234 
THE GEOLOGIST. 
volcanic rocks. From pseutloinorplis, we find Leucite is often con- 
verted into Sanidine. In this alteration the Si remains untouched, 
ajid the Al and alcalies are removed in combination as alcaline 
aluminates. 
25. Albite, Soda-felspar, Periclme [.333]. A very important felspar species. 
Composition Ka gi' + Al Si^ l^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. Nepheline ; ElaoUte [1.0]. Essentially a silicate of Al, iJa (13), 
K (o) with a small quantity of Ca. The wliite variety is Nepheline 
and the colom-ed Elwolite. They are not abundant minerals. 
27. RvAcoLiTE [.666]. A glassy felspar. A silicate of Al, Na (10.5), 
K (6), and Ca. 
28. Oligoclase, Sodorspodimene [.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 sUicate of Al, Na (6), 
Ca (5), and K (1). 
30. Spodumene, Triphane [.5]. Essentially a silicate of Li (4 — 7), and 
some Na. 
31. Petalite [.25] ; Castor. Essentially a silicate of ^1, Li (3), and 
Na (2.5). Castor is the crystallised variety found 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 Si. 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 Al anil 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. Sausswite 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. Hadyne. Sodalite is a silicate of Al, and Na combined 
with Na CI ; the proportion being about Al (32), N'a (25), and CI 
