STRUCTURE AND COMPOSITION OF A BASALT FROM BONDI. 223 



A fresh portion of the powder was ground to the finest possible 

 condition, in an agate mortar, moistened with water and just 

 covered with strong hydrochloric acid in a small watch glass. In 

 six hours the whole mass had gelatinised rigidly at the ordinary 

 temperature of the laboratory 65-70 F. The mass was just 

 covered with water and let stand for forty-eight hours. The 

 edges of the gelatinised silica then showed under the microscope, 

 numbers of cubes of sodium chloride — far in excess of what one 

 should expect, even if all the felspars were completely decomposed 

 by the acid. A drop of strong hydrochloric acid was placed on a 

 polished slab of the basalt. In two hours the acid had become 

 rigid and gelatinous and showed cubes of sodium chloride under the 

 microscope. A portion of the finely powdered rock was digested 

 for half an hour with acetic acid, hydrochloric acid added to the 

 filtered solution, and on evaporating on a glass slide cubes of 

 sodium chloride were formed in abundance. 



In order to guard against the possibility of the sodium chloride 

 being contained in the rock as an impurity, derived from its 

 proximity to the sea I determined the presence of chlorine in four 

 separate samples of the basalt, in the manner recommended by 

 Messrs. Fouque and Levy.* A gram of the basalt was powdered 

 and ground in an agate mortar, washed well on a filter with 

 boiling water, and then acted on by nitric acid. The acid was 

 evaporated to dryness to render the silica insoluble, and taken 

 up with water, on the addition of silver nitrate a precipitate of 

 chloride of silver forms immediately. 



There can be no question then as to the presence of chlorine 

 and some easily soluble soda bearing mineral in the Bondi basalt. 



The soda minerals found in basalt are — soda felspars, nepheline, 

 hauyne and nosean, and analcime as a secondary product. 



The soda felspars do not gelatinise with acids, and certainly are 

 not rapidly soluble in weak hydrochloric acid or acetic acid. So 



* Mineralogie Micrographique, p. 449. 



