145 



Silicate o£ alumina and potash. Pseudomorphs o£ felspar Leucite. 

 after leucite ; Arkansas, 



Hydrous silicate of alumina and soda. Analcite. 



Until recently it was believed that analcite is always a secondary 

 mineral, but it is now known to be an original component of certain 

 igneous rocks. It has been found by means of the microscope in 

 some of the rocks round Sydney (see page 184). 



Silicate of alumina and soda. Small crystals only, in rocks from Nepheline. 

 Lue and the Snowy River. 



Silicate of alumina and soda with sodium carbonate; Maine. Cancrinite. 



Silicate of alumina and lime ; characterised by its crystalline Scapolite. 

 form, which gives essentially square outlines in cross section.* 



St. Lawrence County (New York), Perth (Canada), llussia, 

 Tasmania. 



Silicate of alumina, lime, &c. ; also characterised by essentially idocrase. 

 square sections. Kevv England, Tasmania. 



Silicate of alumina and soda with chloride of sodium ; a pretty Sodalite. 

 mineral, taking a good polish ; frequently blue in colour. 3597, 

 Dungannon, Ontario. Sodalite has recently been detected in 

 rocks from near Lue. 



Silicate of alumina, soda and lime with sulphide of lime. Lapis Lazuli. 

 Frequently of a deep blue colour ; a handsome, ornamental 

 stone. Chili ; Eussia. 



Zeolites. — In certain volcanic rocks, more particularly Case 97, 

 scoriaceous basalts, secondary minerals very frequently form as 

 a result of the chemical alteration of the rock. When cavities 

 exist, these secondary minerals have room in which to develop 

 their crystalline outline, and beautiful forms may result. The 

 greater numberf of the minerals formed in this way are hydrated 

 silicates of alumina, with potash, soda, or lime, and they are all 

 grouped together as zeolites.X 



Zeolites are very common in many altered basalts and allied 

 rocks in New South Wales. The Survey collections include a 

 good many of these, more particularly from the Northern 



* A good crystal of scaoolite will be found in case 33. 



t Carbonate of lime and various forms of silica, e.g., agate and precious opal, also occur 

 in this way. 



I This name refers to an apparent boiIin<r-up when zeolites are heated in the blow-piije 

 flame ; this is due to the large quantity of water they contain. 



