.Remeivs — Prof. Hull's Physical Geology of Ireland. 125 



largely eroded, and an irregular bed of flint-gravel, with red ochre filling up the 

 hoilaws in the rocks, generally occurs between the chalk and the sheets of volcanic 

 lava by which it is covered. There has not been much contortion or disturbance of 

 the beds, which are, for the most part, nearly horizontal, but as a whole they form 

 a shallow basin-shaped area (the central depression of which is marked approximately 

 by the course of the Eiver Bann) , dipping towards this depression from the sides. 

 We do not see that our author notices the fact, which is certain to be among the 

 first to strike the stranger, that the Chalk of Ireland is a hard, compact, white 

 limestone, without any of the soft granular and powdery character distinctive of the 

 English Chalk. 



The occurrence of the Cretaceous formation in this isolated locality, so far from the 

 nearest north-west point of its area in England, is of much interest, as bearing on 

 the question of its former greater extension. Prof. Judd has also found Cretaceous 

 rocks in the Isle of Mull, far away to the north, and the sharply truncated edge of 

 the formation along its western and eastern faces in Ireland would seem to establish 

 that it formerly covered a considerably larger area. It is not improbable that it 

 was even connected with the English area at the time of its deposition ; and if so, it 

 would appear probable, further, that this connexion occurred over the lower country 

 or depression which now separates the mountains of "Wales from those of Cumberland 

 and North Lancashire. To the west, it was probably limited by the highlands of 

 Donegal and Sligo. 



The old volcanic rocks of Antrim, long classical ground to the geologist, which 

 cover nearly all the area in that county, as well as considerable portions of London- 

 derry and Tyrone, forming generally a very well marked and elevated plateau, 

 succeed to the Cretaceous beds just noticed. Prof. Hull (Brit. Asso. 1874) had already 

 pointed out that these were capable of a three-fold division : the earliest stage 

 marked by trachyte porphyry (Sandy Braes, Temple Patrick, Hillsborough). The 

 masses are very limited in extent as compared with the succeeding overflows. The 

 rare mineral Tridymite has been found in the cavities of these rocks. The actual 

 age of these outbursts is unknown, but Prof. Hull thinks they probably belong to 

 the later part of the Eocene period. They are thus possibly the only Eocene rocks 

 in Ireland. The middle stage of the volcanic rocks is characterized by vesicular 

 amygdaloids, and basalts, often separated by beds of red bole, and succeeded by vol- 

 canic ashes and larger bombs of trap. These are supposed to have been subaerial 

 discharges of volcanic matter, the intensity of the volcanic action being succeeded 

 by a period of repose, during which peculiar beds of pisolitic iron ore, and layers of 

 fine debris of the rocks, containing plant-remains, were deposited, probably in lacus- 

 trine waters. These plants have proved similar to those found in the Isle of Mull, 

 and have thus been taken to indicate a similar Miocene age. And then, finally, after 

 this period of repose, the volcanic forces again became very active, and gave forth 

 vast flows of lava, now successive beds of columnar basalt, which are often seen 

 resting directly on the thin beds of iron ore, or on a bed of lignite. 



The total thickness of these volcanic rocks is often more than 1000 feet : all 

 actual craters and cones have long disappeared by denudation, but ' necks ' are still 

 to be traced through which some of the volcanic outbursts were forced up. These 

 have been noticed by other observers, but om* author brings forward as an instance, 

 a locality visited yearly by hundreds of tourists, namely the now detached and ruined 

 Castle of Dunluce between Portrush and the Giant's Causeway — which he states is 

 built on a mass of agglomerate, originally forming one of these ' necks.' In a sub- 

 sequent page, however (p. 70), he says his views on this point have been recently 

 altered during a visit to the localities along with Prof. Ramsay and Mr. Traill, and 

 that he is now disposed to view these cases as simply pipes formed by filtration out 

 of the chalk, into which the basaltic masses have fallen or slipped down, thus giving 

 rise to their fragmental appearance.^ 



* Those who are curious to trace out accurately the successive steps by which 

 opinions are formed, put forward and obtain currency, in such matters, will be 

 amused to follow up this case of supposed ' necks ' in the basaltic country of Antrim . 

 Let them look first then to Portlock's Report on Londonderry, etc., 1843 — more than 

 thirty years since, and at p. 92 they will find a carefully drawn view of one of the 

 instances referred to by Prof. Hull, and an equally carefully drawn up description of 

 the appearances presented by the rocks. Portlock says: "The trap penetrates 

 into the chalk, and forms a conglomerate with the flints and fragments of chalk it 



