60 Trof. Dr. Ferdinand Roenier — A Visit to Ireland. 



basalt waslied by tbe waves, and during winter storms overflowed 

 by the sea. It is simply the regularity of its columns that has 

 made it celebrated. This regularity is particularly striking, as one 

 sees not only the prisms from the side, but, by going over their tops, 

 the broken ends, plainly showing the six-sided or polygonal sections. 

 This peculiarity distinctly characterizes the phenomenon. The 

 thickness of the columns, usually amounting to over a foot, corre- 

 sponds to the breadth of the crevices between neighbouring columns, 

 the spaces being mostly half an inch broad. The concavo-convex 

 jointings of the columns are frequently developed in great per- 

 fection. On the land side the Giant's Causeway is bounded after 

 the manner of an amphitheatre by perpendicular walls of basalt 

 several hundred feet in height. Here, however, the regular 

 prismatic formation is absent. The rock being partly amygdaloidal, 

 and the cavities covered with various kinds of zeolites. Other- 

 wise, the basalt of Northern Ireland is altogether of the self-same 

 age with that of Germany and Central Europe, which is proved by 

 the occurrence with it of a layer of red clay containing impressions 

 of leaves of Miocene trees. Mr. W. H. Baily has particularly de- 

 scribed such a stratum exhibiting this kind of plant-growth in the 

 neighbourhood of the town of Antrim. It is only a few inches 

 thick, and rests proximately on a ten to twelve foot deep series of 

 strata holding lumps of iron-ore (" Notice of Plant-remains from 

 Beds interstratified with the Basalt in the County of Antrim," Quart. 

 Journ. Geol. Soc. 1869, vol. xxv. p. 162). 



Having returned from the Causeway to Portrush, we pursued our 

 journey from the latter to Belfast. On the way thither we had to cross 

 in its entire length the great basaltic plateau of the North of Ireland. 

 It is almost a level or rather undulating district, on whose surface the 

 great dark blocks of basalt are everywhere scattered. But in the 

 neighbourhood of the town of Antrim there rises a group of hills 

 consisting of igneous rocks of an entirely diiferent kind, and the 

 central point of these is the round bowl-shaped Tardree mountain, 

 a few miles from Antrim. Several quarries, in which the rock is 

 worked for economic purposes, are situated on its borders. It is a 

 light grey felspar rock, rich in quartz, and of the trachyte group. 

 Professor E. Hull, who has lately fully described its characteristics 

 ("Memoirs Geol. Survey: Explanatory Memoir to accompany Sheets 

 21, 28, and 29 of the Maps of the GeoL Survey of Ireland, including 

 the country around Antrim, etc., Dublin, 1876," p. 17 et seq.), 

 has named it Trachyte-porphyry, and J. Eoth (Beitr. Petrogr. 

 pluton. Gest., 1873, p. xxxiii) places it among the liparite group, 

 following a published analysis by E. Hardman. According to Prof. 

 Hull, the rock is covered by basalt, and is considerably older than the 

 latter. We had not the opportunity of fully examining these con- 

 ditions of the strata on our flying visit to the quarries. The isolated 

 appearance of the rock in such a limited space is at any rate well 

 worth observation. In no other part of Ireland are trachyte rocks 

 known, [except to the west of Hillsborough, Co. Down.] 



The environs of Belfast are also deeply interesting to the geologist. 



