Geanite. 
289 
description. The granites around Bathurst are decom- 
posed in situ to a depth of 40 to 100 feet. Before the 
present water supply for Bathurst was provided, the 
town depended on wells sunk in this decomposed 
granite. The wells could usually be put down with a 
pick and shovel. The explanation is that the felspars 
of the granite had decomposed, leaving the quartz 
crystals intact, but with their cementing material 
removed. If we can picture a noble building of brick, 
the mortar and binding materials of which have been 
converted into a non-cohering pasty mass, we have the 
condition of granite when the felspathic constituents 
have been decomposed. The Wyalong goldfield is 
another illustration. Here the granite has decom- 
posed to a depth of quite one hundred feet. No 
crystal has been disturbed. The quartzes lie just 
where they were first formed. The mica plates are 
still where they were born, so to speak. But the 
felspar! It is there too, so altered, however, by 
disease as to be scarcely recognizable, even by those 
benevolent scientists who take a kindly interest in the 
family of the felspars. The optical properties that the 
physicist was so proud to develop give no intelligible 
response to the vibrations of ether. Cleavage faces 
have lost their lustre, and the strong square shoulders 
of the orthoclase are rounded and weakly. Thin 
sections of the mineral show their glassy clearness 
dimmed. The crystals are cloudy, and transmit light 
with difficulty, and the light that does struggle through 
has its undulating battalions travelling helter-skelter as 
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