64 
Geology of Sydney. 
When the piece of basalt is examined in the same 
way as the sandstone and granite,, a very decided 
difference is apparent. Few, if any, individual minerals 
can be seen without the help of a lens. The contrast 
in this respect with the other rocks is in itself 
characteristic. If a piece of basalt is polished, or if a 
thin slice is cut and examined under the microscope, 
quite a number of minerals are seen to make up the 
Fig. 17.— A thin slice of granite as seen under the microscope. 
The rectangular crystals are felspar, the clear mineral is 
quartz, and the shaded portions represent mica. 
mass, but quartz , which was so prominent in sand- 
stone and granite, is absent. One can hardly help 
noting the almond-shaped cavities common in basalt, 
and these resemble in more ways than one the steam 
cavities we find in lava. On comparing our specimen 
with a lava from Vesuvius or Mt. Etna, for example, 
the resemblance is so complete we at once conclude 
that basalt is a lava. When basalt is studied in the 
field, we learn that it flowed into the position it 
