306 Chronicles of Science. [April, 



these stones fell within a very limited area, and thus ample oppor- 

 tunity was furnished of collecting specimens. A large numher of 

 these have lately been examined by Vom Eath, who shows that 

 they consist of about 86 per cent, of silicates — probably Olivine and 

 Shepardite — associated with 10 per cent, of nickehferous iron, and 

 nearly 4 per cent, of magnetic pyrites, together with a small 

 percentage of chrome iron-ore.* 



Although copper-pyrites is the most widely diffused of our 

 copper-ores, it is by no means common to meet with well-defined 

 crystals ; and hence observations upon its crystalhne forms are of 

 much value. Formerly the mineral was referred to the cubic 

 system, but in 1822 Haidinger showed, by measurement of the 

 fundamental octohedron, that it must be removed to the tetragonal 

 system. Herr Sadebeck, of Berlin, has paid considerable attention 

 to the crystallography of this species, and has recently pubhshed the ■ 

 results of his study.t He first separates the two opposite tetrahedra, 

 or hemihedral forms of the tetragonal octohedron, distinguishing 

 them as tetrahedra of the first and second orders respectively. He 

 then discusses the laws according to which twins are combmed — a 

 point of considerable interest, since twins of this mineral are much 

 more common than simple crystals. Finally, he gives the charac- 

 ters of typical forms from difierent localities, and shows that these 

 are in many cases so peculiar, as to enable the mineralogist, by 

 a study of form alone, to refer a given crystal to its original 

 locality. 



Every collector must be familar with the beautiful crystals of lim- 

 pid quartz, which, in company with calcareous spar, line many of the 

 drusy cavities in the famous snow-white marble quarried at Carrara. 

 More than 200 specimens from these quarries have been examined 

 by Dr. Scharfi", who has lately published a paper on their irregu- 

 larities of gi'owth.^ He shows that the crystals, although appa- 

 rently simple, are in many cases formed by the union of several 

 individuals, by which the aggregate crystals possess an apparent 

 rhombohedral cleavage. The two opposite rhombohedra, forming 

 together the six-sided pyramid that caps each crystal, are often 

 developed to a very unequal extent; the faces of the positive 

 rhombohedron being enlarged at the expense of the corresponding 

 negative form. 



Dioptase — a beautiful and rare silicate of copper found in the 

 wilds of the Kirghese Steppes, and so strongly resembling the 

 emerald as occasionally to be mistaken for that gem — has recently 



* Leonhard und Geinitz's ' Neucs Juhrbuoh.' 18G9. Heft I., p. 80. 



t ZcitschriFt der deutschen gcologisehcu Gosellschaft. Baud xx. Heft III., 

 p. 395. 



X Leonhard uiid Geinitz's 'Jahrbueh fiirMineralogic,' U.S. w. 1868. Heft VII., 

 p. 822. 



