The Geology 



Sometimes a joint or a crack in the rock has given entrance to 

 the waves ; more often a vein or dyke is softer than the rock 

 which it traverses, and yielding more easily to waves and 

 weather has caused the beginning of an opening. In several of 

 the caves at Moulin Huet may be seen the dark stripe which 

 indicates a dyke ; the Creux Mahie seems partly due to a land 

 slip, while its entrance is choked with fallen blocks and rubbish. 

 A cleft west of the Doyle column is opened in a vein of quartz. 

 The causes of the Bays are less easy to find ; most of them are 

 the mouths of long valleys. Down the valley is a rivulet which 

 wears away the rotten rock, the solid rock is so decomposed 

 below the soil that often for some feet down it can be dug with 

 a spade ; this may be seen inland in many road cuttings. 



Evidences of ancient changes of sea level exist; raised 

 beaches have been found at Lihou, L'Eree and elsewhere. 



A student of Eock- structure and Petrology will find an 

 ample field for collection and observation. The table-land of 

 the island is entirely Gneiss which forms the cliffs and shore, 

 from Castle Cornet round the south and west up to Vazon Bay. 

 This is white at Saints Bay and Lihou, red at Petit Bot and the 

 Gouffre, mottled at Castle Cornet and Yazon Bay, while yet 

 other varieties occur. The cliffs again give excellent opportuni- 

 ties for studying this muck controverted rock. True Granite is 

 found round Cobo and Grandes Rocques, in FAncresse Bay, in 

 some knolls and in many dykes. " Granite " is the name locally 

 given to the grey rocks so largely quarried over the northern 

 part of Guernsey, but scientific writers would call these Diorites 

 or Syenites. They make excellent cubes for street paving, 

 and very durable metal for macadamising. A dark rock called 

 by Quarrymen, " Bird's Eye" extends along Belgrave Bay and 

 round St. Sampson's ; it also is extensively quarried. This is 

 of very unusual constitution, and has been described scien- 

 tifically as Horn-blende Gabbro. 

 In primeval times igneous agencies must have been pro- 



