Ixxxiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. Ixxiii, 



])rincipal trough was what is often called the Midland Valley of 

 Scotland, that is, the lowland tract, some 50 miles broad, which 

 divides the Highlands from the Southern Uplands. Subsidence of 

 this area, with correlative elevation of the contiguous tracts, con- 

 tinued through Old Red Sandstone times, and eventually passed 

 into important trough-faulting. It was along the two opposite 

 boixlers of the depressed area, where differential stress reached its 

 maximum, that volcanic eruption broke out. It began, as 

 Dr. Campbell has proved at Stonehaven, in the Downitonian, but 

 belongs in the main to the Lower Old Ked Sandstone. On the 

 north-western side, where activity was most vigorous, it affected 

 a stretch of countrv which, including its prolongation into Ireland, 

 has a length of about 300 miles. Of the other detached or semi- 

 detached areas of depression the most irajwrtant are the Lome and 

 Glencoe district on one side and the Cheviot district on the other- 

 In the far north two or more minor volcanic centres are indicated 

 in the Orkneys and Shetlands, while smaller outbreaks in the Kil- 

 larney district and in County Waterford mark perhaps the limit 

 of volcanic action towards the west and south. 



In these subsidiary basins the linear trough-form was much less 

 pronounced, and a marginal arrangement of volcanic vents is not 

 clearly discernible. None the less, the intimate relation between 

 igneous action and crustal displacement is very apparent. It is 

 illustrated in the most striking fashion l)y the cauldron -subsidence 

 of Glencoe with its accompanying outflow of lavas, as described by 

 Dr. Clough, Mr. Maufe, & Mr. Bailey, The plutonic phase which 

 succeeded, not in the Midland Yalle}" but in Lome, Cheviot, and 

 the Shetlands, tells the same story. In each area the plutonic 

 rocks have broken through as abruj^t bosses in the midst of the 

 volcanic outpourings ; and the cylindrical plugs of Beinn Cruachan 

 and Beinn Nevis, associated with a peculiar piston-like faulting, 

 are the analogues in this phase of the neighbouring Glencoe 

 subsidence. The numerous dykes clustered round these two centres 

 present a sheaf -like arrangement about a north-easterly and south- 

 westerly axis, showing the regional stress modified in some degree 

 by the local. In each case there is a second, inner plug of 

 granite, A^ounger than the majority of the dykes, imphdng a 

 duplication or alternation of the plutonic phase and that of minor 

 intrusions. In the Cheviot district stress of the regional type was 

 of little moment during the final phase, for the dykes about that 

 centre approximate to a regular radiate disposition. 



