55 



New South Wales, will be found iu case 82. Large specimens 



of galena, cerussite, and pyromorphite are placed in case 46 



near the entrance. 



Lead minerals are "all very heavy, and can be in every case readily, 

 scratched with a knife-blade. 



Ce7nissite — carbonate of lead (PbCOs) : a colourless trans-, 

 parent mineral with a lustre approximating to that of the 

 diamond. Erom the Broken Hill Silver Mines have been 

 obtained in abundance some of the very finest specimens of 

 cerussite ever found; unfortunately, they ai'e very fragile, so 

 that it is very difficult to safely transport some of the more 

 delicate varieties. The beautiful arrow-headed twin crystals* are 

 particularly noteworthy (4107, 7585 ; Plate 11). 



I'is. 1 



Fis. 2 



7585, Block 11, Broken Hill, shows arrow-headed twins coated 

 with small crystals of anglesite — sulphate of lead. 



Crystallised cerussite is also found at Mount Costigan (see 

 case 125) and Lewis Ponds (8272), while massive cerussite is of 

 very common occurrence. 



5552, Broken Hill, admirably illustrates the replacement of 

 one mineral by another {pseudomorphisni) . The specimen now 

 consists of cerussite with cubical interspaces ; originally it was 

 lead sulphide (galena), but the action of water holding carbonic 



■'•• The formation of these twin crystals may be illustrated by cutting- out a piece of paper' 

 similar to A B C D (Fig-. 1), and then dividing- it along- the broken line A-I>. Hy turning- 

 the ADC portion completely round so that C comes to C (Fig-. 2), an arro-w-headed form ' 

 results. It must not be understood that such a rotation has actually taken place ;_but:the 

 molecules of one portion of the twin are reversed with regard to those of the other. , - - '"•' ^ 



