Intelligence and Miscellaneous Articles. 69 



its specific gravity is •2-86 ; it resembles petrosilex. Its composition 

 on an average of several analyses is as follows : — 



Silica 82-6 



Alumina ''^ 



Protoxide of iron 1'2 



Lime ^'^ 



Soda 4-2 



Potash 0'7 



Water 1'6 



The rock when reduced to powder still retains a certain quantity 

 of water which can only be got rid of at a red heat. In this cir- 

 cumstance, and its composition, this rock approaches the aquiferous 

 sranites and the petrosilex of China, of which analyses have been 

 liven by Ebelmen and Salvetat. In its transformation by the action 

 of the water it first acquires a very bright red colour, due to some 

 peroxide of iron; it then cracks as though it had undergone the 

 action of a very high temperature, and afterwards becomes white 

 and friable, acquires the appearance of pumice-stone, and consists 



onlv of pure silica. „ „„ ,. ji ..i 



Analyses of decomposed rocks gave 95-97. 98-00, 99-5, andlastly 

 100-00 'per cent, of sihca. according as their change advanced ine 

 siUca is usually snow-white, friable and porous, sometimes coloured 

 by iron and manganese. In the clefts of the rock the sihca is formed 

 in stalactites, consisting of concentric layers which indicate the re- 

 sult of a gradual deposit. This variety of silica is often covered with 

 crystals of sulphate of lime ; it sometimes also contains very small 

 crystals of quartz, a circumstance which may perhaps be appealed to 

 by those geologists who admit that quartz has been formed by 

 aqueous crystallization. The surface of the stalactiform silica is 

 often of a very fine green colour, owing to the presence of crypto - 

 gamic vegetation. Lastly, the silica is met with m a gelatinous 

 ttate constituting transparent masses which are often confounded 

 with glairine, or sometimes imbedding plants growing upon the 

 rocks, and forming by spontaneous desiccation a gray felt-like sub- 

 stance, which might readily be mistaken for pasteboard. 



llie silica is consequently derived from two different sources : in 

 one case it is due to the action of the water on the rock removing 

 all the other elements ; in the other it is deposited from the water 

 holding it in solution by the aid of a high temperature, just as hme 

 in incrusting waters is dissolved by carbonic acid. This view is con- 

 firmed by the fact that the porous silica is insoluble in weak alkaline 

 or acid solutions, whilst the stalactiform silica dissolves with the 

 greatest facility in these reagents. 



The water carries off the soluble matters, and deposits a reddish 

 mud ; this is attacked by muriatic acid, which removes aU the iron ; 

 the remainder consists of white plastic clay. This mud is com- 

 posed of — 



