Geology and Geography. 383 



altered by the granite. On the S. and S. W. sides the strata 

 are somewhat distorted, but on all sides the rocks are materially 

 altered at their contact with the granite. The characters of the 

 slates are changed, and the grey wacke is converted into a quartz 

 rock. Veins proceed from the granite into the slate, and pro- 

 duce alteration of characters. The district of Antrim consists 

 of mica slate containing granite, which is often porphyritic, and 

 may sometimes be termed porphyry. The masses of granite 

 sometimes resemble beds in the mica slate. Mr Griffith entered 

 into a variety of details regarding the sienites, traps, and por- 

 phyries of the north and other parts of Ireland. He seemed to 

 consider that the phenomena he had observed prove that the 

 sienite of the' north of Ireland is of posterior origin to the chalk 

 and the porphyry. The granite of Galway and the porphyry 

 of Lowth were described, and some instances given of the con- 

 version of limestone into dolomite at its junction with trap. In 

 Donegal, a point was mentioned where a columnar quartz rock 

 lies between two portions of non-columnar trap. The ochre beds 

 in the island of Magee, &c. and their relation to the porphyries, 

 were then discussed. Many interesting views were exhibited of 

 the coast of the northern trap district. The trap-dykes and 

 horizontal beds of trap were noticed, and their general direction 

 given as follows ; — those of Mayo E. and W., of Fermanagh 

 N. of W., of Down S. of E. and N., of Antrim N. and S. A 

 geologically coloured copy of the mining index map of the county 

 of Down was exhibited. 



Mr Bryce remarked, that an idea seems to prevail among 

 many geologists that the Giants 1 Causeway rests upon chalk, 

 but this notion is erroneous. Ballygilly Head, near Larne, is a 

 precipice of columnar basalt, washed by the sea on the east, and 

 resting on sandstone. The chalk abuts against it on the south, 

 the upper surfaces of both rocks being on the same level ; the 

 chalk encircles it completely on the western side, and is resumed 

 in the same place as before on the northern side, the basalt be- 

 ing thus isolated in the chalk. At the Causeway the same seems 

 to take place, though the parts are not so clearly shewn. The 

 chalk abuts against the basalt at Port Bradin, both rocks being 

 at the same level, and from that part westwards the whole range 

 of magnificent precipices rests on lias or sandstone. In the in- 



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