HORNBLENDE- SCHISTS, GNEISSES, ETC. OE SAKE. 127 



In the more fine-grained variety of gneiss the bands (pale reddish 

 grey and greenish grey) are broader, but less distinctly defined. 

 Microscopic examination shows it to consist of quartz, felspar 

 (orthoclase and plagioclase), flakes of a green mineral (most of 

 ■which is an altered biotite, but possibly a little fibrous hornblende 

 is also present), apatite, a little sphene and iron oxide, and a few 

 grains of pyrite. The banding is due to slight differences in the 

 proportionate amounts of the constituents. The grains of the first 

 two minerals are commonly about '03" in diameter, though smaller 

 occur ; the flakes of the third are about the same length. The 

 grains present the slightly irregular outline already mentioned, and 

 enclosures of quartz occasionally occur in the felspar. The rock 

 exhibits little or no sign of having suffered from mechanical 

 disturbances subsequent to consolidation ; and its structure is 

 identical with that of many specimens from the Granulitic Group 

 of the Lizard. 



(2) Included masses of Hornhlendic Rock and their relations. 



Masses of a somewhat coarsely crystallized rock may be seen in 

 several places on both sides of the island, which on closer examina- 

 tion appear to consist almost wholly of a lustrous deep-green 

 hornblende. These, in some places, might be regarded at first 

 sight as the ends of intrusive ' tongues,' which had been afterwards 

 somewhat compressed by earth-movements, but, as will presently be 

 seen, many cases cannot be thus explained. These hornblendic 

 lumps are not restricted to any very definite horizon. At the same 

 time they appear to be most common either just above the base- 

 ment-gneiss or at no great height up in the overlying series. They 

 are well exhibited in the cliffs all about Port du Moulin, and on the 

 opposite coast between Creux Harbour and Derrible Bay. 



In these lumps the colour of the hornblende is a rich green. Of 

 two slides examined microscopically, one, from Point Derrible, 

 consists (except for a few granules of iron oxide) only of hornblende. 

 The other (from just N. of Port du Moulin) contains in parts of 

 the slice (a large one) a very few granules of quartz, with a 

 linear arrangement, and a grain or two of felspar. The hornblende 

 crystals are often fairly idiomorphic, with characteristic cleavage, 

 and generally exhibit some tendency to a parallel ordering. They 

 are on the whole rather uniform in size, often being about '06" long 

 and -04" wide (in a transverse section). In a thin slice they are of 

 a fairly strong green colour and dichroic, the tint varying from a 

 pale sap green to a rather deep green. The rock obviously is the 

 ' hornblendite ' of certain authors. 



It is by no means easy at first sight to determine the relations of 

 this peculiar rock to its associates, and difficult to express in a 

 description, necessarily brief, the effect produced by the cumulative 

 evidence of numerous instances. Perhaps the best method, after a 

 few general remarks, will be to describe in detail one or two sections 



