DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



105 



dolerite, those filled with fragmentary materials bring before us, perhaps more vividly, 

 the volcanic conditions in which they were formed. The agglomerate is generally 

 exceedingly coarse, and without any trace of structure. Blocks of all sizes up to masses 

 some yards in length, and of the most diversified materials, both volcanic and non- volcanic, 

 are dispersed confusedly through a granular paste of similar miscellaneous composition. 



One of the most instructive examples has been already alluded to as occurring at the 

 island of Carrick-a-raide, on the north coast of Antrim. It forms that island, and a 

 portion of the opposite mainland. Its visible mass is about 1000 feet in diameter, but 

 the boundaries, except on the land side, are concealed by the sea. The material filling 

 up this vent is a coarse agglomerate, in which blocks and bombs of basalt, with pieces of 

 chalk and flint, are stuck at all angles in a dull dirty-green granular tuff. Some large 

 and small intrusions of basalt rise through it. Owing partly to these intrusions, and 

 partly to the grass-covered slope that separates it from the line of cliff, the actual contact 

 of this neck with the volcanic beds of the escarpment cannot be seen. I have no doubt, 

 however, that the tuff, which has already been referred to as so conspicuous a member of 



Fig. 29. — Diagram to show the probable relation of the Neck at Carrick-a-raide, Antrim, to an adjacent group of Tuffs, aa, 

 Chalk ; bb, Lower group of bedded basalts ; c, Vent of Carrick-a-raide, filled with coarse volcanic agglomerate ; dd, bedded 

 tuffs ; ee, large veins of basalt traversing the agglomerate ; ff, zone of tuffs and pisolitic iron ore ; gg, Upper group of 

 bedded basalts. 



the series here, was discharged from this vent.* The materials are as usual coarser in 

 the pipe than beyond it, but the finer portion or matrix of the agglomerate is similar to 

 many bands of the tuff. The structure of the locality may be diagrammatically repre- 

 sented as in fig. 29. The bedded tuff is thickest in the neighbourhood of the vent, and 

 gradually dies away on either side of it. 



But another important inference may be drawn from this locality. I have already 

 pointed out that the lower basalts here reach their minimum thickness. Their basement 

 beds thin away towards the vent as markedly as the tuff thickens. Obviously they 

 cannot have proceeded from that point of eruption. Yet, that they had begun to be poured 

 out before the discharge of the tuff, is shown by their underlying as well as overlying that 

 rock, though westward, owing to the thinning away of the undermost basalts, the tuff 

 comes to lie directly on the Chalk. Hence, we may legitimately infer the existence of one 

 or more other vents in the neighbourhood that supplied the sheets of the lower basalts. 



In the promontory of Ardnamurchan, where the basalt-plateau has been so obscured 



* See Explanation of Sheets 7 and 8, Geol. Survey of Ireland (1888), p. 31. 



