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Dr. Mac Culloch on a 



tudes, they form an uneven columnar floor resembling the 

 well-known causaway of Staffa. This very perfect part of the 

 structure does not, however, occupy a space of more than a 

 few square yards. 



As the columnar division takes place in the manner most 

 common in the trap rocks, namely, at right angles to the stra- 

 tum, the vertical inclination of the protruding columns is ten 

 degrees, or thereabout. Their diameters vary from eighteen 

 inches to two feet, or more ; and, as far as can be discovered, 

 their greatest altitude is about ten or twelve feet. It is not 

 certain, however, that this is their total height, or the greatest 

 thickness of the prismatic part of the sandstone bed; as a 

 great part of the surface has been broken or worn away during 

 the lapse of time. The angles of the prisms vary in number; 



Fig. 1. Concentric Structure of the Columns at Dunbar 



but, like the cases most frequent in the trap rocks, forms of 

 five, six, and seven sides are the most numerous. They do not 



