10 THE VOYAGE OF H.M.S. CHALLENGER. 



to the cubic system ; for, instead of resembling crystallographic forms of this system, 

 they appear elongated and fusiform, sometimes presenting the appearance of a line 

 tapering at both extremities, and with their greater axis always parallel to the direction 

 of the band structure. This elongation and disposition of the particles of chromite is 

 very similar to the arrangement of the magnetite particles in the Cambrian slates of the 

 French Ardennes, so strikingly exhibited in the magnetic schists of Deville. This elonga- 

 tion of form, though so inconsistent with the crystalline system to which I refer them, 

 is, however, an anomaly so frequently met with in minerals of the cubic system, that it 

 offers very little difficulty : cuprite, among others, affords very remarkable examples 

 of this inconsistency. 



In the larger sections of chromite we observe fissures whose direction seems to corre- 



O 



spond with that of the cubic cleavage, but they are mere breaks in the ground-mass, into 

 which they may be traced. These same sections are also observed to contain enclosures, 

 probably of magnetic iron, which cause the grains of chromite to be attracted by the 

 magnet. If a fragment of the rock be pounded, and the grains extracted by the magnet 

 be examined with the microscope, it will be seen that they often consist of an opaque 

 nucleus, generally surrounded by a transparent zone of chesnut-brown, very similar to 

 the transparent sections of chromite. 



The question now is, whether this mineral is chromite, or whether it should be re- 

 garded as picotite, a mineral to which are generally referred the black grains found in con- 

 stant association with olivine ? "We must here remark respecting these two mineral species 

 that they present great analogies in their microscopical characters, and that the relations 

 between their physical properties perfectly illustrate the transitions of their chemical 

 composition. Indeed, it is well known that the chemical composition of the picotite 

 found in the lherzolites and other olivine rocks, as well as in serpentines, approaches very 

 closely to that of chromic iron. The picotite found in dunite, and which Petersen calls 

 chrompicotite, contains 56 per cent., and even more, of oxide of chrome, with only 12 

 per cent, of alumina, 14 per cent, of magnesia, and 18 per cent, of protoxide of iron. 

 These same proportions are found in most chromites ; that of Freudenbach, for instance, 

 analysed by K. von Hauer, gave him 49 to 52 per cent, of oxide of chrome, 10 to 12 per 

 cent, of alumina, 18 to 21 per cent, of magnesia, and 4 to 6 per cent, of silica. It will 

 thus be seen that the difference between picotite, chrompicotite, and chromite is not very 

 considerable ; they have all the same chemical formula, crystallise in the same system, 

 and, as I shall point out presently, the microscopical characters of sections described as 

 picotite are exactly similar to those of chromite. One may therefore be allowed to ask 

 by what microscopic characteristics these two mineral species are to be distinguished. It 

 has hitherto been admitted, on the one hand, that sections of picotite become transparent 

 when very thin ; and on the other hand, that those of chromite in every case remain 

 opaque. This distinction is due to Fischer, who, in his microscopical study of the 



