409 



which occurs at the lower part of the north and south declivities of 

 the Ox mountains. The pehbles consist of quartz and jasper : 

 they are stated not to exceed an egg in size, and the imbedding 

 cement is a stiff, ferruginous clay. No organic remains were ob- 

 served. On the south side of the mountains, where the conglome- 

 rate rests upon quartz-rock, it is said to graduate into that forma- 

 tion j but on the north side, where the quartz-rock is wanting, the 

 conglomerate rests unconformably on mica-slate. 



4. Quartz-rock. — This formation occurs in many parts of the di- 

 strict, viz. on the south flank of the Ox mountains ; in the Nephin and 

 Birreen Cove mountains; at the headlands east and west of Broad- 

 haven ; on the south shore of Carramore lake ; and on the south 

 coast between Portarlin and Conoghrea. In its simplest state it 

 consists of finely grained white quartz-sand, the beds having a 

 schistose structure; but in some localities it contains white felspar 

 and mica, when it graduates into gneiss. 



5. Hornblende-slate, mica-slate, and gneiss. — No new details are 

 given respecting the characters of these formations. Gneiss occurs 

 principally in the peninsula of Erris, and in the mountains above 

 Coolany, at Mullinashie ; and the hornblende-slate and mica-slate 

 in the Ox chain and the Erris group of mountains. 



6. Granite. — This rock is found only at the southern extremity 

 of Erris, and penetrates in veins the mica-slate which rests upon it. 



7. Trap. — Under this head the author gives a detailed account of 

 the composition, range, and effects of eleven parallel basaltic and 

 amygdaloidal dykes, which traverse the northern part of the district 

 in a nearly east and west direction, and cut through all the forma- 

 tions from gneiss to the carboniferous limestone. One of these 

 dykes he traced between 60 and 70 miles, and was induced to con- 

 clude, from information which he received, that it might be followed 

 much further to the eastward. The distance between the northern 

 and southern dyke is only 11| miles. Two of the dykes are said 

 to be traversed by others having a north and south direction. A 

 detailed account is also given of an extensive bed of trap, which 

 overlies the mountain limestone and the sandstone and shale on 

 the east and west sides of the Bay of Killala. Finally, the author 

 offers a series of general observations on the origin of the different 

 formations, and on the probable extension into England of the trap 

 dykes of Mayo and Sligo. 



A communication was then read from the Rev. Adam Sedgwick, 

 V.P.G.S., and Woodwardian Professor in the University of Cam- 

 bridge, respecting certain fossil shells overlying the London clay 

 in the Isle of Sheppey. 



Mr. Sedgwick, in examining a series of fossils from the Isle of 

 Sheppey, lately presented to him, found several specimens differing 

 from the rest, both in their specific characters and state of preser- 

 vation. These shells were derived by Mr. Crow of Christ College, 

 Cambridge, from a bed in Warder Cliff", about 15 feet below the 

 surface of the ground, and had lately been laid bare by a small 

 land-slip. The bed in question is from 8 to 12 inches thick, and 



