171 



la case 109 are several large specimens of concretionary iron- 

 stone from New South "Wales. Also a concretionary mass of 

 chert — an impure form of silica often found forming bands in 

 sedimentary rocks- — ^in which the original planes of bedding may 

 still be traced. 



In which rocks such as ironstone, bauxite, and limestone are pisolitic 

 made ixp of an aggregation of pea-shaped grains. ™° ^^^' 



Pisolitic ironstone from Ph, Bumballa (Wingello District), 



in case 109. 



Similar to pisolitic, but the component grains are much Oolitic 



siimcturG 

 smaller. Oolitic limestones will be found among the sedimentary 



rocks. 



The replacement of one mineral substance by another is of Pseudo- 

 very common occurrence, the replacing substance being generally 

 silica {silicification') . Case 113 contains a number of very 

 interesting specimens of this (see page 142). 



Coral replaced by chalcedony , also case 111. 



Coral replaced by phosphate of lime (see page 128). 



Limestone replaced by silica {Beehite structure). 



Ehyolite replaced by chalcedony (see page 141) . 



Wood, fossil shells, and various minerals replaced by some 

 form of silica. 



Calcite replaced by quartz, which still shows the cleavage 

 structure of the original mineral. 



Chalybite replaced by Umonite — a very common form of 

 alteration. 



Quartz-Porphyry, the base of which has been completely re- 

 placed by topaz. 



Thinolite, a peculiar pseudomorph of carbonate of lime, after 

 some other mineral. Specimens from Jervis Bay, Singleton, and 

 Mount Vincent. 



Case 112, Lava; New Hebrides, Europe. Volcanic 



rocks. 

 Bomhs. — New Zealand (recent eruptions) : Erance (eruptions 



of Tertiary age) : Wollongong, in tuE of Permo- Carboniferous 



age (also in case 111). 



