122 EEV. E. HILL AND PEOF. T. G. BONNET ON THE 



11. On the HoRNBiiENPE-scHisTS, Gneisses, and other Crystalline 



Hocks of Sark. By the Rev. Edwin Hill, M.A., F.G.S., and 



Prof. T. G. BoNNEz, D.Sc, F.R.S., Y.P.G.S. (Bead January 



27th, 1892.) 



Contents. 



Page 



Introduction .• 122 



Part I. 



(1 ) Description of the Gneisses and Hornblende- schists 123 



{a) The Basement Gneiss. 

 (h) The Hornblende-schists. 

 {c) The Banded Gneisses. 



(2) Included masses of Hornblendic Eock and their relations 127 



(«) Section at the eastern corner of Point Derrible. 

 (6) Section near Port dii Moulin. 

 (c) Conclusions. 

 Partir. the later Intrusive Eocks 139 



Introduction. 



A PAPER on the Geology of Sark by one of the Authors has already 

 appeared in this Journal.^ In 1889 he introduced the other to 

 the island. Their visit was a very short one, but in 1890, after 

 working at the Lizard hornblende-schists, as to the genesis of 

 which they felt doubtful, they entertained some hope that a careful 

 study of those of Sark might help in solving the riddle ; so 

 they determined to examine carefully de novo all the more important 

 sections in Sark. They spent nearly a fortnight there shortly 

 after Easter 1891, weather and tides being on the whole alike 

 favourable. The present communication gives the results of this 

 work and of the study of the specimens collected on that and 

 former occasions. 



As Mr. Hill's paper was published so lately as 1887, the Authors 

 beg leave to refer to it for a general account of the physical struc- 

 ture and petrology of Sark ; but it may be well to summarize at the 

 outset the facts bearing most on the present investigation : — 



(1) The gneisses and hornblende-schists of Sark are fairly markedly 

 foliated, and sometimes also are rather conspicuously banded. 



(2) These structures, whatever be their genesis, are older than 

 the intrusion of the two great granitic masses and of the ' green- 

 stones,' mica-traps, and other dyke-rocks. 



(3) Thus these structures, for reasons indicated in former papers, 

 are of great antiquity. The rocks in which they occur maybe 

 classed, with reasonable probability, among the Archaean gneisses 

 and schists. \, 



(4) The planes of foliation, as described by Mr. Hill, generally 

 dip both at moderate angles and outwards from the middle part of 

 the island. This structure has no connexion with the faults, gene- 



1 Vol. xHii. (1887) p. 322. Mr. Hill wishes to state, with regard to the 

 present paper, that he has taken part only in the fiekl-work. 



