BY THE REV. J. MILNE CURRAN. 



219 



structure so characteristic of the jointings in basalt is nowhere to 

 be seen. The joints are planes, sometimes normal to the sides of 

 the columns and sometimes forming small angles with them. As 

 regards thickness, there is no uniformity in the columns. The 

 average size might be taken as eighteen inches across. The 

 weathering of these columns is rather noticeable. As the basalt 

 decomposes it peels off in layers, and the centres of these films are 

 fairly fresh. Plate xvn. shows this peculiar weathering. 



The columns are, for the most part, tetragons, pentagons, and 

 hexagons. With regard to the relative frequency of the various 

 kinds, the following may be taken as a fair estimate — tetragons 

 4 per cent., pentagons 20 per cent., hexagons 65 per cent. 



I made some measurements of the angles of the basaltic columns 

 with these results : — 



Tetragons (sum = 360°) : 



(i.) a 93° (ii.) a — 



b 110 b — 



c 88 c — 



d 68 cl — 



(i.)a 

 b 

 c 

 d 

 e 



359° 

 Pentagons (sum = 540°) : 



-112° (ii.) a 



-121 b 



- 81 c 



-115 d 



■ 95 e 



■113 c 



■ 81 



■ 83 

 . 82 



359 c 



-133 c 

 ■118 

 -100 



- 98 c 

 ■ 89 



524 c 



(iii.) a 

 b 



-114 c 

 130 

 ■ 80 

 •105 

 . 96 



538 c 



525 c 



