132 GEOLOGY OF ALDERNEY. 



1 . The portion of the Island lying west of the grits, in- 

 cluding L'Etac a la Quoire, the Harbour, and the rear of 

 Braye Bay, is mainly diorite. It inclines to gabbro in Roselle 

 Bay. This portion includes the largest quarries in Alderney. 



2. Immediately south of this area, on both sides of the 

 Island, notably in Crabby Bay and at Tourgis Point, is a 

 hornblende-granite; north of the same area at Bibette Head, 

 Saye Bay, and Corblcts Bay (west), is fine-grained granite 

 with rare hornblende and biotite. 



3. South and west of a line joining Tourgis Point to 

 the Noire Roque is a granite-porphyry, covering a granite and 

 faulted against a tonalite, in places also covering the latter. 



1. The Diorite Area. 



Diorite is seen typically in Roche's Quarry, West Man- 

 nez, and L'Etac a la Quoire. The colour is dark grey. 

 There is much hornblende, less and variable biotite, and 

 white felspar. All the constituents are small. In Roche's 

 Quarry is a slight inflow of pink orthoclase and quartz in 

 the S.S.W. corner. In the south of the quarry is seen the 

 hornblende-granite of the Islands, invading a fine dolerite 

 mass 100 feet wide. The east face gives well developed cal- 

 cite-rock. This quarry has a large fault face with E.-W. 

 trace. The next quarry to north has also a fault trace in 

 this direction, hadine slightly to south. A fault plane in 

 West Mannez runs W.N.W.-E.S.E. hading 8o Q to S.S.W. 

 The slickensided effect is very marked, producing mainly a 

 display of sap-green epidote. 



At L'Etac a la Quoire, the diorite resembles a dingy brown 

 sandstone where weathered. There are three varieties, 

 grading into one another. A dark diorite is followed by a 

 white hornblende-diorite, succeeded by a quartz-diorite or 

 tonalite. The second rock is quarried most. A large fault 

 plane E.N.E.-W.S.W., hading 70 9 to S.S.E. is seen with 

 repetitions. The slickensiding has again produced a light 

 green hornblende-epidote rock. In this quarry one variety 

 is almost a <s birdseye," a dark rock rich in rounded horn- 

 blendes somewhat radially displayed. 



At Roselle Point, inflows in the diorite of granite, of 

 aplite, and of quartz, are seen. There is more resemblance 

 here to the Ronez area of Tersey and the Belleereve Bay 

 diorite of Guernsey than to anv other part of Alderney. 



Between Roselle Point and Bibette Head there is less fel- 

 soar. The rocks show development of large hornblende 

 crystals, somewhat tabular, a half-inch in length. There 

 is also augite with some olivine, It is the most basic rock in 



