56 THE BLACK HILL OF EARLSTON 



Hence is formed an intrusive sheet, or a dyke, as the case 

 may be. 



It was in this way, and at this period, I think, that the 

 great sheet of trachyte was intruded, whose remains now form 

 the capping to which the peculiar shape of the Black Hill of 

 Earlston is due. It has to be remembered, in this connection, 

 that not only was the Upper Old Red Sandstone which now 

 forms the base of the Hill continuous far and wide around, but 

 that the Carboniferous Eocks were almost certainly present 

 in considerable thickness when this great trachyte sheet ate 

 its way into the rocks around. 



The shaping of the Hill into its present form is, geologically 

 speaking, quite a recent event. The agents concerned have 

 certainly been little else than "rain and rivers," though 

 glacial action, during the Age of Snow, must have modified 

 the work of the former agents to a considerable extent. 



The two figures of fossil fishes are copied from restorations 

 by Dr Traquair, and are from Harvie Brown's " Vertebrate 

 Fauna of the Moray Firth." The sections are intended to 

 illustrate the structure of the Hill, with especial reference to 

 the part played by the trachyte in determining its present 

 form. 



