136 GEOLOGY OF ALDERXtfY. 



Fort axe of quartz-diorite, but there appears porphyry a few 

 yards inland in numerous places. 



Quartz-diorite is seen fay the drinking trough on the road 

 to St. Anne's, and also in two outstanding knolls in the upper 

 part of Trois Vaux. These points may indicate the roof of 

 the sill. Elsewhere, notably along the Tourgis Mill stream, 

 farther inland at Rose Farm, and at the Brick Kilns, the 

 surface appears to be porphyry. Saline Bay shows no signs 

 of the porphyry. 



The remarkable dyke of similar material, appearing in 

 Hannaine Bay, Clonque Bay, and Saline Bay, is nearly a 

 mile long and is 30 feet wide. As a source of material for 

 the sill, a kind of fissure eruption or irruption, it presents an 

 interesting speculation. 



The Later Dykes., 



There is little to add regarding the dolerite and lampro- 

 phyre dykes. Those that are known in Alderney are referred 

 to in the previous pages. They are later than the Cambrian 

 Grits, and are regarded in Brittany and Normandy as of 

 Permo-Carboniferous Age. They were sufficiently late to 

 invade the Alderney rocks after the folding imposed on them 

 in the Armorican main movement, and probably resulted 

 from the movement and fracture of that period. 



Tectonic Evidence. 

 Block-faulting of granite-porphyry occurred prior to the 

 deposit of the grits. The fracture lines take the direction 

 roughly N.E.-S.W. in Hannaine Bay, N.-S. in the north of 

 Trois Vaux Bay, and N.W.-S.E. in the south of Trois Vaux 

 Bay, and in Telegraph Bay, Saline Bay, a low, sand silted 

 expanse, has no rock features except at the extremities. If 

 the limiting line of the block faulting runs from here to Noire 

 Roque, it comprises in the subsided area the whole of the 

 quartz-porphyry of Alderney, with an isolated Cambrian 

 capping of grit in situ. The Cambrian Grit also lies in situ 

 on the diorite of the eastern area and no reason is provided 

 in this patch of grit to suggest anything but normal succes- 

 sion. The dip however is not quite the same, the following 

 being noted : — 



L'Etac a la Quoire S.S.E. 40° 



La Tchue S.E. 20 



Hanging Rocks S.E. 30 



Essex Castle S.E. 20? 



Longy Bay S.E. 18 



At the southern grit area the dip is to E.S.E. at 30?. 



