HORNBLENDE-SCHISTS, GNEISSES, ETC. OF SAKK. 141 



with iron oxide, sphene and a little apatite ; small crystals of felspar 

 are numerous, — of these most are plagioclase, but some in their 

 form and their twinning on the Carlsbad type suggest orthoclase. 

 The intervening groundmass shows a frequent approach to a micro- 

 graphic or spherulitic structure. Thus the rock is a mica-porphyrite. 



A rather similar but less coarsely porphyritic rock occurs on 

 Brecqhou, on the E. side of the Port, and a dyke, rather like that 

 at the Eperqueries Landing, but too rotten for examination, runs 

 up the cliff S. of the Coupee (W. side). As pebbles rather like the 

 Eperqueries rock are not rare on the shore about here, the identi- 

 tication is rendered more probable. 



In 1889 we discovered, on the shore at Port du Moulin, one or 

 two blocks of a variety of picrite ; ^ but time and tide prevented 

 us from seeking the rock in situ. On our next visit we searched 

 carefully. At last, at the base of a slightly projecting crag which 

 lies roughly east of the Great Autelet, two or three small humps of 

 a similar rock were seen projecting from the shingle. The height 

 of the part exposed is not more than a couple of feet, and it can 

 only be traced for a very few yards ; but another low boss was 

 visible among the shore-boulders some fifteen yards away. The 

 rock occurs in the banded gneiss, the layers of which it somewhat 

 distorts, so it is clearly intrusive ; but it does not, as is usual with 

 the ordinary greenstone dykes, run up the cliff. The eye is attracted 

 to it by a rather unusually rounded outline, like a seal's back, and 

 by a peculiar aspect, the dull greenish-grey rock being mottled with 

 light-coloured blur-like spots. ^ The rock, as in the boulder, though 

 soft is not easily broken, as it is tough, and ' pounds ' under the 

 hammer. A freshly fractured surface presents some slight varietal 

 differences from the rock of the boulder. The groundmass of the 

 latter is rather more like that of an ordinary dark serpentine. This 

 is rather paler, more fibrous in aspect, and speckled with small 

 glittering crystals, but is less distinctly porphyritic. There are some 

 corresponding microscopic differences. In the boulder, a considerable 

 amount of olivine still remains unchanged, but this is not so in the 

 specimen from the rock in situ. That mineral is replaced by a 

 number of secondary products, minute serpentinous minerals of more 

 than one kind. There are also numerous brown, subtranslucent 

 grains, of earthy aspect; these probably are an impure chalybite, 

 and replace the magnetite which in the other rock is abundantly 

 associated with the partially serpentinized olivine. The rock in situ 

 contains also a larger quantity of hornblende ; this mineral varies 

 from a rather light green, fairly dichroic hornblende (occurring in 

 crystals with ragged outlines, but showing distinct and very charac- 

 teristic cleavages) to a fibrous colourless actinolite. On the other 

 hand there is less of the micaceous constituent. This rock, in short, 

 is much more difficult to describe since (to use a homely term) " it is 



^ T. G. Bonney, ' On the occurrence of a variety of Picrite (ScyeHte)in Sark/ 

 Geol. Mag. for 1889, p. 109. 



^ This especially attracted my eye to the boulder in Port du Moulin. — T.G B. 



