Vol. 59.] MAGNETITE-MINES NEAK COGNE. 59 



mixed rock from both mines. It may be convenient, before 

 describing them, to recapitulate briefly the general characters of 

 the Alpine serpentines, which I have examined at not a few loca- 

 lities from Monte Yiso to the Gross Glockner, and of which I have at 

 least thirty slices in my own cabinet. Though all are some shade 

 of dark green in colour (I cannot remember to have seen a really 

 red specimen), we can distinguish at least three types : — One dark 

 and compact, probably an altered dunite 5 not, I think, very common. 

 Another with crystals of bastite and more or less augite — not 

 unlike the dark serpentine on the south side of Kennack Cove (the 

 Lizard) : this also is not very common, and rather sporadic in 

 distribution, for I have obtained it on the Col de Sestrieres, on the 

 west side of the Julier Pass, near Davos, and in one or two other 

 localities. The third type is rather rougher to the touch, and 

 tougher under the hammer, not porphyritic, but somewhat granular 

 in structure. This is found on microscopic examination to be 

 rather rich in augite, and is abundant about the head-waters of the 

 Visp, as well as to the west, and for a considerable distance to the 

 south, including the Cogne district. All three types are occasion- 

 ally but little affected by pressure, though in the majority it has 

 developed either an elongated, slickensided phacoidal, or a fissile 

 structure, owing to which the rock breaks into slabs, like roofing- 

 tiles, or even slates, exfoliated pieces being sometimes hardly thicker 

 than a visiting-card. This slaty variety, so far as I 'can tell — 

 obviously it is not easy to be sure — may be produced from the 

 augitic type, though that mineral has entirely disappeared, and 

 the rock under the microscope consists of more or less filmy flakes 

 of mineral serpentine arranged in a generally parallel order, with 

 lines or very elongated patches of magnetite-granules. We meet 

 occasionally with flakes showing oblique extinction, and the more 

 pressure-modified specimens often show high polarization-tints, but 

 I cannot ascertain that the variety antigorite has any direct 

 connexion with the alteration of augite. Where the latter mineral 

 has been abundant, the commoner type of the (mineral) serpentine 

 appears to be one with a general resemblance to a mica, which 

 gives low polarization-tints, as we shall see in the rocks about to be 

 described. 



The specimens of ordinary serpentine collected, as already 

 mentioned, at each mine, prove on microscopic examination to be 

 mainly composed of two minerals : one in transparent flakes, some- 

 times prismatic, but with less conspicuous cleavage than a mica, 

 which give straight extinction and low polarization-tints — a milky 

 white or occasionally yellow ; the other, magnetite, 1 in granules 

 and small grains, occurring often in streaky clouds and occasionally 

 in clusters, when they sometimes form a matrix for the first 

 mineral, like augite does for felspar in an ophitic dolerite. The 

 specimen from the Pilon Licone is slightly ferrite-stained and 

 contains some associated granules of residual augite in one part ; 

 1 possibly also a trace or two of bastite. In that from the Pilon 



1 In these Alpine rocks the ferriferous grains are almost always opaque. 

 I have rarely met with picotite or a similar spinellid. 



