250 Prof. Sedgwick on the Geological Structure 



On the Geological Structure and Relations of the Frontier 

 Chain of Scotland. By the Rev. Professor Sedgwick, 

 F.R.S., &e. 



I. — General Remarks on the Chain. 



This chain, as is well known, extends from St Abb's Head, 

 on the east coast, to the Mull of Galloway, on the west coast 

 of Scotland, and it reappears, with identical mineral compo- 

 sition and in the line of its strike, on the east coast of Ire- 

 land. Whatever, therefore, may be proved respecting the 

 general relations of the chain in Scotland, must apply to, at 

 least, a considerable portion of the prolonged chain in the 

 north of Ireland. 



Leaving out of account all igneous and intrusive rocks, the 

 chain is essentially composed of a peculiar form of grey- 

 wacke, often coarse, and sometimes, though rarely, passing 

 into a very coarse conglomerate, not unlike some of the con- 

 glomerates among the old rocks of South Wales. These hard, 

 coarse beds, alternate indefinitely with a peculiar soft, earthy, 

 and often pyritous alum-slate, which frequently has undergone 

 such compression and induration that it passes into an earthy 

 flagstone, and more rarely into a pretty good roofing-slate ; 

 but in no instance is the slaty structure distinct from the 

 stratification in any quarries that are worked for use. 



From one end of the chain to the other the beds are highly 

 inclined, strike generally in the mean direction of the chain, 

 and are thrown into contortions and undulations. The great 

 protruding granitic masses never form any true mineralogical 

 centre, though producing, as might be expected, considerable 

 local derangements, local changes of structure ; and, in a few 

 instances, they are accompanied, near their junction, with 

 the phenomena of mineral veins. The axis of the chain, the 

 centre of the vast undulations, seems to be very ill defined ; 

 and the difficulty of determining this point is greatly increased 

 by the bogs and extensive vegetable accumulation by which 

 the sections are much covered. 



The author crossed the chain in 1841 with Mr J. Carrick 

 Moore (now one of the Secretaries of the Geological Society 

 of London) ; and in 1848 he again crossed it, both on the 

 western coast line and on two other traverses, in the hopes 



