134 GEOLO&Y OF ALDfeRN^Y. 



size of 1/4 inch. There are smaller white felspars, very tiny 

 hornblendes 1/10 inch by 1/40 inch and biotites 1/10 inch. 

 The rock contains aplite veins and narrow dolerite dykes. 

 The large dolerite dyke seen in Roselle Bay is met again. 

 North of it is a kersantite dyke, both running E.N.E.- 

 W.S.W. At Bibette Head the former is 100 feet wide. 

 Mica diorite is seen again in Saye Bay. Here a band of 

 gneiss is developed twelve inches wide. It is part of the 

 diorite and runs parallel to the dolerite dyke. Chateau a 

 L'Etoc is built of Bibette granite. The west side of Corblets 

 Bay is the same material with basic inclusions. Here granite 

 is seen invading diorite. The east side of this bay contains 

 the sedimentary beds, faulted down. 



3. The Granite-Porphyry Area. 



Almost a third of the Island, comprising practically the 

 south-western portion, stretching from Tourgis Point south- 

 west to Telegraph Bay, and from this bay north-east to the 

 shore opposite the JNoire Roque, is an area of acid material 

 suggesting in texture a hypabyssal rock. As the Island pro- 

 vides no evidence of volcanic activity in the form of ashes 

 or How lavas, although true rhyolite pebbles exist in the grits, 

 it seems more in consonance witii held evidence to regard the 

 acid material as a sill. In one locality there is flow struc- 

 ture at the high angle of 8o Q to S.S.W., probably due to an 

 early movement. The Coque Lihou grits, dipping to E.S.E., 

 lie on an old sea floor of this rock, and their dip is only 30 . 

 Near this granite-porphyry there are aplite veins in plenty, 

 but always quite distinct in appearance. They weather a 

 bright pink colour whereas the porphyry weathers white or 

 cream. Abandoned " china quarries " in this rock are seen 

 in the heights above Clonque J? ort. In problematic areas like 

 Telegraph Bay it is difficult to say whether the porphyry 

 invades the aplite or whether the reverse occurs. The ba- 

 lance of evidence suggests posteriority of granite-porphyry. 



If the long dyke, seen in the shore from near Tourgis 

 Fort, past Clonque Fort, to Hannaine Bay, is a part of this 

 sill, then it is clearly of later date, for it cuts its way through 

 dolerite, quartz-diorite, and aplite. 



The granite porphyry and aplite could belong to the 

 same original mass, with different periodic movement. 



The sill is much faulted against quartz-diorite, and in 

 places quartz-diorite stands out in relief surrounded by por- 

 phyry, but as a rule the underlying rock is a fine-grained 

 granite which may grade into the porphyry. The vertical 

 fault planes noted at the boundaries of this area suggest a 

 subsidence or block faulting. 



