FLOOR.] 
MINERALS. 
55 
while the isoraorphous elements under that type replace each other in 
unlimited variety. Among the familiar forms of this mineral group, the 
violet-tinted Almandine, and the rich red Syriam Garnet, are ferrpus- 
aluminic varieties ; the yellow and hyacinthine Garnets, known as 
Cinnamon-stone and Essonite contain calcium and aluminium : the 
calcium is substituted by magnesium in the deep blood-red Bohemian 
Garnet and Pyrope, varieties which when cut en cahochon, i.e., with 
a concavo-convex, or with a convex form, 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 Case 42. 
by specimens from Ala and from the Obersulzbachthal. To the Ca.se 44. 
Epidote group also belong the minerals Allanite, Zoisite, and Jadeite. 
To these succeed the various minerals, Phlogopite, Biotite, Musco- Cases 44 
vite, Lepidolite, &c., included in a group under the name of Mica. ^^^^ '^^^^ 
The group of Felspars follows, among which will be found Labradorite, ^'"^^^ '^^ ^^^'^ 
with its beautiful play of colours ; the Moonstone, a partially decom- 
posed Orthoclase ; and a fine specimen of the Orthoclase called ^'^^^^ (^•) 
" Valencianite " from Mexico. Dichroite (the Sapphire d'Eau of 
jewellers), is remarkable for its pleiochroism, a character due to the diffe- 
rent degrees to 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, per- 
pendicularly to the faces 010 and 100 it is of a bluish white, and of a 
pale straw colour respectively. The Beryl includes the Emerald, and 
also the Aquamarine of the jewellers, and with Euclase occupies half 
of Case 46. It is an alumino-gluciuum silicate, the Aluminium being 
in the Emerald apparently displaced to a minute amount by Chro- 
mium, 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 interest. These are followed by by d rated silicates, in- Cases 47 to 
eluding a very complete collection of the Zeolites, among which the 49. 
Natrolite from 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 othei-, also a very large 
specimen, and of deep colour, was presented in 1S09 by C. S. J. L. 
Guthrie, Esq. Tliese are succeeded, in Case 51, by a class of minerals 
of great mineralogical interest, containing some of the rarest of the 
elements, and themselves of rarity ; much uncertainty, however, still 
attaches to the chen^'''al formula3 of several of these species. The Case 51. 
titanates, the tantalates, and niobates, and these combined with silicates 
y-irconates, and st- nnates, thus link the silicates to the molybdates ^''^-^^ 
