HORNBLENDE- SCHISTS, GNEISSES, ETC. OF SARK. ] 23 



rally not of a great throw, which are as common in Sark as at the 

 Lizard. It gives no indications, macroscopic or microscopic, of 

 being a result of crushing, for it has no real resemblance to the 

 pressure-structures of the Alps or of the jN^orth-west Highlands, 

 already described in the pages of this Journal. It is, in short, a 

 ' stratification-foliation,' not a ' cleavage-foliation.' 



(5) At first sight, it seems more natural to interpret the structure 

 as indicative of an original sedimentation, in a rock which has sub- 

 sequently undergone such great chemical changes as to cause a re- 

 crystallization of the constituents. 



In Sark, as stated in Mr. Hill's paper, a fairly coarse, reddish 

 biotite-gneiss, moderately foliated but not banded, is lowest in 

 position and apparently the oldest. This is exposed for a con- 

 siderable distance along the eastern side of the island, at, and south 

 of, Creux Harbour, and occasionally on the western, in, and to the 

 north of, Port du Moulin, and to the south as far as Port a la 

 Jument.^ This is overlain by a series of stratified aspect, varying 

 from a hornblende-schist, very like some of those at the Lizard, to 

 a banded biotite-gneiss, which is generally more compact in struc- 

 ture and greyer in colour than the subjacent one. Occasionally the 

 second member presents a rather close resemblance to some of the 

 banded rocks in the Granulitic Group at the Lizard. Associated 

 with this series, and commonly rather low down in it, is a dark- 

 green rock, consisting almost wholly of distinctly crystallized horn- 

 blende, to which we shall frequently refer in the following pages. 

 Sometimes the banded gneiss appears to form a zone between the 

 basement-gneiss and the hornblende-schist, sometimes it seems to 

 replace the latter, — that is the two rocks apparently occur in a 

 rather lenticular fashion, and shade off one into another, often so 

 gradually that in some localities it is almost impossible to separate 

 them. So closely are they related that Mr. Hill, in writing his 

 paper, deemed it needless to attempt a distinction, and was content 

 — as Prof. Bonuey had been in his first paper on the Lizard — to 

 designate the whole series as ' hornblende-schist.' 



Part I. 



(1) Description of the Gneisses and Hornhlende-sclilsts. 



Time will probably be saved by giving a brief description of the 

 three types of rock mentioned above. As the purpose of this paper 

 is petrological rather than petrographical, the authors have thought 

 it needless to spend much time in trying to identify every microlith 

 or to ascertain the precise species of every felspar. They have, 

 however, carefully studied the microscopic structure of the more 

 important rocks, and have intentionally suppressed many minera- 



^ It is mentioned in Mr. Hill's paper, though it is not identified with the 

 Creux Harbour gneiss, but after a closer examination we find it difficult to 

 separate one rock from the other. The only difference perceptible is that the 

 Port a la Jument gneiss is a shade more micaceous and the felspar is paler in 

 colour. [For localities mentioned in this paper, refer to the map facing p. 124.] 



