly vertical, constituting the seaward 

 face of a singular hill in this island, called " Chalky Mount," 

 I have detected minute portions of alumine, lime, fixed alkali, 

 and phosphate of lime. In the fine-grained compact sand- 

 stone of Craigleith quarry, near Edinburgh, I have detected, 

 also, a very little carbonate of lime, and magnesia, and oxide 

 of iron, with a trace of phosphate of lime and organic matter. 

 The one stone, that of this island, disintegrates in water, ren- 

 dering it slightly turbid, falling to pieces, reduced to sand, as 

 the water penetrates between the grains ; and more rapidly 

 so when acted on by an acid. The other stone, after the ac- 

 tion of an acid, retains its original firmness unaltered. I 

 speak of the pure siliceous kind, such as I examined. 



On what does this difference depend ? 



When the two sandstones are reduced to powder or sand 

 (the more compact one is easily so reduced by gentle attri- 

 tion under water), and they are placed under the microscope. 



