278 
Geology of Sydney. 
polarity was induced in this by placing it between the 
poles of an electro-magnet, with the result that the 
needle of rock became sensitive to opposite poles of a 
magnet. It is too heavy to respond to the directive 
influence of the earth’s magnetism, but on subjecting 
the whole apparatus to a gentle vibratory motion, the 
bar will set north and south. 
Chemical Composition. 
The peculiar colour of the weathered basalt 
immediately around the freshrock suggested the deter- 
mination of its solubility in acids ; *5 gram of finely- 
powdered basalt was digested in dilute hydrochloric 
acid for two and a half hours, then it was heated with 
strong acid for one and a half hours, with the result 
that *218 gram remained insoluble. This gives 56*4% 
of the rock as soluble in hydrochloric acid. 
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 tempera- 
ture 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 
