142 



Pseudomorphs. Silica is very frequently found replacing other substances, the 

 replacement being sometimes so gradual that the structure of 

 the original may be perfectly preserved. This is frequently the 

 case with wood, the cell-structure being sometimes almost as 

 perfect as in the wood itself. It is well shown in some of the 

 specimens chosen for exhibition. Silicified, or petrified wood, 

 as it is often called, is of very common occurrence in New South 

 Wales. 



8699, Schneebei'g, Saxony : quartz pseudomorphous after 

 harytes. 



8654, Schneeberg, Saxony : quartz pseudomorphous after 

 calcite. 



3191, in case 119, Morida : coral replaced by chalcedony. 



Numerous other cases of replacement by silica will be found in 

 Structure case 113. 



GROUP 11. 



95. Minerals consisting largely of silicates of alumina with 



potash, soda, or lime. 



Felspae. — The great group of minerals known as felspars is of 

 the very greatest importance in the composition of igneous rocks. 

 The group comprises a great many varieties differing in chemical 

 composition, crystallographic form, and even in physical properties. 

 They all contain silica and alumina ; and a convenient classifica- 

 tion is that which depends upon what other silicates are present. 

 Thus we have 



Ortlioclase '■ — Silica, alumina, potash. 



Alhite : — Silica, alumina, soda. 



Oligoclase : — Silica, alumina, soda, lime. 



Labradorite: — Silica, alumina, lime, soda. 



Anorthite : — Silica, alumina, lime. 

 They all agree in readily cleaving into little flakes, and in being 

 just scratched with a good knife. 



Orthoclase. This is found in granites and syenites, or veins traversing 



them, in porphyries, and in rhyolite. It is usually pink or creamy 



