342 Professor GrenviUe A. J. Cole— The Red Zone 



■urge that the pisolitic layer was produced by contact-action proceeding 

 from the subsequent Upper Basalts. This suggestion may have affected 

 the reception accorded to their views on the decomposition of basalt 

 in situ ; and the larger ovoid blocks of basalt embedded in the litho- 

 marge came to be treated as "bombs of rounded and exfoliating 

 basalt," ' while the series generally was spoken of - as " beds of tuff or 

 ash, together with bole, lithomarge, and pisolitic iron ore, which 

 always occur below the Upper Basalt." Much of this material was 

 believed to have accumulated in lakes.^ 



The appearance of bedding in the 'pavement' — a name for the 

 variegated ferruginous rock underlying the rich pisolitic ore — is 

 certainly clear, and seems highly conclusive in hand- specimens. But 

 successive visits to the famous sections at the Giants' Causeway, which 

 are now rendered so accessible by the railed and carefully constructed 

 path, have convinced me that there, at any rate, tuffs and ashes played 

 no part in the red zone. The neighbourhood of true ash and tuff, 

 splendidly displayed at Carrick-a-rede, served to prevent hasty 

 generalisation ; and I confess I was hardly prepared, when arranging 

 for a general examination of the interbasaltic beds by the staff of the 

 Geological Survey of Ireland, for the way in which section after 

 section declined to show us more than is revealed along the cliff-path 

 at the Giants' Causeway. 



In that typical locality every kind of ' red rock ' that is found 

 in what has been claimed as a stratified interbasaltic series occurs 

 in intimate association with unmistakable basaltic lava. Red 

 bole, without a trace of original structure, is found in, and not 

 merely on the surface of, the basalt. At times this bole is pisolitic, 

 showing that the structure most readily conceded to be of lacustrine 

 origin may also arise by concretion in a decomposing mass. The 

 jointing of the basalt is continuous with that of the overlying jointed 

 lithomarge, and the ' onion structure' between the main joints of the 

 latter is clearly a relic of that which arose from contraction in the 

 basalt. This onion structure occurs in very fresh basalt near Carrick- 

 a-rede, and is evidently only emphasised, and not originated, by 

 decomposition. The lithomarge, of strange purple-grey and violet 

 tints, now and then retains pseudomorphs of the felspars of the original 

 basalt, and consequently traces of the mesh-structure of the basaltic 

 ground. Unreddened iron-ores remain as black specks ; but this type of 

 rock passes into one recognisable only in the field as the product of 

 decay of basalt. The great ' bombs ' of residual lava are surrounded 

 by zones of red and brown and orange, which are the same as those 

 that stripe the lithomarge and 'pavement,' and thus give it an 

 appearance of lamination. At the Causeway such colour-bands run 

 in various directions, and often a concentric group of them alone 

 represents the site of a ' bomb ' that has completely decomposed. 



The alleged 'bombs' can be nothing else than unweathered cores 

 of basalt, in a zone of deep lateritic decomposition, which spread 



1 Mem. Geol. Suit, to Sheet 14 (1886), pp. 20 and 22. Cf. Sir A. Geikie, 

 " Ancient Volcanoes of Great Britain," 1897, vol. ii, p. 204. 

 - Mem. Geol. Surv. to Sheets 7 and 8. 

 3 Ibid, to Sheets 21, 28, and 29, p. 29. 



