332 



KEY. E. HILL ON THE EOCKS OP 



came against a westerly slope of schists than where, as at the north 

 and south ends of Sark, the forces acted parallel to the planes of the 

 junctions. This is the same as saying that the corrugations were 

 produced where the upper rock, if it slid, would have to slide up a 

 slope of the lower, but not where its slip could be horizontal. This 

 would seem to show that the granite was the readier to yield ; but, 

 as has been mentioned, the schists also are in some places corru- 

 gated in the like direction ; besides, the underlying unconformable 

 gneiss may be near the surface and influential, though concealed. 



5. Veins and Dykes. — A brief notice of the veins and dykes may 

 be added to complete this account of Sark. It has been mentioned 

 that faults abound ; they have generally opened into fissures now 

 filled with a reddish earthy material. These have been everywhere 

 burrowed into in search of ores, but nothing has ever been found 

 worth attempting to work except a vein of silver in Little Sark, and 

 this has ]ong been abandoned. Other veins are few. 



The dykes have not nearly the variety seen in Guernsey. I 

 have seen none of the granite dykes and elvans common there. A 

 brownish microcrystalline quartz-felsite with microscopic mica 

 occurs at the sea-end of Les Boutiques caves, and may be the same 

 as the whitish decomposed intrusive dyke which has originated both 

 the fissure and the cavern. A dyke six or eight feet broad cuts the 

 rocks at the Eperqueries landing, a compact dark felsite with 

 porphyritic felspar, showing under the microscope also a fair 

 amount of mica. The pink rhyolites or glassy felsites of Guernsey 

 and Jersey seem to be entirely wanting. The cliffs south of Dixcart 

 Bay are traversed by some large dykes conspicuous from the water, 

 but mostly inaccessible; I believe they are identical with a specimen 

 collected in the Bay Terrible, which is a beautiful, highly crys- 

 talline syenite or diorite, with a good deal of epidote intermixed. 



The majority of the dykes belong to the Guernsey group of 

 basaltic or diabasic intrusions, and to that section which are of 

 medium fineness. A slide cut from one of these which traverses 

 the cliff near the Coupee is described by Prof. Bonney to me as 

 " probably a hornblendic diabase rather than a true diorite." 



There is a fine mica-trap dyke, a kersantite, at the N.E. corner 

 of the cliffs of Port du Moulin, and apparently a smaller one much 

 decomposed at the natural gateway in the southern wall of that bay. 

 There seem also traces of another in the road-cutting at the Coupee : 

 doubtless others may be found. 



6. Herm and Jethou. — As these two small islands lie between 

 Sark and Guernsey, which I have already described, it may be 

 convenient to append here a brief account of them. Herm, the 

 northern, is the larger, about 1| mile long and | mile broad. 

 Jethou, the southern, is separated by a narrow but deep channel, and 

 though not one quarter the size of Herm, is rather higher. Both 

 consist of a beautiful crystalline white rock, usually regarded as a 

 granite. Jethou possesses a faint foliation in N.-S. vertical planes, and 

 Herm also shows a tendency to the same structure at its end nearer 

 Jethou. On the east side of Jethou is a very fine raised beach, 



