Geological Survey of the Island of Jersey. 367 



in the island. In the south-east it lies within the space 

 marked out by Mount Orgueil, Granville church, a', and La 

 Hougue Bie. In the south-west there is hardly any other 

 rock (except 43), and it is generally of a large, coarse-grained 

 description. 



The well-known Mount Madou, quarry 52, is the last point at 

 which this sienite shews itself, where it overlays the pink-red 

 porphyry, as decidedly as it does at Mount Orgueil. It reaches 

 also from l'Etac to half-way between Greve-de-Leiq and 

 Plemont, and from X Bay to Mount Madou. At X Bay all 

 the three constituents are in large grains, which apparently 

 present the most decomposable description of sienite. I have 

 seen the hornblende fairly washed out by the sea, leaving a 

 frail honey-combed skeleton of quartz and felspar. 



Although this rock alternates with the argillaceous schist, 

 yet there appears the utmost jealousy in keeping itself quite 

 distinct; 46 was chipped from the junction at a, (l'Etac). 

 (See Fig. E, p. 39). 47 shews the quartzose nature of it as 

 as composing the vein, and which becomes gradually though 

 decidedly sienite at about 20 feet from the point where it 

 enters the argillaceous schist. At a are numerous veins of 

 white quartz (48), 6 or 8 inches thick. 



Felspar, Quartz and Chlorite Mica. 



14, Found in the space contained by Le Hoc St. Clement's, 

 Granville church, and thence eastward. 



Felspar, Quartz and Common Mica. 



53, From half-way between Plemont and Greve-de-Leiq to 

 X Bay. At Greve-de-Leiq there are two large veins (54) of 

 mica 10 feet wide ; close to, and parallel to one of them, is 

 another vein of argillaceous schist. 



With the exception of the green porphyry, all the preceding 

 felspar rocks have a peculiarly columnar structure, especially 

 Nos. 33, 36, 41, 42, 43 ; the finest example of this lays 

 between Le Corbiere and the western point of St. Brelade's 

 Bay. They form hills of from 400 to 200 feet in height : thp 

 green porphyry no where exceeds 200. 



