HORNBLElfDE-SCHISTS, GNEISSES, ETC. OP SAEK. 135 



the final consolidation of the mass. At Point Norman we find that 

 the curious ledges into which the rock weathers are caused by the 

 ' streaking ' of the normal rock (a diorite) by a rather more fel- 

 spathic and fine-grained variety.^ Again, a quarry in the ' bird's- 

 eye' diorite," south of the Saumarez monument, shows a curious 

 interstreaking of slightly different varieties, cloudlike stratulse of a 

 more hornblendic rock occurring in a more felspathic one. Another 

 case occurs on the shore to the north of St. Peter's Port, about 

 20 yards south of Hogue h la Perre battery. Here, in a mass of 

 'bird's-eye' diorite, the normal rock is occasionally streaked with 

 lighter-coloured bands, the latter being formed by a finer-grained, 

 slightly more felspathic rock, in which now and again a crystal of 

 the larger hornblende may be seen, and the latter mineral has a 

 tendency to occur in streaky clouds.^ Obviously the rock is of 

 igneous origin, and these bands are not the result of subsequent 

 mechanical disturbances, but must be due to the mixture of two 

 slightly difierent magmas.^ 



But we are indebted to Messrs. W. Sharp and J. Whitehead, F.G.S., 

 of Guernsey, for a still better example. In some of the ' bird's-eye ' 

 quarries a rather foliated variety of the rock occurs, called ' longrain ' 

 by the workmen. The specimen, sent to us, is a slab about 6" x 3" 

 square, composed of a layer of ' longrain ' and one of a pale grey 

 felspathic rock. The breadth of the latter layer is slightly under 

 haK an inch, and that this is its whole thickness is proved by a 

 ' skin ' of the hornblendic variety on the outer surface of the slab 

 (a joint face). The other (fractured) surface, that of the ' longrain ' 

 layer, exhibits the usual, generally parallel ordering of the horn- 

 blende crystals, and it has an interrupted silky lustre.^ Under the 

 microscope the hornblende in the dark layer is moulded on the 



^ Quart. Journ. Geol. Soc. vol. xl. (1884) p. 408. 



- The name given by the quarryinen to the variety with porphyritic crystals 

 of hornblende. 



^ One band was about 2'' wide ; it was followed by a more hornblendic band 

 (without distinct 'eyes') about "5", and that by a felspathic streak on an 

 average about -25" wide. Then came, as at the bottom, perfectly normal rock. 

 After rather less than a yard the white band is succeeded by a band like the 

 second named. 



^ We quote these instances because they occur within a few miles of Sark ; it 

 will be enough to refer to the paper by Prof. Bonney and Major-Genera] 

 McMahon on the Lizard (vol. xlvii. (1891) p. 464) for others, and the former 

 author wishes to add that during his last journey in the Alps he has obtained 

 evidence corroborative of the views advanced in that paper with regard to the 

 banding of the gabbro. 



^ [It was suggested by Mr. Barrow in the discussion that the planes visible 

 in the hornblende on the flat surface of the ' longrain ' were pinacoids, so the 

 specimens in our possession have been carefully examined. As the felspar 

 crystallized first, the faces of the hornblende crystals are often not well formed, 

 but those of the primary prism are certainly better developed, in this and in 

 the hornblende-rock, than either pinacoid (the orthopinacoid is often, as usual, 

 suppressed) ; the surfaces visible on the flat sides of the ' longrain ' are those of 

 cleavage-planes. As the hornblende crystals lie with their longer axes roughly 

 in one plane, the rock obviously cleaves more easily in this direction. — T. Gr. B., 

 February, 1892.] 



Q. J. G. S. No. 190. L 



