130 



REV. E. HILL AND PROF. T. G. BONNEY ON THE 



exhibits a very peculiar structure (fig. 2). It 

 composite ; fairly angular masses of 

 rock, without any distinct foliation, 

 foliated and darker-coloured mass 



a brighter 



too, appears to be 

 green hornblende- 



Fig. 2. — Hornblende - rock 



hrecciated structure, 



Natural Arch, Port du Moulin. 



ivith 

 nortJi of 



being included in a somewhat 

 of the same material. This 

 in the upper part of the mass 

 appears to go ' stringing ' ont, 

 and to assume a more definitely 

 bedded aspect. Over this hrec- 

 ciated portion, the cliff, so far 

 as can be seen, consists of 

 banded hornblende-schist, often 

 of the usual character, but 

 sometimes rather gneissoid in 

 aspect. 



The microscopic structure of 

 these several rocks must now 

 be briefly noticed. In a rather 

 large slice both the light and 

 the dark green varieties of the 

 hornblende-rock can be studied. 

 The difference between them is 

 very slight. In the latter the 

 crystals are perhaps slightly 

 larger than in the former ; cer- 

 tainly they are more distinctly 

 orientated, are of a richer 

 colour — those in the former 

 having a pale sea-green tint — 

 and they are more dichroic. 

 With the darker rock, aa 

 already mentioned, a little quartz and felspar are associated. The 

 lighter one seems to be pure hornblende, but this distinction may 

 be accidental. 



A slice from the gneiss (B' in fig. 1) shows that the chief constitu- 

 ents are quartz, felspars, and biotite ; as accessory minerals (all small 

 and sparse), we find a little iron oxide, apatite, sphene, epidote (?), 

 zircon (?), and possibly a grain or two of tourmaline. The quartz 

 contains rather numerous, very minute enclosures ; the felspar is 

 somewhat decomposed, but the twinning of plagioclase is frequently 

 quite distinct ; the biotite generally is in good condition. It exhibits 

 a slight parallelism, and dominates, as a matter of course, in certain 

 bands. In the irregular form of the grains of quartz and felspar, 

 in the occasional encircling of the one by the other, or their micro- 

 graphic association, and in the not unfrequent elongation or slightly 

 streaky aggregation of the grains we have an exact repetition of the 

 structure (noticed above) which is frequently found in the gneissoid 

 bands of the Granulitic Group at the Lizard, and in other rather 

 fine-grained banded gneisses. Another slide, cut from a rather 

 darker variety, in which some streaks of hornblende occur, shows a 

 similar mica-gneiss, containing ' stratulae ' of hornblende identical 



Grranite. B. Hornblende- rock. 



The sketch is diagrammatic, and exag- 

 gerates the distinction between the 

 two varieties of hornblende-rock. 



