A GLANCE AT THE ROCKS OF ALDERNEY. 



BY C. G. DE LA MARE. 



On the 25th. June, 1891, 1 took advantage of the excursion by 

 the Courier to pay a visit to the above island, and, during the 

 few hours I spent there, noted a few points in connection with 

 its geology which I thought might be of interest to this 

 Society; Alderney, though not distant, being comparatively 

 little known. 



I first directed my course towards the east end of the 

 island. After passing Fort Albert I noticed that the head- 

 lands and outlying rocks were composed of a coarse granitic 

 or syenitic rock, presenting a tabular appearance, and 

 contrasting strongly with the diorite forming the main body 

 of the island. This granite appears to be intrusive in the 

 diorite. 



On the west side of Corbelets Bay I noticed a dyke of 

 mica-trap cutting the diorite. It is about 2 feet wide, of 

 reddish colour, somewhat schistose, and lies about E. and W. 



The sandstone or grit formation for which Alderney is 

 noted commences abruptly on the east side of Corbelets Bay. 

 There is a flat beach between the diorite and the grit, and this 

 probably conceals a fault separating the two formations. It 

 is therefore impossible to say to what depth the sandstone 

 may exist beneath the beds first exposed. The dip is about 

 30° E. or N.E. The sandstone formation continues to the 

 eastern extremity of the island, the beds being passed over in 

 ascending order, and their shelves form on the sea-shore an 

 interesting type of rock scenery not seen elsewhere in the 

 Channel Islands. The road passes through Maunez Quarries, 

 whence the chief part of the materials for the construction of 

 the breakwater and forts has been extracted. These quarries 

 present fine sections of the grit series — which there shows 

 alternating bands of purplish and greenish tints. The rock 

 varies in texture, as the Rev. E. Hill states in his paper on 

 Alderney, from a coarse sandstone to a fine mudstone. The 

 sandstone in these quarries is cut by two dykes of greenstone, 

 whether diorite or diabase I cannot say. These dykes are 

 only a few yards from each other and parallel, so that they 

 are probably connected with one another. Their direction is 

 approximately E. and W. The adherence between the green- 



