of Edinburgh, Session 1881-82. 
599 
geological horizon. The striking unconformability of these Upper 
Old Red Sandstones is connected with great inequalities in the 
surface upon which they were deposited. In some places they 
swell out to a thickness of many hundred feet; but elsewhere they 
may be seen rapidly to die out against an underlying prominence of 
older rocks round which the earlier Carboniferous groups overlap. 
The general contours of the ground must consequently have been 
remarkably varied. It is possible even yet to define approximately 
the outlines of some of the shores of the period. 
The Upper Old Red Sandstone is overlaid by a volcanic zone, 
consisting mainly of various porphyrite lavas, with occasional tuffs. 
This zone forms a marked feature in the geology of Berwickshire 
and Roxburghshire, running in a line of conspicuous eminences, 
and serving as a convenient platform to mark the base of the 
Carboniferous system. It can be followed from the southern edge 
of the Lammermuir chain to near the mouth of the Kith, and it 
reappears even on the west side of that river in Kirkcudbright. 
Associated with it are numerous “ necks,” marking some of the vents 
from which the volcanic material was ejected, and as these “ necks ” 
occur among Silurian hills, at some distance from the present edge 
of the porphyrites, it is evident that these latter once covered a 
much wider area, from which, together with much of the overlying 
Carboniferous rocks, they have been denuded. The volcanic rocks 
in the south of Scotland agree in general character with those which 
occupy a similar geological position in Ayrshire, Renfrewshire, 
Dumbartonshire, and Stirlingshire, and both are probably referable 
to the same period in the volcanic history of the country. Iu the 
Eskdale district the porphyrite band consists of one or two 
separate flows, with an aggregate thickness of from 40 to 100 feet. 
From the top of the volcanic zone the Carboniferous system may 
be followed in many excellent natural sections up into the Scar or 
Main limestone of the English Carboniferous Limestone series. 
In Berwickshire there is a great development of that peculiar phase 
of the Lower Carboniferous rocks to which the name of “ Cement- 
stone group ” has been given by the Geological Survey of Scotland. 
The strata consist of grey, white, and yellow, less frequently reddish, 
sandstones, with blue, grey, and greenish shales and clays, bands 
and nodules of cement-stone or impure limestone, and occasional 
veinings of gypsum. On the west side of the Cheviot Hills, which 
