THE GEOLOGY OF ALDERNEY. 



BY GEORGE HORACE PLYMEN, M.Sc, Ph.D., F.G.S. 



ALDERNEY is three and a half miles long, from N.E. to S.W., 

 and its width is about one mile. The western portion reaches 

 a height of three hundred feet, sloping to one hundred and 

 forty feet to the south of Longy Bay. From this point, 

 northward and westward, the land inclines gently to the sea. 

 The geological succession is as follows : — 



PERMO-CARBONIFEROUS : 



Dolerite and Lamprophyre Dykes. 

 Cambrian : 



Grits, arkose, and conglomerate. 

 PRE-CAMBRIAN : 



Granite porphyry. 

 Aplite veins and sills. 

 Granite and hornblende-granite. 

 Quartz-diorite. 

 Dolerite. 

 Gabbro. 

 The later dykes are best considered after the Cambrian 

 and pre-Cambrian rocks. 



The Cambrian Grits. 



The grits occur in three areas, north-east, east, and 

 south-east. The first stretches from Corblets Bay to Longy 

 Bay. The second, separated from the former by modern 

 sands, reaches from Longy Bay to L'Etac a la Ouoire. The 

 third, an isolated outlier, is a small patch opposite Coque 

 Lihou Island. The complete series shows the following, a 

 succession of at least 1,200 feet of conglomerates, sandstones, 

 grits and arkose : — 



Feet. LOCALITY. 



10. — Faint pink arkose 110 



9 — White ankose 40 Horn met Herbe. 



8. — [Dark purple arkose 50 



S. of Homme t Herbe, and 



7. — White arkose 400 Mannez Quarry, E. end 



(90ft. seen). 



6. — Pink arkose 15 Mannez Quarry. 



5. — Pink arkose, variegated 



with green sandstone .... 575 Mannez Quarry, Berry's 



Quarry, Hanging, Rocks 

 and north shore. 

 4. — White arkose with grit ... 40 La Quoire, Corblets Bay. 

 3. — Red arkose, variegated 



with green sandstone .... 48 Coque Lihou. 



2. — Red grit, pebbly and sandy 10 Coque Lihou and La 



Quoire. 

 1 . — Co nglomer a te 5 Coque Lihou . 



