SAKK, HEKM, AND JETHOTJ. 



333 



15 feet above high-water mark, four or five yards thick, a mass of 

 rounded pebbles in general as large as a man's head, but some of 

 them two or three feet across. In the south-east of Herm is a fine 

 " creux." This name is generally applied in these islands to a 

 shaft in the hill-side communicating at its bottom with the sea ; 

 but the Creux Mahie in Guernsey is a cave, and at the Creux 

 Harbour in Sark there is now at all events only a tunnel through 

 the rocks. On the south-west shore of Herm, below some cottages, a 

 greenish compact seam, often less than half an inch wide, runs for 

 many yards through the granite, and becoming wider shows its real 

 nature by developing into an ordinary compact dyke. This helps 

 to explain some of the appearances noted in the Sark granite. In 

 Jethou in like manner there are a few platy dykes, and a small much- 

 decayed dyke of mica-trap which shows its later date by traversing 

 one of the above. The singular " shell beach " at the north end of 

 Herm, the only one of the kind in these islands, illustrates the local 

 nature of fossil accumulations. 



The granite of Herm is a highly crystalline rock, consisting of 

 white felspar, both orthoclase and plagioclase, hornblende, quartz, 

 and biotite, with a little apatite ; in my slide mica encloses a horn- 

 blende crystal. The felspar often shows plagioclase striping even 

 to the unaided e} T e, but generally is rather amorphous ; it occurs in 

 grains about -Jr inch long, but runs up to half an inch or more in 

 some specimens. The hornblende is in well-formed crystals, often 

 rather lath-shaped, but seldom exceeds J inch. The quartz is in the 

 usual interstitial glassy grains. The mica is the least abundant of 

 the principal constituents ; it shows well-shaped hexagonal plates. 

 Dark nodes are frequent, and vary much in shape and in sharpness 

 of definition. The rock of Jethou is much the same as that of 

 Herm, both to the eye and under the microscope; but Prof. Bonney 

 remarks to me on a slide that it is a little crushed, while a slide 

 from Herm shows no crushing. But, as above mentioned, on a large 

 scale there is a very faint structure visible also in Herm at the end 

 nearer Jethou. Prof. Bonney writes of both that they are holo- 

 crystalline and indubitably igneous. 



7. General Conclusions. — Comparing these granites of Herm and 

 Jethou with the granitic overlying rock of Sark described above and 

 shown also to be igneous, the differences appear very slight and 

 smaller than may be found in less widely separated parts of many 

 continuous masses of granite. The probability, then, seems con- 

 siderable that they are all remnants of one great irruption. No 

 Jersey rocks that I know, and none of the principal Guernsey rocks, 

 appear to be the same as these ; but the granite of Alderney, which I 

 hope at some future time to describe, I expect will prove also to belong 

 to this great mass. Certainly some of my specimens could not be 

 distinguished as different. If so, since Alderney, the northernmost 

 island, shows no sign of crushing, while Herm and Jethou exhibit 

 faint traces, and Sark, the southernmost, presents clear proofs of 

 compression, it would seem that the compressing forces had acted 

 more powerfully in the south of the area concerned. 



Q. J.G.S. No. 171. 2 a 



