290 
Geology of Sydney. 
they will, no molecular power remaining to order 
“halt,” “dress,” or “forward.” The crystals in their 
debility and decay no longer send the pulsating 
ether along by companies and columns, or in single 
file, as was their wont. This is only another way of 
saying that the decomposed felspar crystals cannot 
now polarise light, and have lost their doubly refract- 
ing properties. 
When thin sections of granite are examined with 
the microscope, under high powers, the quartz crystals 
are found to contain liquid cavities. Often a gas 
bubble is seen floating in the imprisoned liquid. 
These bubbles are supposed to be carbon dioxide — a 
gas that is particularly active in attacking felspars 
and converting them into kaolin. This gas was 
caught up when the granite was formed, and it is 
thought that the felspars also had their share, but it 
has long since united with some of the elements of 
the felspars, thus weakening their constitution. 
Dolomieu's remark has therefore a great deal of 
truth in it. Granites do suffer from la malaclie as he 
surmised, but it was reserved for the petrologist, 
with his microscope, to discover that the disease was 
internal, and it may be added incurable. The sketch 
shown on page 254 is fairly typical of the boulder- 
crowned hills that characterize granite country. It 
may be noted here that these weathered boulders are 
not to be compared with the perched blocks due to 
glacial action. The boulders we are dealing with are 
not transported or carried from a distance, as is the 
