MICA TRAP DYKES. 153 



of small glittering flakes of Mica. They occur as dykes which 

 never exceed a few feet in thickness, and are sometimes to be 

 measured only by inches. Several have been already described. 

 In Jersey there is one at the Creux de Vis, which I have 

 seen, and M. Noury gives a list of others under the name of 

 Porphyrite Micacee. In Alderney one may be found at the 

 north end of the Mannez Quarry ; in Jethou a small one on 

 its eastern side : but in Herm I have not found any yet. In 

 my paper on Sark is mentioned the fine one which occurs in 

 Port clu Moulin, whence come the large reddish blocks which 

 lie in the north-east corner of the bay. But there are others 

 besides. In the spring of this year Professor Bonney, F.B.S., 

 and myself, were engaged in a re-examination of the rocks of 

 Sark, and in the course of this discovered several more of 

 these peculiar rocks. One in Saignie Bay runs up the cliff 

 a few yards to the north of the path. Another has hitherto 

 escaped notice, though situated in so frequented a locality 

 as the entrance to the Gouliot Caves. It intersects the cliff 

 between the two northern openings, and probably crosses the 

 roof of the " Chimney." Blocks of it lie on the floor, and on 

 the shore at the mouth of the " Main Cave." A singular 

 rock which appears to have affinity with the group forms 

 a dyke near high water mark in the bay below the cottages 

 called Fregondee, the innermost recess of Havre Gosselin. 

 Yet another Mica Trap occurs on the western shore some- 

 where just below the Coupee. A single day in Guernsey 

 added one more to our list. This occurs in a cleft of the 

 rocks at Point Norman, close to the quarry, but rather on its 

 southern side. Here a rather reddish-brown dyke about 

 4 feet thick runs down to the sea, which upon examination 

 proved to be also a decomposed Mica Trap. 



This considerable addition to the number known renders 

 it probable that there may be various others of this peculiar 

 group to be found on the shores of Guernsey, and I venture 

 to suggest them as an interesting subject for search. They 

 are not difficult to recognise. The surface is almost invariably 

 reddish-brown and soft. Any rusty-looking rock easily 

 scratched with a knife should be subjected to further exami- 

 nation. If on being broken the thin glittering flakes of Mica 

 are seen scattered through its mass, it belongs to the group 

 in question. If the pieces at first discovered be only detached 

 blocks, a search for the parent rock should be made. A 

 memorandum of the position of each dyke should be made 

 with sufficient completeness to secure its certain and easy 

 rediscovery. 



