170 



while, and a portion of the carbonate of lime they contain is 

 deposited on the ceiling. In course of time this accumulates 

 after the manner of icicles, giving rise to the pendant form 

 known as stalactites. After hanging for some time the drops 

 of the water have increased to such a size that eventually a 

 portion drops to the floor, splashing out more or less. On the 

 evaporation of this w^ater, the remainder of the carbonate of lime 

 is deposited. In this way massive columns, known as stalagmites, 

 are built up. 



In case 134 stalactites and stalagmites from some of the 

 Limestone Caves of New South "Wales are mounted in a natural 

 position. 



Similar stalactites of iron-pyrites, ironstone, and manganese 

 wad are shown in case 113. Other specimens from the upper 

 levels of the Broken Hill Mines are contained in case 127. 



Travertine: A deposit of carbonate of lime, frequently pro- 

 duced by springs — the so-called "' petrifaction." It is commonly 

 the case that pieces of stick, leaves, shells, or other objects 

 become encased in these deposits. Kattai Eiver (case 113), 

 Manilla (case 111). 



Sinter : A deposit of silica produced by hot springs. This 

 differs from travertine principally in consisting of silica instead 

 of carbonate of lime. IS'ew Zealand, Yellowstone Park (case 

 113). 



Limonite : Hydrated brown oxide of iron is frequently deposited 

 from water, giving rise to hog-iron ore. 



In case 106 is a mass of stick, bark, and leaves, encrusted and 

 cemented together by hydrated oxide of iron, from the base of a 

 rock-fern, part of which was still growing, and part similarly 

 encrusted. This highly interesting specimen is from Mount 

 Victoria. 

 Concretions Carbonate of lime, ironstone, and other substances have a 



tendency to aggregate together in rocks, giving rise to more or 

 less spherical masses, often with a concentric banded formation ; 

 this is known as concretionary structure. Small specimens in. 

 case 113. 



