141 



Lord Howe Island (several colours), bundle, Cooma, Bald 

 Hills (Byrock). 



3946 : silicified spherulitic rhyolite, now almost in the condition 

 of jasper. The rock has been replaced, molecule bj molecule,* 

 by silica, without losing its original structure. 



5568, Broken Hill, with malachite : the red jasper and the 

 green malachite form an effective combination. 



HtDROUS ISTON-CRTSTALLINE SILICA. 



Specimens of j^recious opal will be found in G-em Cases 48 and Opal. 

 78. Common opal is exhibited from Cowra, Gundagai, Balranald, 

 Wbitecliffs, Glen Innes, Tamworth, Barraba, and elsewhere. 



10111 and 3742, Whitecliffs Opal-field: showing the association 

 of common and precious opal. 



3979, 3981, Balranald : showing the transition, under atmos- 

 pheric agencies, of opal into opaque earthy silica : material like 

 this is liable to be taken for kaolin. 



Hyalite. — A transparent form of opal occurring somewhat like 

 drops of dew. It occurs principally in igneous rocks, e.g., the 

 specimens from Mullumbimby and Ben Lomond. 



9269 : a pretty blue variety from the capping of the Brown's 

 Creek Lode, Blayney. A specimen from the Gwydir Eiver 

 also exhibited. 



. Menilite. — An opaque liver-like variety occurring in the 

 gypsum-bearing marls near Paris (9702). 



Compact forms of silica, sometimes very impure, occurring in Flint- 

 Chert 

 limestones and othei* sedimentary rocks. Flint is very common 



in the Chalk Formation of Europe. It was used by prehistoric 



man for making weapons and implements, since it breaks w^ith 



sharp cutting edges. 



In addition to Chalk flints, there are specimens from Pambula, 

 "Wallsend, Capertee, South Australia, and New Zealand. 



* In Structure Case 113, is an instructive specimen from Bos'gabri (9040) showing a 

 spherulitic rhyolite that has luidergone partial replacement by chalcedony, a core or 

 comparatively unaltered rock still remaining'. 



