1868. | Mineralogy. 257 
far from being entitled to a specific distinction, is simply a mixture 
of the basic sulphate of copper called Langite, with a hydrous 
alumina such as Gibbsite. By way of confirming this assertion, he 
points to another Cornish mineral, which in its general characters 
—colour excepted—bears a close resemblance to Woodwardite, but 
which on analysis proved to be a mixture of Langite with Allo- 
phane or some such silicate of alumina. It would seem then that 
this Langite has the sociable habit of mingling itself with other 
minerals, thus giving rise to a number of indefinite mixtures, which, 
however deceptive in appearance, must by no means be regarded as 
distinct species.* 
For the last twelve years large quantities of Cryolite have 
been worked in Greenland, partly for use in the manufacture of 
aluminium, and partly for the production of a pure soda-ash ; but 
although extensively employed in commerce, the mineral has never 
been found in a crystallized form. Recently, however, small crystals 
have been discovered, and some of these specimens we have had an 
opportunity of examining. They are coated with a thin film of 
hydrous peroxide of iron, but on the removal of this incrustation 
the vitreous lustre and transparency of the crystals are at once 
evident. At a cursory glance they appear related to the cubic 
system, but on careful measurement Dr. Websky finds that they 
must be referred to the doubly-oblique system.t It is pleasing to 
note that this determination verifies a conjecture thrown out some 
years ago by Descloiseaux, who was led to his conclusion by study- 
ing the optical characters of the massive mineral. 
In a recent memoir Dr. Dana discusses at length the chemical 
composition of the felspars, in order to explain the close relation 
which exists between the different members of this natural group— 
a relation extending not only to crystalline form and chemical 
composition, but also to the colour, hardness, optical characters, and 
other physical properties of the minerals. Among these felspars 
the species Jeucite oceupies an anomalous position ; for, although 
clearly related to the family in most of its features, it yet crystal- 
lizes in cubic instead of oblique forms. To reconcile this anomaly 
Dana ingeniously shows that the cubic and oblique crystals are in 
truth intimately connected ; that “ the monoclinic crystals of ortho- 
clase and the triclinic of anorthite, &c., are in fact nothing but dis- 
torted, or, rather, clinohedrized dodecahedrons, variously modified 
by cubic, octohedral, trapezohedral, and other planes.”t 
Every right-minded student cherishes so profound a respect for 
* “Sur la Woodwardite du Cornouailles,’ ‘Comptes Rendus,’ Ixy., No. 27, 
p. 1142. 
+ “Ueber die Krystallform des Kryolith’s,” ‘ Leonhard’s Jahrbuch,’ 1867. 
Heft, vii., p. 810. 
{ “Crystallogenie and Crystallographic Contributions :” ‘Siliman’s Journal,’ 
vol. xliv., No. 132, p. 398. 
