Frof. G. A. J. Cole — Red Zone in Basalt of Co. Antrim. 341 



of silica and a fair quantity of oxide of manganese. I have not found 

 any certain indication of arsenic, but small quantities of antimony are 

 present, and I have detected a trace of copper." Possibly, then, these 

 pebbles may have been derived from the ' blackband ' (beds impreg- 

 nated "with iron-carbonate) of the Coal-measures, much of the metal 

 having been removed and the rest altered to limonite by the action 

 of water. 



II. — The Red Zone in the Basaltic Series of the County of Antrim. 

 By Grenville A. J. Cole, F.G.S., Director of the Geological Survey of Ireland. 



ONE of the most beautiful features at the Giants' Causeway, from 

 an artistic as well as a geological point of view, is the broad red 

 zone that divides the Lower from the Upper Basalts. As is well 

 known, this zone of lithomarge, bole, and laterite is remarkably 

 persistent in north-eastern Ireland, and represents an interval of 

 Eocene time when volcanic activity was lessened and when the basalts 

 ceased to appear at the surface. At the same time, however, sporadic 

 eruptions of rhyolite occurred, and some of the cones of acid lava 

 supplied material for an interbasaltic conglomerate of rhyolite pebbles, 

 which was discovered several years ago by Mr. A. McHenry near 

 Glenarm.' 



The occurrence of leaf-beds, lignites, pisolitic iron-ores, and pale 

 bauxitic muds on this horizon has led to the prevalent opinion that 

 the whole red zone and the associated rocks of lighter colour are of 

 detrital origin. Portlock^ seems to have laid the foundation for this 

 view in 1843, when he regarded the amj'gdaloidal basalt as an alteration- 

 product derived from a compacter form, and suggested that some of 

 the amygdaloids were poured out "rather in the state of volcanic 

 mud than of lava." But at the same time he regarded the bole as 

 resulting from the induration of amygdaloids that "have been decom- 

 posed on the surface," and pointed out the intimate association of the 

 hard and soft rocks in the uppermost portion of the Lower Basalts. 



Prom Portlock's time onward, the tendency has been to emphasise 

 the appearances of stratification and fragmental structure along this 

 zone ; and John Kelly,^ in 1869, in his remarkable paper on the geology 

 of Antrim, refers to the deposits as " volcanic ashes thrown up in the 

 eruption, and disseminated in the water, making literally a red sea." 

 On the other hand, Messrs. Tate and Holden,* almost in the same year, 

 maintained that the lithomarge and bole are but decomposed basalts, 

 and traced the passage from unaltered blocks into the soft rock round 

 about them. They held that the basalt was poured out under Avater, 

 and rotted there, so as to produce the red zone, and went so far as to 



1 See Sir A. Geikie, Anniversary Address, Quart. Journ. Geol. Soc. London, 

 vol. xlviii (1892), Proc, p. 168; and McHenry, Geol. Mag., 1895, p. 260. 



* " Report on Geology of Londonderry, etc.," Dublin, 1843, pp. 145-6. 



^ " On the Geology of the County of Antrim, with parts of the adjacent Counties, ' ' 

 Proc. P. Irish Acad., vol. x, p. 307. 



* " On the Iron-ores associated with the Basalts of the North-East of Ireland," 

 Quart. Journ. Geol. Soc. Lond., vol. xxvi (1870), pp. 155 and 158. 



