313 



stones ; but this could not go far. There are 40 feet of brecciated trap, 

 forming a very steep precipice over it, so that, if quarried to any extent, 

 it should be excavated in the manner of a mine. The height of this 

 mass is from eight to ten feet. 



There is another example of rudely columnar trap at Ardihannon 

 also, on the east side of the by-road leading from the hotel down to the 

 Giant's Causeway. The columns are about two feet diameter, and 14 

 to 16 feet long. It is in the same layer of brecciated trap as the last 

 described, but lower down in the face of the cliffs. This dyke appears 

 also to have been quarried for building to some extent. The face of the 

 mass may be 40 or 50 feet long ; and it thins out at the ends, so that 

 the whole is of a rudely lenticular form. ~No side of a column is a 

 plane, and no solid angle is on a straight line through. The columns 

 are therefore irregular, and indicate perhaps a want of time in cooling 

 to produce the regular articulation of the Causeway columns. In all 

 the varieties of trap, it appears that the thin masses, which are sup- 

 posed to have cooled the most rapidly, are the finest in the grain. The 

 greenstone dykes of Donegal show this fact well. A very thin dyke is 

 almost as fine-grained as coarse black glass. Dykes of pitchstone are 

 often of this kind. Dykes 30 or 50 feet thick are very coarse-grained, 

 and often show the white crystals (a component of the rock) like grains 

 of oats in the black mass of the hornblende. 



Fig. 9. 



Columnar Basalt underlying the tabular Basalt at Craigahulliar. 



Fig. 9 is a sketch of a basaltic range of columns at Craigahulliar, 

 half a mile east of Bally willin church. This is one of the most beauti- 

 ful colonnades that can be seen. It is under a mass of tabular basalt. 

 The extent is 190 feet, presenting its fagade towards the north-north- 

 west. The pillars are from 15 to 18 feet high; and the joints of 

 which they are composed about a foot and a half. Most of them are 



