140 



88, Uralla: shows doubly-terminated crystals lying on and 

 partly embedded in orthoclase felspar. 



Amethyst. — Mexico, Cornwall, New Zealand, Victoria, Penn- 

 sylvania, "Wellington, Abercrombie Eiver, Sidmouth Yalley (near 

 O'Connell). The amethyst sometimes merely forms an envelope 

 to normal white quartz. This is the case with the Sidmouth 

 Valley specimen, 3793, and that from Oban in case 144. See- 

 also case 121. 



JRose Quartz. — South Dakota (also in case 119) ; Russia. 



Eisenkiesel. — A spotted variety of red quartz. 932, Eairfield. 



Paetiallt cetstallised silica. 

 Agate. — Banded chalcedony. Tamworth, Wee Waa, Single- 

 ton, Boggabri, Cowra, Narellan, Bowning. Lismore, Tweed 

 River, Cooma. Many of these have been cut and polished^ 

 and among them are some of considerable beauty. Agate 

 generally originates from the deposition of silica as successive 

 linings on the interior of cavities in lavas such as basalt and 

 rhyolite. This is notably the case with the Boggabri and 

 Tweed River agates. They therefore have the same shape as the 

 cavity in which they were formed. It is only when they are cut 

 across that the concentric or banded structure, to which agates 

 owe their beauty, is revealed.* 



113, Tamworth, fortification agate : so-called from the zig-zag: 

 form of the bands. 



Ordinary cliaJcedony is shown from Boggabri, Murwillumbah, 

 Tweed River, Ph. Tilbuster. Several specimens, notably 2593^ 

 Murwillumbah, show the characteristic mammillated form of 

 chalcedony well. 



2686, Richmond River : the variety known as " water-stone." 

 These are hollow, and can sometimes be seen to contain a liquid, 



8807, Angastan, S.A.: the spongy variety known as "float, 

 stone." 



Jasper. — An opaque variety. 



* The colouration of agates can be much modified bj" artificial treatment. 



