HORNBLENDE-SCHISTS, GNEISSES, ETC. OP SAEK. 



129 



Fig. 1. — Dtagrammatic sketch 

 of relations of rocks at hase of 

 cliff, N. of Natural Arch, Port 

 du Moulin. 



which forms a sort of step or terrace, three or four yards high above 

 the shingle, &c., to the top of which it is easy to climb (fig. 1). It con- 

 sists wholly or almost wholly of the ' basement-gneiss ' (A), above 

 which, forming as it were the lowest ' course ' of the actual cliff (B), 

 come some bands of pale-coloured 

 aplite, including ' lumps ' or 

 ' streaks ' of the hornblende-rock, 

 followed by a fairly constant mass 

 of gneiss, consisting of pale grey or 

 buff- coloured quartzo-felspathic 

 and dark micaceous layers, each 

 of which may be of any thickness 

 from about an inch downwards, 

 but are very commonly about ^ 

 inch thick. Still, thin lines of 

 mica commonly occur in the 

 quartzo-felspathic bands and vice 

 versa. A hand-specimen is 

 exactly like hundreds of other 

 banded gneisses, in which there 

 is so little fissility that a specimen 

 is easily broken off with the 

 broader face transverse to the 

 plane of the apparent bedding. 

 Here and there in this gneiss is 

 an elongated ' eye ' or even a 

 thin streak of coarse hornblende, 

 identical with that in the ap- 

 parent fragments. One band of 

 gneiss, about two feet thick (B'), 

 could be traced for some yards, 

 being defined, top and bottom, by 

 a seam from half an inch to an 

 inch thick, consisting generally of 

 foliated, dark hornblende-rock.^ 

 Above and below this band lumps 

 of the latter rock occur. The 

 whole mass from the top of the 

 terrace is from 2 to 3 yards thick. 

 It is limited by a band of rather 

 fine-grained grey gneissoid granite 

 (C), which, from its varia-ble 

 thickness (from 1 to 2 feet) and 

 slightly irregular course, seems to 

 be undoubtedly an intrusive rock. 

 Over this, for about a couple of 



yards more, we again find the dark hornblende-rock traversed by some 

 granitic veins (D). Here, however, the former in some jjlaces 



^ Erom tin's a specimen was taken, the microscopic structure of which will 

 presently be described. 



B 



A. Easement -gneiss. 



B. Aplite-bands, with eyes and 



streaks of hornblende-rock, 

 passing into grey banded bio- 

 tite- gneiss, with occasional 

 streaks of hornblende, reverting 

 at the top rather to the cha- 

 racter of the lower part. 6 to 8 

 feet. 



Fairly regular band of gneiss, 

 defined at top and bottom by a 

 streak of hornblende. Nearly 

 2 feet. 



Vein of rather fine-grained grey 

 granite, slightly gneissoid. 



Nodular hornblende-rock, seem- 

 ingly running into bands, with 

 some gneissoid bands, becoming 

 more bedded in aspect in about 

 2 yards, and passing up into 

 the normal hornblende-schist. 



