Yol. 68,] PEIEOLOGICAL NOTES ON GTJEENSEY, ETC. 55 



in Guernsey (as pointed out by Mr. Parkinson in 1907) and towards" 

 the north-east in Alderney. Thus the Channel Islands form 

 a kind of petrographical province, characteri'zed by 

 richness in soda-bearing felspars and by a fair amount 

 of ferromagnesian minerals in all but the most acid 

 type of rocks. 



The hornblende-schists of Sark and the Lizard are practically 

 identical, and the granulitic group in the one closely resembles that 

 in the other, but with this important difference, that it underlies 

 those schists in the former district and overlies them in the latter. 

 It is also produced in a similar way, though not altogether from the 

 same material. In Sark an aplitehas broken into and partly melted 

 down a coarse-grained hornblendite ; at the Lizard a rather more 

 normal granite has similarly affected the hornblende-schist. The 

 last rock, however, may in both cases represent incipient differ- 

 entiation in a magma, the extreme products of which are horn- 

 blendite and aplite ; for a few small lumps of the former rock occur, 

 though very rarely, in the hornblende-schists of the Lizard. The 

 main difference appears to be that, in Sark, the basic portion of 

 the magma became completely separated, and even solidified, while 

 the acid residue remained liquid ; so that the latter, in consequence 

 of earth-movements, shattered and carried upwards the former, with 

 local melting-down. Following this came the less perfectly differ- 

 entiated residual mass, which is represented by the hornblende- 

 schists. But, at the Lizard, the process of differentiation became 

 most complete in the later stage. ^ We think, as stated in our 

 paper on Sark in 1892, that the fairly coarse, slightly foliated gneiss, 

 which is seen beneath the above-named groups on both sides of the 

 island, is really an intrusion somewhat, though perhaps not much, 

 later in date. 



The gneiss of Southern Guernsey must be the oldest rock in the 

 islands, for it had already been made by pressure from a porphy- 

 ritic granite, prior to the intrusion of the diorite ; and the same 

 may be said of the gneissoid rocks (also locally porphyritic) between 

 Cherbourg and Cape La Hague. In fact, as Prof. Bonney pointed 

 out in 1887,^ many of the foliated rocks on the eastern side of the 

 English Channel are very similar to those in the Lizard district. 

 Further examination has confirmed this opinion, so it may suffice 

 to call attention to one point of some importance — that the 

 structure of the gneissoid rocks in Guernsey and 

 j^ormandy indicates very early and important earth- 

 movements acting almost at right-angles to those 

 which gave rise to the great Armorican chain. 



The latter produced as their ultimate result a great extrusion of 

 acid rocks, beginning with the granites in the South-West of England 

 and in Brittany, followed by more limited ejections of basic rocks 



1 But, as the hornblendite was often easilj' melted by the aplite, these stages 

 may have followed in rather quick succession. 



2 Q. J. a. S. Tol. xliii (1887) p. 320. 



