98 GEOLOGY OF THE LOTHIANS. 



however, the most usual forms, and on decomposition, these 

 columns appear to consist of globular concretions, regularly 

 superimposed on each other. On examining this columnar 

 rock at right angles to the axis of the prism, the globules 

 exhibit an alternating series of layers of a very ferruginous 

 trap, with others having a less quantity of iron disseminat- 

 ed through them, and separating the columns : there very 

 generally occur veins of iron flint. 



The late Dr M'Culloch, in his System of Geology, vol. i. 

 p. 172, describes this rock as a sandstone rendered colum- 

 nar by a slow refrigeration from a state of igneous fusion. 

 In a theoretical point of view, this is an error of much im- 

 portance, as it is awarding a changing power to igneous ac- 

 tion, of greater magnitude than, in this instance, nature war- 

 rants us to do. The situation of this rock, erroneously des- 

 cribed as columnar sandstone, is one which is the reverse of 

 harmonizing with a metamorphosis on so gigantic a scale as 

 exists in the altering of a stratified sandstone into one per- 

 fectly columnar. It is interposed between beds of trap-tufa, 

 a rock which does not, in the generality of cases, appear to 

 have been in so highly-heated a state as to change, to a 

 great extent, the original structure of strata. In regard 

 to its chemical composition, we may mention, that this is as 

 incongruous with its being an altered sandstone, as are its mi- 

 neralogical characters, all of which are too plain to allow it 

 to be a subject on which two opinions can with propriety 

 be held.* The only other trap rock which occurs on this 

 shore appears about a mile to the south of Dunbar ; it is 

 composed of greenstone, either compact or amygdaloidal, and 

 is many yards broad. It contains numerous veins of calca- 



* A rock having the same mineral characters as that of Dunbar, but 

 amorphous, occurs at Usan, a fishing village near Montrose. It is asso- 

 ciated with claystone amygdaloid. 



