TBAMSACTIONS OF THE SECTIONS. Ill 



rubble, wiiicli was tbe same formation witli a less admixture of clays. Bones and 

 teetb of the Mammoth, the Horse, Ox, Deer, and Whale, &c. were found imbedded 

 in the two deposits, which showed that they were not far removed from each otlier 

 in g-eolog-ical sequence. In speaking of the old sea-beach, the author doubted its 

 existence, as stated by Mantell, 50 feet beneath the surface of the Western Road, 

 which consists almost entirely of chalk from its commencement to Western 

 Cottages, where the Coombe rock makes its appearance, followed by brickearth, 

 which there overlies it. Mr. Howell inclines to the opinion that the clays of the 

 Coombe rock and brickearlhs, as well as the lignite, breccia, and sandstones of the 

 former deposit, were derived by denudation from tiio tertiary strata of the South 

 Downs, in the same manner as those tertiaries might have been derived from the 

 denudation of the Wealden. 



Coming down to the recent periods, the author described the Brighton valley as 

 consisting of silt and flints resting upon Coombe rock, in which were imbedded im- 

 mense quantities of water-rolled sandstones, similar to the " grey weathers," the whole 

 deposit pointing to a time when the valley was an estuary of the sea as high up as 

 the London and Lewes Roads, then the beds of rivers, one possibly issuing from 

 the Weald, the other probably from the high hills round about the village of 

 Fahner. Pebbles, exactly the same as those lying on the Brighton beach, were 

 dug up at a depth of 11 feet in the valley, above the new church, lying in Coombe 

 rock, which had everj^ appearance of haA'ing been the bed of a stream. Few or no 

 specimens of paleeozoic pebbles were met with similar to those in the old sea- 

 beach, the author inclining to the views of Mr. Godwin-Austen, of a coast-line 

 extending from Colvados across the Channel to Sussex in Postpliocene times, dry 

 land being to the east of this coast-line, whose beach was the same as that now 

 found in the Brighton cliffs, along wbicli the palosozoic pebbles found in the old se.a- 

 beach travelled from France, giving illustrations of pebble -travelling that came 

 under his own observation in the Isle of Win-lit. 



On fhe Traclujte Porjjlii/ries of Antrim and Down, in ilie North of Ireland. 

 Bij Professor Edwaed Hxtll, M.A., F.li.S., F.G.S., Director of tlie Geological 

 Survey of Ireland. 



Trachyte is one of the rarest of the British rocks, and it is as j'et uncertain whether 

 it is to be found amongst these islands except in the north of Ireland. In this 

 district it was identified by the late Professor Jukes and Mr. Du Noyer during the 

 progTcss of the Geological Survey in the year 1807. Under the name of "clay- 



J)orphyry " of Sandy-brae, it is described by Ih-. Berger in his paper " On the Geo- 

 ogical Features of the North-east of Ireland " * ; and the author gives a short 

 account of its characters and relations to the surrounding formations as it occurs 

 both in Antrim and Down f- 



Trachyte Porphyry of Antrim. — The principal mass forms a group of emineneea 

 about four miles to the north of the town of Antrim, called Tardree Mountain, 

 Carnearny Hill (104.3 ft.), Brown Dod Hill, and Scolboa Hill. The tops of three of 

 these hills are formed of basalt in beds capping the trachytic rocks; and it is sup- 

 posed that basaltic sheets enclose the whole of the trachytic district, though the 

 survey of the district being incomplete, the actual limits have not been determined 

 in every direction. 



The mineral constitution of the trach3'te is generally uniform, although the 

 relative proportions of the individual minerals occasionally vary. In general the 

 rock consists of a nearly white or grey felspathic base, with individual crystals of 

 sanidine and a triclinic felspar, blebs or grains of smoke-quartz, and rarely a little 

 mica. In some places the giaius of silica are exceedingly abundant, giving the 

 rock the appearance of rhyolite orperlite, as described by L. von Cotta. Minute 

 crystalline grains of magnetite appear in a sliced section. It is in this state that 



* Geol. Trans, lut ser. vol. iii. p. 1&9. See also Note to ITall's 'Irekind,' vol. iii. 

 t The trachyte porphyry of co. Down, near Hillsborough, is described in the Descriptive 

 Memoir of the Geological Survey to accompany Sheet 36i 



0* 



