1922.] GEOLOGY OF ALDERNEY. 133 



Alderney, named a picrite by Rev. E. Hill. Now containing 

 hornblende, it was originally an olivine-gabbro. 



North of Fort Albert a large vertical dyke, 25 feet wide, 

 lies E.-W. It widens to an erosion channel 100 feet wide 

 across on the north fof Roselle Bay. This dyke is met again 

 to the east. It is an apparent Permo-Carboniferous intru- 

 sion, basic in character. 



2. The Granite Area. 



At Crabby Bay, north, the rock is highly biotitic with 

 some quartz, but the felspar is plagioclase. The dimensions 

 of the minerals are biotite 1/8 inch, felspar 1/7 inch, quartz 

 i/5inch. It is invaded by aplite dykes, followed by quartz. 

 I here is much secondary sap-green epidote accompanying 

 the dykes. The " mixing " of differentiation is well seen on 

 the north side of the bay. Later than the rock above-named 

 there intruded a granite dyke six feet in width, in the direc- 

 tion N.E.-S.W., seen under Fort Doyle. The texture is 

 small, no element being larger than 1/16 inch. There is 

 more pale pink felspar than quartz or hornblende. The latter 

 is the smallest and rarest constituent. To southwards, under 

 Fort Tourgis, where there is much " mixing," is a granite 

 with fine small hornblendes (1/8 by 1/20 inch), tiny black 

 mica, 1/5 inch pink felspars, smaller yellow felspars and 

 small, badly-shaped quartz. The main rock is faintly 

 gneissose, the banding being exhibited by the hornblende. 

 Here also the gneissose structure is seen in banded dolerite 

 and in micaceous diorite, the Crabby rock recurring. There 

 is also banded aplite, with pink felspar, in elongated streaks. 

 Most of the indications of gneissic structure are given in the 

 beach pebbles. As in other parts of the Island, whenever 

 diorite shows a local gneissose structure there is a dolerite 

 dyke in proximity. There are two- diorites at Tourgis Point. 

 One is a hornblene-biotite-pale- felspar rock enclosing dole- 

 rite. The other is a, fine quartz-dioarite, related to dolerite, 

 absorbed by a coarse granite with 1/5 inch pink felspars, 

 very small hornblendes, and quartz, followed by the inflow 

 of aplite. 



South of L'Etac a la Quoire is a granite with large 

 greenish quartz 1/6 inch, small pale felspars 1/8 inch, less 

 abundant hornblende 1/10 inch, and biotite 1/10 inch. 

 Aplitcs invade this. The granite porphyry is met here as at 

 Tourgis Point. 



In the north of the Island the handsome granite of 

 Bibette Head is abundant. Its colour is pale pink, and its 

 numerous patches of basic inclusions, generally of the nature 

 of diorite, present a greyish tint. Pink felspars reach the 



