ROCKS OF THE CHANNEL ISLANDS. 35 



be modifications of granitic rocks. The banded rocks and 

 breccias are confined to Jersey. 



I now come to the remaining sedimentary rocks of the 

 Channel Islands, viz,, the Jersey conglomerate and the 

 Alderney grits. I group them together, because though very 

 different in appearance and composition, there is some reason 

 for supposing they may be approximately of the same age if 

 not contemporaneous. The Alderney grits consist of quartz 

 with a large proportion of felspar grams, and are evidently 

 derived from the disintegration of granitic rocks. Pebbles of 

 quartz, granite, and quartz felsite occur in them. In one 

 spot these grits are seen to rest on the quartz felsites intrusive 

 in the granite, thus proving them to be younger. Rocks 

 absolutely similar to these grits have been identified near 

 Cape La Hague and also east of Cherbourg, by Rev. Hill and 

 M. Bigot. Near Cape La Hague they rest directly on gneiss, 

 in other places they overlie uncomformably the Schistes de 

 St, L6. This position corresponds with that of the " Conglo- 

 merat pourpre " in other localities, and notwithstanding some 

 difference in different localities, it is doubtless the same rock. 

 Rev. Hill considers the Alderney rock to be contempora- 

 neous with the upper Cambrian of Lapworth, that is to say 

 the Tremadoc Slates and Lingula Flags of Wales. The Jersey 

 conglomerate rests on the quartz felsites or rhyolites, and 

 contains pebbles derived therefrom, also granite ones ; but by 

 far the greater portion consists of Argillite pebbles derived 

 from rocks similar to the Schistes de St. L6 I have previously 

 described. The pebbles are of all sizes up to boulders of a 

 yard in diameter. These boulders are found chiefly at the 

 base, where curiously enough they are underlaid by or alter- 

 nate with fine-grained pink sand-stones or sandy shales. 

 Ansted suggested that this conglomerate, as well as the 

 Alderney grits, may be triassic, while M. Noury considers it 

 not older than the Permian period, but in Messrs. Le Yasseur 

 et Carez's Geological Map, recently published, it is marked 

 as Upper Cambrian Conglomerat Pourpre. I have noticed a 

 large area of a very similar conglomerate between Granville 

 and Coutances. 



There now only remain to be considered some few dykes 

 which cut both the Alderney grits and the Jersey conglo- 

 merates, and are therefore of more recent date. The chief 

 part of these consist of what Rev. Hill refers to in his papers 

 as mica traps. Examples of these are found in Jersey, 

 Guernsey, Alderney, Sark and Jethou. They are composed 

 chiefly of mica and felspar, and from their colour and rough- 



