302 



pal layers of columnar trap at Plaiskin, and along the north coast, two 

 miles long and 60 feet thick, extending from the Causeway to Port 

 Moon. This layer, however, is not brecciated throughout ; there are 

 some small lenticular layers of red ochre in it, and also layers of black 

 tabular trap in parts of it, full of ca vities and air-holes ; but the brec- 

 ciated character is prevalent. The lignite bed, six feet thick, got close 

 to the Causeway, occurs in it. 



3. Columnar Trap is that kind which occurs in columns. The 

 columns seem, in every case, to assume the position of being at right 

 angles, as near as may be, to the two surfaces between which the melted 

 mass was injected ; for it appears to me that layers of columnar trap, 

 whether in a vertical position, sloping or horizontal, have been always 

 injected into fissures made in larger solid masses. 



At Bengore Head, Point Plaiskin, and the Causeway (see PI. 

 XXIV.), columnar trap shows itself in two grand layers ; the upper 

 one, No. 10, standing in rather coarse pillars, 60 feet high, like a vast 

 colonnade ; next is the layer of brecciated trap, 60 feet thick, No. 9, just 

 described ; and below this is the lower range of columns, JSTo. 8, about 

 40 feet high. This latter is much more perfect in its articulation than 

 the upper layer. The Giant's Causeway is at the west end of it, and 

 forms a part of it. 



Fig. 6. 



Concretionary Trap, or " Onion" Basalt on the Eoadside, near the Causeway. 



4. Concretionary Trap is that kind which appears in round balls, 

 and decomposes in concentric layers, which shed off like the coats of an 

 onion. Those balls occur from three inches or less in diameter to three 

 feet ; they are sometimes ten feet or more. They are very hard when 

 quarried fresh, but in time, by exposure, shed off layers by degrees, till 

 the ball is reduced to nothing. The sketch (Pig. 6), taken at the road- 

 side leading from the hotel down to the Causeway, shows the appearance 

 of a mass of this kind, and gives an idea of how it decomposes. It is 

 common through the country. The finest I know of this kind is on the 



