Basalt. 
271 
There is a distinct advantage in a student pre- 
paring his own rock slices. For example, in grinding 
a slice of 'nasal t from the Bald Hills, Bathurst, long 
before the slice is thin enough for microscopic exaini- 
Fiu. 77. Remarkably Weathering of Sandstones, Maroubra Bay, 
near Sydney. 
nation the felspars will have become transparent, 
showing the beautiful streaming or flow structure so 
characteristic in this rock. Tire felspars are seen to 
have been caught as they floated in the molten magma. 
In flowing along, the groups part on meeting with an 
