146 



Burgundy Pitch, and is such as is often found in the ligni- 

 form state. T have an example in a foreign specimen 

 mingled with Pitch-stone. 



Opals are chiefly Silex with from 5 to 10 per cent, of Water 5 

 and, under certain circumstances, are so operated upon, 

 that moisture and change of temperature more or less sud- 

 den seem to cause the various appearances ; and those that 

 become degraded below the Precious, are less and less pure 

 with various adulterations. Generally found in more or 

 less perfect Porphyry. 



Analysis of the Opals : 



Precious Opal. Common Opal. Semi-Opal. 

 Silica - - 90' - - 93-50 - - 43*50 



Oxide of Iron — - - POO - - 47*00 



Water - - 10- - - 5-00 - - 7'50 



Loss - - — -- 50-- 2-00 



100- 100-00 100-00 



