formed during various Metallurgical Operations. £55 
following query of Breislac, in the Introduction to his Geology, 
p. 451., ” 44 Les cristaux contenus dans les laves se sont-ils 
forme's dans le lave meme, et ont-ils ete produits par les ma- 
tieres qu’elle contenait, ou bien lui sont-ils etrangers ; et etai- 
ent-ils formes anterieurement, preexistant dans les matieres et dans 
le couches prerreuses fondues par le feu volcanique ?” Koch 
also remarks, that the glassy and opaque varieties of enamel 
pass into each other, and both varieties or kinds occur in the 
same crystal ; and, what is of importance in a theoretical view, 
in crystals exhibiting these kinds, the glassy kind forms the 
centre of the crystal, while the opaque forms the crust or outer 
part; and a gradual transition from the vitreous centre to the 
completely opaque exterior is observed in such crystals. 
Constituent Parts. 
Silica, 
56-40 
Lime, 
26-24 
Alumina, 
8-33 
Magnesia 
3-96 
Oxide of Magnesia, 
1-81 
Oxide of Iron, 
0-18 
96-92 
Loss, 
3-08 
100-00 
Salts. 
Muriate of Soda. 
Alkaline salts, which are almost always formed from aqueous 
solution, sometimes appear along with metallurgic products, as 
the result of the action of high temperatures. Muriate of soda^ 
we are informed by Koch, occurs in some smelting furnaces, not 
only in massive forms several inches in magnitude, but also beau- 
tifully crystallised in cubes, which are in druses, but seldom iif 
single crystals. They are met with in fissures of the furnace/ 
and appear to have been formed by sublimation. 
Art. III.— Account of the Parallel Roads in the Valley of 
Coquimbo. By Captain Basil Hall. 
The parallel roads , or natural terraces of Glen Roy, in this 
country, have, for some time, engaged the particular attention of 
