﻿Vol. 6 1.] MICROSCOPIC STRUCTURE OF SERPENTINE. 709 



composed of flakes, rather irregular in outline, of the usual mineral, 

 without any distinct orientation, which give the faintest possible 

 trace of pleochroism (green to buff) and polarization-tints ranging 

 through the first order, once or twice reaching the violet of the 

 second order. 



Specimens of this kind of serpentine from the summit of the 

 Riffelhorn have been already described, 1 so it will suffice to repeat 

 that they (and one from the moraine of the Gorner Glacier) contain 

 the mica-like minerals with the lower and the higher polarization- 

 tints, and though the latter are generally, they are not invariably, 

 the larger. One slice includes only the d nil-tinted, and all of 

 these show residual augite, sometimes clear, sometimes dusty, 

 occasionally with diallage- cleavage ; in the flakes extinction is 

 usually straight, but, as stated, there are exceptions which some- 

 times at least may be actinolite, for they apparently have a slightly- 

 higher refractive index. Magnetite, and occasionally awaruite, is 

 present ; crushing, crumpling, and the ophitic structure have been 

 noticed. Additional work has convinced the author of the original 

 description that augite has been even more abundant in these rocks 

 than he had supposed (for he fails to find any certain trace of other 

 ferromagnesian minerals), and that it has frequently produced the 

 above-mentioned flaky mineral. He pointed out (in 1896) its 

 resemblance to antigorite, as then described, and has now no 

 hesitation in assigning that name to at least the variety with low 

 polarization-tints, between which and the other one he has not yet 

 found any valid distinction. 2 



XI. Slaty Serpentines. 



These are so schistose, as to resemble green tiles or slates, the 

 broken surface of which affords no clue to the original composition 

 of the rock. The least fissile of them comes from the Fee Alp, near 

 Saas, where it forms a dyke in the calc-mica-schist series. 3 Two 

 slices have been cut from it, one of which (a little thicker) shows a 

 slightly-stronger tint of green with parallel than with perpendicular 

 vibrations. They contain the usual iron-oxide (with occasional 

 ophitic structure) and a streak of a granular mineral (? a carbonate), 

 but are mainly composed of the two mica-like minerals, variable in 

 size, though generally rather small, and with so marked a structure 

 that the slice exhibits fairly-definite streaks of colour (generally 

 pale yellow) where the former makes an angle of 45° with the 

 vibration-planes of the crossed nicols. Here also size and thickness, 



i E. Aston & T. G-. Bonney, Quart. Journ. Gcol. Soc. vol. lii (1896) p. 452. 



2 A specimen collected in 1880 'from old moraine of Furggen Glacier' 

 (it might come from the Furggen Grat or the northern spur of the Matterhorn 

 on the whole resembles a serpentine from olivine, but contains no certainly- 

 recognizable bastite or other residual mineral, and is almost wholly composed 

 of rather minute antigorite, with a structure ranging from ' thorn ' to streaky. 



3 The Alpine serpentines, indeed all that I have seen, very rarely form 

 dykes. [T. G. B.] 



