1868. | Mineralogy. 583 
25 per cent.; these ores cost from 8s. to 10s. per ton delivered at 
Denain. Spanish iron-ores sent from Bilboa, yielding 50 per cent. 
of iron, and costing 1/. 4s. per ton, are used. Algerian iron-ores, 
said to give 65 per cent. of iron, are sent to France from Mokta- 
el-Hadid, and costs 12. 8s. 10d. a-ton delivered at Dunkerque. 
Wolfram steel some years since attracted some attention, and 
M. Jacob,;the patentee of a process for effecting the combination, 
produced some beautiful examples of cutlery made from steel pro- 
duced by his process. At Messrs. Cockerell and Co.’s Works in 
Belgium they are now using 24 tons of wolfram per month in this 
manufacture. 
Several French engineers have reported more favourably on 
the strength of iron produced by alloying ordinary pig-iron with 
tungsten. It is evident that some good experiments, made on a 
large scale, and under varied circumstances, are required to settle 
this question. 
9. MINERALOGY. 
In our Chronicles of last quarter we called attention to a recently- 
discovered form of silica, described by Vom Rath under the name of 
Tridymite. It now remains to describe the characters of the new 
mineral, and to point out the interest attaching to its discovery. 
Hitherto it has been the fashion to recognize two distinct forms of 
silica: the one, represented by common quartz, having a specific 
gravity of about 2°6, and crystallizing in the hexagonal system ; 
the other, represented by opal, having a specific gravity of 2°2 
or 2°3, and being non-crystalline or amorphous. A relation was 
thus established between density and form; and on this relation 
many deductions were based. We now find, however, that a low 
density does not necessarily connote an amorphous condition; for 
Vom Rath discovers a species of silica which has a specific gravity 
of only 2°2 or 2°83, and yet assumes well-defined crystalline forms. 
These forms, although belonging to the rhombohedral system, are 
in no wise related to those of ordinary quartz; and hence silica 
turns out to be dimorphous. Our new mineral occurs in small 
six-sided tabular crystals, which are usually grouped together in 
(to use a solecism) twins of threes, or, as the Germans more aptly 
express it, in Drillingen. It is this peculiarity of a three-twinned 
srowth that has suggested the name “Tridymite.” Associated 
with specular iron-ore and acicular hornblende, these crystals bestud 
the cavities of a volcani¢ porphyry from the Cerro San Cristobal, 
near Pachuca, in Mexico. ‘That they are not pseudomorphs of 
amorphous silica after some hexagonal species, is clearly proved 
by the property which they possess of double refraction.* 
* *Pogeendorif’s Annalen,’ No. 3, p. 507; ‘ Geol. Mag.,’ No, 48, p. 281. 
