the Colouring Matter of the Green-sand Formation. 37 



having passed into a higher state of oxidation ; whereas the 

 black-looking grains are met with in highly calcareous sand- 

 stone, where the texture is too firm to admit of the percola- 

 tion of water. From either kind of rock the green matter may 

 be obtained by washing with water and subsidence, since the 

 colouring matter subsides less readily than grains of quartz, and 

 more readily than calcareous and argillaceous substances. For 

 the purpose of analysis it is best procured from those calcareous 

 sandstones where the cement predominates, as in the neigh- 

 bourhood of Hythe and Folkstone in Kent. On reducing 

 such samples to powder, washing away the finer particles with 

 pure water, and separating any adhering carbonates by dilute 

 muriatic acid, the colouring matter is left, mixed only with 

 small grains of quartz. It then always appears in the form 

 of earthy particles of a deep green tint. 



The green matter, when not previously weathered, is very 

 feebly attacked by concentrated acids, even by the nitro-mu- 

 riatic. It gives out water when heated, and becomes brown 

 from its iron passing into the state of peroxide. As it has 

 been supposed to owe its green colour to the presence of phos- 

 phoric acid, it was carefully examined, with the view of de- 

 tecting that acid, if present. It was accordingly fused with 

 carbonate of soda, the alkaline filtered solution neutralized by 

 nitric acid, and evaporated to dryness, and the neutral so- 

 lution tested by nitrate of silver and nitrate of lead. Of two 

 samples of green-sand, thus examined, one was found to be 

 quite free from phosphoric acid, and traces only were detected 

 in the other. The former was also free from lime, and the 

 latter contained but a small portion. It is hence obvious, that 

 neither lime nor phosphoric acid are essential constituents of 

 the colouring matter of green-sand, and their presence must 

 be regarded as casual. 



In order to determine the chemical constitution of the co- 

 louring matter, I collected some green particles from the cal- 

 careous sand of Eastware-bay, near Folkstone, removing all 

 foreign matter as far as possible, by washing with water and 

 dilute acid. The only impurity which I could detect after 

 this treatment consisted of small grains of quartz, the quantity 

 of which varied in different samples. 



A portion of green particles thus purified, very free from 

 oxidation, and dried at 212° Fahr., lost 7*0 percent, of water 

 when heated to redness. 



Another portion of the same sample was fused with carbo- 

 nate of soda and the earthy ingredients subsequently separated 

 and weighed in the manner usual in such analyses. 



A third portion was heated with carbonate of baryta, and 



