FLOOR.] 
MINERALS. 
197 
remainsconstant, while the isomorphouselementsunder that type replace £) 1V . IV. 
each other in unlimited variety. Among the familiar forms of this mineral 
group, the violet-tinted Almandine, and the rich red Syriam Garnet are 
ferrous-aluminic varieties; the yellow and hyacinthine Garnets, known as 
Cinnamon-stone and Essonite contain calcium and aluminium : the 
calcium is replaced by magnesium in the deep blood-red Bohemian 
Garnet and Pyrope, varieties which when cut en cabochon are the 
Carbuncle of jewellery. Idocrase, a mineral with a smaller range of 
chemical variation than the Garnet, is represented by a series of 
crystals (of pyramidal forms) of remarkable variety and perfection. 
Epidote is also well represented by specimens from Ala and from ^ ^ 
the Obersulzbachthal. To the Epidote group also belong the minerals 
Allanite, Zoisite, and Jadeite. To these succeed the various mine- dSe ' 
rals, Phlogopite, Biotite, Muscovite, Lepidolite, &c, included in a Cates 41 
group under the name of Mica. The group of Felspars follows, among and 45. 
which will be found Labradorite, with its beautiful play of colours ; Case 43 (ii.) 
the Moonstone, a partially decomposed Orthoclase; a fine specimen of Cage ^ ,j > 
the Orthoclase called " Valencianite," from Mexico'; also, fine speci- 
mens of Amazonite and other varieties of microciine felspar. 
Dichroite (the Sapphire d'Eau of jewellers) is remarkable for its 
pleiochroism, a character due to the different degrees in which the 
crystal absorbs the light of different colours according to the planes 
of their vibration ; the crystal when looked through perpendicularly 
to the basal face 001 is of a rich blue, perpendicularly to the faces 
010 and 100 it is of a bluish white, and of a pale straw colour respec- 
tively. The Beryl includes the Emerald, and also the Aquamarine of 
the jewellers, and with Euclase occupies half of Case 46. It is an 
alumino-glucinum silicate, the Aluminium being in the Emerald 
apparently displaced to a minute amount by Chromium. Euclase 
is a mineral composed of the same elements, and containing a small 
quantity of water: the specimens of it from Siberia are of high inte- 
rest. These are followed by hydrated silicates, including a very cases 47 to 
complete collection of the Zeolites, among which the Natrolite from 49. 
India, the Scolecite from Iceland and India, and the Edingtonite 
from Scotland are remarkable. 
The silicates proper are succeeded, in Case 50, by minerals in which Case 50. 
silicates are associated with boric-oxide or borates. Among these 
the Tourmalines present a rich assortment of valuable and beautiful 
specimens, conspicuous for crystals of Rubellite, from Siberia and 
Ava. Two very fine specimens of the Rubellite from the latter 
country are seen in this Case. The one remarkable for its magnitude 
and form was brought from Ava by Colonel Symes, to whom it 
was a present from the King of Ava. The other, also a very large 
specimen, and of deep colour, was presented in 1869 by C. S. J. L. 
Guthrie, Esq. These are succeeded, in Case 51, by a class of minerals (\ lse 5^ 
of great mineralogical interest, containing some of the rarest of the 
elements, and themselves of rarity ; much uncertainty, however, still 
attaches to the chemical formula? of several of these species. The 
titanates, the tantalates, and niobates, and these combined with silicates 
