330 MEETINGS. 



places with the gneiss, a peculiarity also observed at La Moye 

 and other places along the southern coast. The country rock 

 from Pleinmont to near Mont Herault is granitoid gneiss, 

 similar to that of Richmond. At Mont Herault the gneiss is 

 porphyritic and very compact, almost resembling granite or 

 syenite ; thence to Les Thielles and further east, the prevail- 

 ing rock is coarse porphyritic gneiss, to which the abundant 

 large red orthoclase felspar crystals give a Avarm tint of 

 pleasant and picturesque appearance. 



4. — Creux Mahie Cave. 



The roof appears to be a fault plane partly coinciding 

 with the boundary of a diorite vein. The slope of the cliff 

 west of the cave is the upper surface of another vein of the 

 same nature. The following measurements were taken : — The 

 cave descends from the mouth imvards until a depth of about 

 20 feet is reached, then the floor continues at the lower level 

 to a distance of 76 feet from the mouth, then rises at an angle 

 of 29° for a further distance of 109 feet, making about 180 

 feet for the total length of the cave. The greatest width is 

 60 feet. The height at the mouth is only 5^ feet, but in the 

 centre at least 18 feet. At the further extremity of the cave 

 the floor meets the roof, at a point about 50 feet in vertical 

 height above the lower level of the floor. The direction of 

 the level part of the cave is nearly south to north, N. 5 E., 

 but the ascending portion bends considerably to the west. 



5. — East Coast of St. Martin s. 



The marble caves under Doyle's Monument have been 

 visited. They are probably the most picturesque in the island 

 and only inferior in size to the Creux Mahie. They are due 

 to the erosion of diorite veins. The length of the principal 

 one is 70 to 80 feet, its extreme width 18 feet, height 10 to 12 

 feet. The side cave is open to the sky, forming in the 

 side of the cliff a funnel opening, which is overgrown with 

 bushes. The rock is a bluish gneiss very much contorted 

 with white veins of calcite mixed with quartz. From a 

 distance it has the appearance of marble, hence the name. 



6. — Jethou. 



The prevailing rock is a syenitic granite containing 

 numerous inclusions of a darker rock (similar to that occur- 

 ring abundantly in Herm, and probably corresponding to the 

 St. Sampson's and Vale diorite), but nowhere in Jethou 

 occurring en masse. This included rock is, however, distin- 

 guished from the generality of the Guernsey diorite by its con- 



