BY H. C. RICHARDS. 203 



The sandstone is. believed by the writer to have 

 originally contained iron pyrites which in decomposing 

 has caused ferric sulphate and sulphuric acid to be formed 

 and through subsequent attack on the aluminous cementing 

 material, basic aluminium sulphate has resulted while the 

 ferric sulphate has been reduced to ferric oxide and hydrated 

 to form limonite. 



The sandstone is a friable, white sedimentary rock with 

 quartz grains rounded to subangular and averaging about 

 .2 mm. in diameter. The cementing medium is very 

 abundant and clayey in nature and when the stone is 

 moistened the characteristic clay odour is given off. 



The occurrence of alunogen as a decomposition product 

 irom iron pyrites in the presence of aluminous shales is 

 recorded by G. P. Merrill* so that its development under 

 the above conditions can be readily understood. The 

 aluminium sulphate woidd be gradually leached out to the 

 surface of the stone and deposited there as an incrustation 

 while the iron sesquioxide would stain the sandstone in the 

 immediate vicinity of its point of origin. In those places 

 where the pyrites crystals Avere large we may have the 

 nodules formed and Avhere only small grains of p^Tites 

 •existed Ave haA^e the specks of broAvn limonite. The 

 aluminium sulphate, being soluble, would be Avashed off 

 the surface of the stone by the rain unless protected from the 

 weather, so that one can understand the occurrence of the 

 material in the caA'e on the cliff face and in the back of 

 the small amphitheatres scooped out of the sandstone. 



In the caA'e the best protection has been afforded, also 

 'probably the leaching effect has been more pronounced 

 there owing to the downAvard Avarp of the bed abo\"e it giving 

 more drainage in that direction, so that the greatest thick- 

 ness of the incrustation therein is not unexpected. 



In order to ascertain the extent to which the sandstone 

 some distance from the surface was impregnated AAith sul- 

 phate material, a specimen of sandstone was obtained two 

 feet in from the surface for analysis. This sandstone was 

 analysed in duplicate b}^ Mr. R. Graff, B.Sc, and the 

 folowing results Avere obtained : 



*Non Metallic Minerals. New York, 1910 Ed., pp. 352, 358. 



