172 ON THE CONCRETIONARY ANJD 



stones, it must have been overlooked by geologists; 

 and the only two instances with which I am acquainted 

 are found in the island of Rum and at Dunbar, in 

 Scotland. The fact is noticed in the account of that 

 rock; (Chap, xxxv.) the inferences are more usefully 

 placed here. 



The columns that occur in the sandstone of Rum 

 are of small dimensions^ not exceeding a few inches 

 in diameter. They lie in the stratum, in perfect 

 contact, presenting the usual intermixture of polygonal 

 forms; and, what is especially necessary to notice, they 

 are covered by a mass of basalt. At Dunbar, the sand- 

 stone in which the columnar arrangement is found, is 

 that which is known to be the lowest of the secondary 

 strata, and which, throughout a great extent of coun- 

 try, presents only the usual stratified character. The 

 columns are limited to a small space, but are of con- 

 siderable dimensions; attaining two feet or more in 

 diameter, and a length of 15 feet or upwards. Where 

 this columnar structure occurs, the character of the 

 rock is changed in a greater or less degree ; becoming 

 more compact, harder, and, in some places, passing 

 into a perfect but coarse jasper. In addition to this, 

 it presents the indications of an internal concretionary 

 structure, similar to that which might be inferred to 

 exist in the columns of trap, from the mode, already 

 mentioned, in which they are found to desquamate. 

 The transverse sections of each prism are marked by 

 concentric lines of different colours, whitish and 

 reddish; which conform accurately to the sides and 

 angles, towards the exterior, but become gradually 

 curved as they approach the centre; indicating the 

 probable existence of a spheroidal nucleus. This 

 disposition, it is plain, is real, and unconnected with 

 any agency of the atmosphere. 



The columnar shales or argillaceous iron stones 



