238 
THE GEOLOGIST. 
conversion into Chlorite and Steatite, the alcalies present in Tour- 
mahne are separated, and the Mg constantly increased. 
41. Garnet [1.0]. This mmeral, so widely variable in its components, is 
yet — unlike Tounnalme — remarkable for the regularity of its chemical 
relations, all the various rei>lacements of its constituents being in 
strict accordance with the doctrine of isomorphism. Its O.Q. is con- 
stant, and likewise the oxygen-ratio of il to B = 1 : 1, answering to 
the formula II^ Bi^ + Si Hi, where il is Ca, Fe, Mg, Mn ; and 
S, Al or Fe. The following examples will illustrate the common 
variations. The common (alumina-lime) garnet contauis principally 
Al (20), Ca (32), with ¥e (8). The noble {alumina-iron) garnet 
{Almandine), principally Fe (40), Al (20). The black (magnesian) 
variety Al (22), Mg (13), Fe (9), Mn (6), Ca (6). The manganese- 
garnet contains Mn (33), Al (18), (15). The iron-lime variety has 
Fe (31), Ca (28), Mg (7). Two other varieties (Pyrope and Uwaro- 
wite) contain respectively 9 per cent, and 23 per cent, of the oxide 
of Cliromium. Thus every important base, both protoxide and 
sesqui-oxide, which each in their characteristic varieties reach to a 
large per centage, in turn disappears, being replaced by their 
isomorphs. 
Like all minerals containing considerable quantities of Ca and Fe, 
the varieties of Garnet with these bases are very liable to decom- 
position. The principal pseudomorphs of Garnet are Chlorite, Ser- 
pentine, and Steatite. 
42. Idoorase, Vesuvian [1.0]. Like Garnet, to which Idocrase bears 
a great similarity, and with which it is frequently associated, the 
O.Q. is 1.0 ; but the oxygen-ratio of the II to E bases is probably 
different, or nearer 3 : 2. The mineral consists essentially of a 
silicate of Ca (33), Al (16), ¥e (7), and Mg (5), replacing each other, 
and with sometimes a Uttle Fe and Mn. 
43. CoRDiERiTE, lolite, Bichroit [.8] ; Finite; Fahlunite ; and others. 
Cordierite is not frequently met with : its essential constituents are 
a sihcate of Al (31), Mg (10), and Fe (8). The oxygen-ratio of the 
Jl and E bases is 2 : 3. This mineral is prmcipaUy important by 
being regarded by Haidinger, Bischof, and others as the initial 
member of a series of minerals, including Fahlunite and Finite, and 
ending in Mica. The alteration to wliich Cordierite is subject has one 
essential feature, — the elimination of the Mg, and the introduction of 
water and alcahes. Li one series of alterations the water only is 
introduced ; but in the other (wliich includes Fahlmiite and Finite) 
the alcalies come in, and attain the maximum in pinite, which has 
K (8). Ramnielsberg considers the ratio of oxygen in the Si and Al 
