Vol. 6S.'] PETEOLOGICAL NOTES ON GUERNSEY, ETC. 47 



L'Eree Promontory. — A mica-trap vein, striking north-east and south- 

 west^ cuts three veins of felsite which cross the porphyritic gneiss from north- 

 north-west to south-south-east. (1895, p. 23.) In 1910 we noticed here two 

 small mica-trap dykes, both south of the Lihou causeway, visible only in a 

 few outcrops, and too decomposed to be worth slicing, but did not notice that 

 they cut felsite or greenstone veins. The southern one was some 4 feet thick ; 

 the northern aboat half that amount. The latter not only cuts the gneiss, 

 but also encloses a piece about a foot thick, one of the branches being only 

 6 or 7 inches. 



Fort La Crocq and Eichmond. — One about 2 feet wide just below the 

 small fort, running north-eastwards down the rocks. A similar dyke was 

 found on the south-west of Richmond, probably a continuation. (1891, 

 p. 126.) 



Horn met [north of Vazon Bay]. — A similar dyke, on the east side of the 

 fort. (Ibid.) 



Albecq Cutting. — On the land side, through the red granite: thin. 

 (3id.) 



Long Port, Vazon. [Guerin's map marks Long Port in Cobo Bay.] — In 

 places fully 6 feet wide : colour, redder than usual : divides into two branches. 

 (1892, p. 186.) 



Grandes Rocques. — Similar to that at Fort La Crocq. (1891, p. 126.) 



Port So if. — Similar. (Ibid.) 



Fort Doyle. — In the quarry near the fort, three veins of undoubted mica- 

 trap : respective widths=3 feet, 1 foot, and 8 inches. (1893, p. 270.) 



Point Norman. — In a cleft of the rocks at this Point; close to the quarry, 

 but rather on its southern side. (1891, p. 153.) We have noticed this one. 



Homtole, and Hommet Paradis. — Two are suspected on these islets. 

 (1893, p. 270.) 



St. Sampson's Harbour. — One which occurs near the mouth of the 

 Harbour has been mistaken for a sandstone. (1890, p. 36.) Subsequently it 

 was cut through in drainage-excavations. ' A remarkable feature was the in- 

 clusion in it of nodules of diorite, many of which were so rounded, and 

 separated so cleanly from the matrix, as to resemble pebbles.' (1893, p. 270.) 



Herm and Jethou. — Mr. Hill mentioned one on Jethou in Q. J. G. S. 

 vol. xliii (1887) p. 333, The 'Guernsey Ti-ansactions ' say that they 'abound,' 

 and give a list of ten (1894, p. 331) : one 10 feet wide, one 12, and one 14. 

 But not one has so far been recorded from the adjacent and larger island of 

 Herm. 



Sark. — It does not appear that any have been added to those mentioned in 

 our paper of 1892. (Q. J. G. S. vol. xlviii, 1892, p. 142.) 



Alderney. — Mr. Hill mentions one in the Mannez Quarry. (Q. J. G. S. 

 vol. xlv, 1889, p. 384.) Another has been noticed in La Chue Sandstone 

 Quarry on the south coast. (1902, p. 123.) 



Mica-trap d3'kes have, therefore, been noticed in every variety 

 of rock, except (up to the present time) the Lancresse granite. 



YII. The Pleinmont Geit. 



In Mr. Hill's paper (1884) ^ a rather fine-grained dark rock is 

 mentioned as forming the whole cliff of Pleinmont Point, which he 

 regarded as an exceptionally large mass (' fully half a mile in 

 length ') of the dyke-forming diabase. 



We gave it a passing glance in 1888, when Prof. Bonney put it 

 down in his notes ks ' a compact diabase, rather schistose and 

 close-jointed." In September 1910, wishing to see its relations, if 

 any, to the microgranitic dyke-rock, we walked up its northern 

 slope near Fort Pezeries until we found a ' red dyke ' ^ cutting it. 



1 Q. J. G. S. vol. xl, p. 417. - Described below, A, p. 49. 



