on Serpentines in the Pennine Alps. 537 



T have examined microscopically slices cut from a slab (about 6 

 or 7 tenths of an inch in thickness) of the above described slaty 

 serpentine, collected from the western slopes of the Gorner Grat. 

 Both were cut at right angles to the flat surface, one along what 

 appeared to be the 'dip' of the cleavage and the other with its 'strike.' 

 The structure in the two slices differs little, but the former is slightly 

 more streaky. The rock 1 consists almost entirely of two minerals. 

 One, forming the matrix, occurs in small translucent flakes or 

 streaky folia of a very pale olive colour ; the other, an iron oxide, in 

 rather small black grains (Fig. 3). The former, with crossing 

 Nicols, resembles a streaky felted mass, the folia varying from a 

 rather regular to a lobate or irregular outline. Commonly the flakes 

 lie with their longer axes parallel with the cleavage-planes. Occa- 



Fig. 3. — Section of Slaty Serpentine from near the summit of Gorner Grat. X 50. 



sionally the mineral exhibits a cleavage like a mica, parallel to which 

 extinction occurs, and these also lie in the above direction. Hence 

 the slide as a whole is darkest when placed with the boundary of the 

 cleavage surface parallel with the vibration plane of either Nicol. 2 

 Then a darkish field is speckled by irregular granules of low tints, 

 white, greyish, or yellowish. It is brightest when the same lines 

 are equally inclined to the vibration planes, a yellowish tint (yellow 

 to orange) dominating. In this position the streaky interweaving 

 of the mineral flakes is more conspicuous. This structure bears 

 some resemblance to tapestry work in wool, where the stitches are 

 irregular in length and more or less in one general direction. 

 Difficult, however, as it is to describe the appearance, it is 



1 The hardness of the rock is between 3 and 3" 5, and its S.G. =267. 



2 Possibly the mineral may be (in part at least) antigorite. Cf. the description in 

 Teall's British Petrography, p. 113. The larger flakes in my slides show a faint 

 dichroism, but it is imperceptible in the smaller. 



