CHAP. XXV 
LAVAS AND TUFFS OF THE PLATEAUX 
3S5 
or of rod marl. In this, as well as in other respects, they piesent much 
resemblance to the lavas of the Tertiary plateaux of Antrim and the Inner 
Hebrides. They are generally marked off from each other by the slaggy 
upper and under portions of the successive Hows, and this structure gives a 
distinctly bedded aspect to the escarpments, as in the Campsie and Largs 
Hills, or still more conspicuously in Little Cumbrae (Fig. 107) and the 
southern end of Lute. Considerable diversity of structure may be noticed 
among these sheets. Some present a compact jointed centre passing up and 
down into the slaggy material just referred to; others liave assumed a vesicular 
character throughout, the vesicles being often elongated in the direction of flow. 
Where, as usually occurs, the vesicular is replaced by the amygdaloidal 
structure, some of the rocks have long been famous for the miueials found 
in their cavities. The beautiful zeolites of the Kilpatrick and Kenfrewshire 
Hills, for example, may be found in every large mineralogical collection in 
the country. AVell-developed columnar structure occasionally appears among 
the lavas of the plateaux, but chiefly, so .far as I have observed, in the lower 
or more basic group, as in the basalts along the cast side of the Dry Dam 
at Arthur Seat. 
In each plateau the lavas may be observed to thicken in one direction, 
or more usually towards more than one, and this increase no doubt indicates 
in which quarters the chief centres of discharge lay. Thus in the Clyde 
plateau, several areas of maximum development may be detected. In 
the Kilpatrick Hills the total thickness of lavas and tuffs exceeds 3000 
feet (Fig. 120). Above Largs it is more than 1500 feet, rapidly thinning 
away towards the south. The continuation of the plateau far to the north- 
east in the Campsie Fells reveals a thickness of about 1000 feet of lavas 
at Kilsyth, which become thicker further west, but eastward rapidly diminish 
in collective bulk, until in about twelve or thirteen miles they disappear 
altogether, and then, as already remarked, the Calciferous Sandstone series 
closes up without any volcanic intercalation. 
In the Solway plateau, the lavas attain a maximum development about 
Birreuswark, whence they diminish in bulk towards the north-east and 
south-west. The Berwickshire plateau rcjaches its thickest mass about 
Stitchill, whence it rapidly thins away towards the north-east, until at a 
distance of some twelve miles it disappears altogether, the last trace of 
it in that direction being a band of tuft which dies out in the Calciferous 
Sandstones to the north of Duns. 
In the Midlothian Plateau, the development of the volcanic series is 
more irregular than in any of the others. As already remarked, tliere 
appear to have been at least two chief centres of discharge in tliis legion, one 
at Fdiiiburgh and one some fourteen miles to the south-west. At the former, 
the volcanic materials attain in Arthur Seat and Calton Hill a thickness of 
about 1100 feet. In Craiglockhart Hill, three miles distant, they are still 
about 600 feet thick. But beyond that eminence they cea.se to be traceable 
for about eight miles, either because they entirely die out, or because their 
dwindling outcrops are concealed under superficial deposits. As we approach 
VOL. I 2 c 
