HOENBLENDE-SCHISTS, GNEISSES, ETC. OE SAKK. 147 



specimens exhibited to the ' Eastern Gneisses ' of the Highlands. 

 One in particular — the rock locally known as ' longrain ' — closelj^ 

 resembled a hornblendic gneiss that he had recently mapped in 

 Forfarshire. The characteristic appearance of this type of gneiss is 

 due to the presence of large numbers of hornblende crystals with 

 conspicuous development of the pinacoid faces ; it thus differed 

 fundamentally from a hornblende-schist, in which such faces were 

 almost totally absent. Tracing the outcrop of this rock, he found it 

 passed gradually into a normal hornblende-schist, and that its 

 gneissose aspect was due to therm ometamorphism — this phase only 

 existing when the basic rock was either penetrated by or in close 

 y)roximity to the more recent and more acid gneisses. The change 

 from schist to gneiss was essentially an aggregation of the con- 

 stituents of the finer-grained mass. The needles of actinolite in the 

 schist are aggregated to more or less definite crystals of hornblende, 

 with marked pinacoid faces, in the gneiss. The tiny felspar grains 

 gradually coalesce and form well-striated plagioclase, the species of 

 which can be easily determined. The sphenes tell the same tale. 

 The quartz is especially important, because there is usually a fair 

 amount of it in this type of hornblende-gneiss, and it is obviously 

 of the same age as the general structure of the rock. If the Authors 

 claimed that a similar structure might be produced during consoli- 

 dation, geologists were fairly entitled to ask for instances, from 

 unmetamorphosed areas, in which rocks, as basic as these gneisses, 

 contained free quartz as an essential constituent. 



The Rev. Edwiis" Hill mentioned, in reply to Mr. Hudleston, that 

 an overlying pre-Cambrian granite indicated the great age of the 

 hornblende-rocks. The beautiful Guernsey specimen which had 

 attracted the speakers' attention is called ' longrain.' He acknow- 

 ledged a change of opinion on Sark. When he formerly worked 

 there the existing hypotheses for the origin of such rocks were only 

 pressure, rolling out, and successive deposition. The first two were 

 clearly inapplicable, and so, though seeing difficulties, he had had to 

 choose the third. General McMahon's papers on the Lizard had 

 now provided a fourth, and this on examination proved to be 

 applicable to Sark. 



Prof. BoNNEY thanked the speakers for the reception accorded to 

 the paper. He said that, as the serpentines and gabbros were 

 intrusive in the crystalline schists at the Lizard, their absence in 

 Sark did not count for much. Mr. Barrow's remarks were of great 

 interest, but he thought instances of contact-alteration among crystal- 

 line rocks were less frequent than the speaker supposed, and were 

 not generally applicable to the cases in the paper, which hardly fell 

 under ordinary contact-phenomena. He thought that the criticism 

 regarding free quartz was founded on a slight miscomprehension of 

 what had been said. 



