1922.] GEOLOGY OF ALDERNEY. 137 



In either area the disposition of the beds would place 

 them on the north-west of the Island at a height of about 

 2,000 feet to 3,000 feet, showing the amount of erosion that 

 has occurred in the igneous rocks alone. The grits occur 

 again at Burhou, an island three miles to the north-west. 



The subsidence was on a large scale, and being prior to 

 the deposit of grits, is pre-Cambnan. 



Turning to the north-eastern grits, evidence of post- 

 Cambrian movement is more amply afforded. We have a 

 succession of at least four parallel fault lines cut off by later 

 movement. 



In Corblets Bay is- down-throw against granite: this 

 fault shows less throw at Berry's Quarry, where the con- 

 glomerate bed is met. Further working may soon disclose 

 diorite, which is worked on the western side of the mineral 

 line which separates the two quarries. 



The bay eastward, Cat's Bay, shows faulting, also seen 

 in Mannez Quarry, giving an eastern down-throw. 



In Mannez Quarry the fault fissure is occupied by a 

 dolerite dyke, twenty feet wide. These two faults seen in 

 the shore rocks roughly N.N.E.-S.S.W. and this direction 

 is repeated west of Quenard Point, in the bay east of Cat's 

 Bay. This line is not seen inland, as it clears Mannez 

 Quarry and is lost in the pasture land east of it. It gives 

 relative upthrow. The most eastern fault, at Hommet Herbe, 

 gives an eastern down-throw. These four fault lines all 

 N.N.E.-S.S.W., seem to correspond sufficiently to the strike 

 to argue a movement synchronous with the uplift. The dips 

 are as follows : — 



Corblets Point E.N.E. 30- Q 



Quenard Point E.N.E. 30? 



Mannez Quarry E.N.E. 3G 9 



Berry's Quarry E.N.E. 40 s 



Hommet Herbe Fort E. 20° 



All of these minor faults are cut off by a transverse fault 

 running E.-W. apparently through the Island. In the 

 southern portion of the diorite quarry west of Berry's, a 

 slickensided fault plane is seen, extending for fifty yards, 

 with a height of forty feet. It passes south of Berry's 

 Quarry with a down-throw to south. In Roselle Bay, where 

 this line should appear, is a dolerite dyke twenty-five feet 

 wide, running E.-W. 



The southern limit of this down-throw is found in Longy 

 Bay, just north of the Longy Quarry. The grits are down- 

 thrown to north, the whole area occupied by flats and 

 modern sands. There is a military shooting range here, 



