234 



THE GEOLOGIST. 



volcanic rocks. From pseudomorphs, we find Leucite is often con- 

 verted into Sanidine. In this alteration the Bi remains untouched, 

 and the A\ and alcalies are removed in combination as alcaline 

 aluminates. 



25. Albite, Soda-felspar, Pericline [.333]. A very important felspar species. 



Composition Na Si^ + Al Si^ I^a (10) ; but almost always con- 

 taining some k. From the preponderance of Na, this felspar is 

 more liable to decompose than Orthoclase ; and is consequently 

 readily converted into Kaolin, the O.Q. increasing as decomposition 

 advances. 



26. Nephelinb ; Elmolite [1.0]. Essentially a silicate of Al, ISTa (15), 



k (5) with a small quantity of Ca. The white variety is Nepheline 

 and the coloured Mceolite. They are not abundant minerals. 



27. Ryacoltte [.666]. A glassy felspar. A silicate of Al, I^a (10.5), 



k (6), and Ca. 



28. Oligoclase, Soda-spodumene [.444]. Essentially a silicate of Al, 



Na (8), Ca (2 — 3), almost always some k. Like orthoclase and albite, 

 is decomposed into kaolin. An important felspar species. 



29. Andesine [.5]. A mineral resembling albite, with which it was 



formerly classed : it is found in the Andes. A silicate of Al, Na (6), 

 Ca (5), and K (1). 



30. Spodumene, Triphane [.5]. Essentially a silicate of Al, Li (4 — 7), and 



some Na. 



31. Petalite [.25] ; Castor. Essentially a silicate of Al, Li (3), and 



Na (2.5). Castor is the crystallised variety foimd in Elba, and differs 

 from Petalite in the absence of Na. 



32. Labradorite [.666] An important felspar species. Essentially a 



silicate of Al, Ca (11), and Na (4), perhaps also K, as it is generally 

 present. Iron is rarely absent. Distinguished from other felspars 

 by its great liability to decomposition ; which results either from the 

 total elimination of the Ca or a partial elimination of the S'i. Many 

 zeolites appear to be the products of the decomposition of Labradorite. 



33. Anorthite, Christianite [1.0], Saussurite [.8]. An important felspar. 



Normally a silicate of M and Ca (16), with small quantities of k, 

 Na, and generally some Mg. The amount of Ca varies greatly, and 

 diminishes as the alcalies and Mg increase. Saussurite is a mineral 

 of irregular composition, and is supposed to be an impure Anorthite 

 or Labradorite. It occurs in the rock Gabbro ; and Bischof considers 

 its unequal composition to be due to the effect of the decomposition 

 of the other minerals in that rock. 



34. SoDALiTE. Hacyne. Sodalitc is a silicate of Al, and Na combined 



with Na CI ; the proportion being about Al (32), Na (25), and CI 



