46 PKOF. T. G. EONXEY A]SD THE EEV. E. HILL: [Feb. T912, 



plagioclase, measuring up to nearly -1 inch in length, not well 

 preserved, which enclose occasionally small felspars and even augites. 

 Some larger grains of iron -oxide (? magnetite) are also present. 



The greenstone dyke in the Mannez Quarry, Alderney, is in 

 one respect interesting, for it cuts the Gres feldspathique. A 

 brief description of it is given in Mr. Hill's paper on that island, 

 but another slice has been examined, because it has been called 

 an andesite by Prof. Bigot. This, hoAvever, as it happens to 

 represent a slightly more compact state of the rock, adds little to 

 our knowledge. In it the grains of iron-oxide assume a slightly 

 club-like form, the small felspars are fairly idiomorphic, and there 

 is a considerable amount of a rather minute greenish material, 

 perhaps both chlorite and secondary hornblende ; some larger 

 grains of iron-oxide (? magnetite) are also present. Though the 

 hand-specimen is a little pale in colour, we still think that the 

 rock should be classed with the diabases ^ rather than with the 

 andesites. Two or three more specimens from other localities in 

 these islands have been examined, but those described may suffice 

 to show that the basic dykes are for the most part either ordinary 

 or hornblendic diabases. 



YI. Mica-Tkap ^ Dykes. [E. H.] 



In Mr. Hill's paper of 1884 two occurrences of these rocks are 

 noticed. Since then they have received attention from the 

 Guernsey Society of Xatural History, and the following list is 

 compiled chiefly from its ' Transactions.'^ The localities are given 

 in order round the coast, starting southwards from the Harbour. 



Putron Village. — Below Mr. Ozaune's property, in the face of the rock. 

 About 1 foot wide. (1892, p. 186.) 



Bee du Nez. — Some distance north of. — Two close together ; one 10 feet 

 wide with mica in rather large flakes : erosion of it has given rise to a cave. 

 (1894, p. 331.) 



Bee du Nez. — One, some wav south of this, is mentioned in Q. J. G. S. 

 vol. xl (1884) pp. 418, 426. 



St. Martin's Point. — In the cave below and north of the battery. 

 18 inches wide. (1892, p. 186.) 



Monument Bay. — South of the so-called 'Marble Caves.' [These names 

 are not on Guerin's map. The spot is said to be under Doyle's monument on 

 Jerbourg.] Very wide. (1894, p. 331.) 



Moulin Huet.— One in this bay is mentioned by us.^ Q. J. G. S. vol. xl 

 (1884) pp. 418, 426. 



Cliffs west of Les Thielles. — V^T'est of the junction between two varieties 

 of gneiss a mica-trap dyke occurs, at least 20 feet wide. (1897, p. 150.) 



^ I disagree with some niodern petrographers, and think diabase (in 

 Hausmann's sense) a convenient term for more or less altered basalts— a horn- 

 blende-diabase being the same thing as a proterobase (J. J. H. Teall, ' British 

 Petrography ' 1888, pp. 134, 135) ; epidiorite is a similai* rock, the formation 

 of which is a consequence of crushiug. [T. G. B.] 



^ I retain the old name mica-trap, though some exception might be 

 taken to the latter half, because I think it better than the modern lampro- 

 phyre, in which the corresponding part is absolute nonsense. [T, G. B.] 



3 The references, unless otherwise described, are to their volumes. 



•* There are apparently two outcrops, one of which is distinctly less mica- 

 ceous than the other. 



