228 



ROBERT B. SOSMAN 



The regularity and symmetry of the columns of the Giant's 

 Causeway suggests the convectional origin. It seemed of interest, 

 therefore, to examine a polished cross-section of one of these columns 

 for evidence of differentiation. Through the kindness of Dr. G. P. 

 Merrill, of the United States National Museum, a polished section 

 was cut for us from a Giant's Causeway column in the Museum, 

 and also one from a column from near Bonn on the Rhine. 



TABLE III 

 Difference in Density between Axis and Surface of Basalt Columns (Delesse) 



Density 



Center 



Outside 



Difference of 

 Density 



Trachyte, Iceland 



Trachyte, Isle Ponce. . 

 Phonolite, Isle Lamlash 



Trap, Antrim 



Basalt 



Basalt 



Basalt 



Basalt 



Basalt 



494 

 469 



S4i 

 911 



930 

 030 



2.924 

 3-053 

 3 044 



■ 478 

 439 

 ■509 

 857 

 •933 

 • 030 

 .916 

 .030 

 .008 



Per Cent 

 O.64 

 1 . 21 

 I. 26 



1.8s 



— o. 10 

 0.00 



o. 27 



0.75 

 1. 18 



The Bonn column was five-sided, with a maximum cross dimen- 

 sion of 18 cm. The cross-joint near which the section was cut 

 showed fracture lines radiating from one corner, and the joint passed 

 straight across. What appeared to be an inclusion about 16 mm. 

 in diameter showed near the center, and another of similar size 

 seemed to have been cut in two by one face of the column. A sharp 

 weathered zone 3 mm. wide showed clearly, but no other difference 

 between center and border appeared. 



The Giant's Causeway column was also five-sided, with a maxi- 

 mum cross-dimension of 37 cm. The section was cut near the 

 convex side of a shallow ball-and-socket joint; the fracture of this 

 joint seemed to have radiated from the center, not from any point 

 of the border. The rough surface gave an appearance of finer 

 grain at the border than at the center. On the polished face, how- 

 ever, no such gradation was visible. There was a sharp weathered 

 zone 3 mm. wide, inside of which was a zone varying from 6 to 



