Yol. 68.1 PETROLOGTCAL NOTES OX &CERNSET, ETC. 31 



4. Petrological Xotes on Guernsey, Herm, Sark, and Alderxey. 

 By Prof. T. G. Bonjs-ey, Sc.D., LL.D., F.R.S., and the Eev. 

 Edwin Hill, M.A., F.G.S.^ (Read :November 22iid, 1911.) 



[Plate I— Microscope-Sections.] 



■ Contents. 



Page 



I. The Gneiss 31 



II. TheDiorites 32 



III. TheAcidDykes 37 



IV. Eocks at Castle Cornet 39 



V. The Basic Dvkes 44 



VI. Mica-Trap Dykes [E. H.] 46 



VII. The Pleinmont G-rit 47 



VIII. Notes on some Paramorphic and other Changes [T. G.B.] ... 51 



IX. Age of the Eruptive Rocks in the Channel Islands [T. G. B.]. 53 



During the nineteen 3'ears which have passed since we visited the 

 Channel Islands, our friend Mr. John Parkinson has examined 

 parts of Jersey with results described in four valuable papers con- 

 tributed to this journah^ He then turned his attention to Northern 

 Guernsey, and in a short but very important summary of his work 

 on its diorites,^ announced his conclusion that the syenite, horn- 

 blende-gabbro, and diorite, which had been separated by the older 

 observers, were not genetically distinct, but were related to each 

 other as the products of the differentiation of a single magma. 

 Our brief visits to the island in 1888 and 1891 had shown us 

 that the above-named distinctions could hardly be maintained, and 

 prepared us for some such conclusion as that just mentioned. 



The principal object of our visit in September 1910 was to make 

 a further examination of this question and to study some of the 

 numerous dykes a little more fully than Mr. Hill had been able to 

 do when drawing up his sketch-map and general account of the 

 geology of Guernsey.^ 



We also spent two full days in Alderney, an afternoon in Herm 

 (both unknown to one of us), and twice visited Sark, in order to 

 re-examine some specially interesting sections/ 



I. The Gneiss. 



The gneiss, which occupies the larger half of Guernsey, forming 

 the grand crags and beautiful bays of its more southern shores, 



"■ Mr. Hill wishes to state, with regard to this paper, that he has taken part 

 only in the field-work. 



2 Q. J. G. S. vol. liv (1898) p. 101 ; voh Iv (1899) p. 430 ; vol. Ivi (1900) 

 pp. 307, 320 ; also vol. Ivii (1901) p. 218. 



3 Geol. Mag. dec. 5, vol. iv (1907) p. 74. 



■* A map of Guernsey, on the scale of 2 miles to the inch, prepared by the 

 Ordnance Survey, was published in 1902. 



^ This paper was completed, with the exception of one section, by the spring 

 of this year (I9II), but examination of the rock-slices brought out a point of 

 much interest, to clear up which Mr. Hill (the other author being unable to 

 accompany him) went to Guernsey after Easter, and again in September. 



