454 PROFESSOR GEIKIE ON THE 
Taking the history of volcanic action as a whole within the basin of the 
Firth of Forth during the Carboniferous period, we can recognise two 
distinct types in the occurrence of the rocks,—I1st, Successive streams of 
porphyrite lavas were poured out until their united mass attained a thickness 
of sometimes more than a thousand feet. Comparatively little tuff was ejected 
over these areas, except here and there, in the earlier stages of eruption. 
The lavas now form continuous sheets covering wide spaces of country, 
and rising into conspicuous ranges of hills. This is undoubtedly the pre- 
valent type of the volcanic accumulations in the Carboniferous system in 
Scotland. The Campsie Fells in the Stirlingshire district, already described, 
are only the north-eastern extremity of the extensive volcanic plateaux 
of Dumbartonshire, Renfrewshire, and Ayrshire. The Garlton Hills, in the 
East Lothian, form a small detached area of the same character, and belonging 
to the same period ; while only about 18 miles to the south-east, on the other side 
of the Lammermuir uplands, the great volcanic zone of Berwickshire begins 
with the same kind of rocks, and swells out towards the south-west into the 
ranges of Roxburgh and Dumfries. 2d, The other type is almost confined to 
the basin of the Firth of Forth. It consists in the protrusion of numerous 
detached masses of tuff, and of various augitic lavas never united into wide 
plateaux or extensive hill-ranges, but all pointing to local and sporadic action. 
The four districts in the Forth basin where this type is exhibited may, indeed, 
be viewed broadly as only one area lying between the two districts of the 
Campsie and Garlton Hills, which so characteristically exemplify the first type. 
The local and independent character of the volcanic activity of the second 
type may be connected here with another feature, which cannot fail to strike 
the most casual observer. While the great hollow of central Scotland, between 
the Old Red Sandstone hills on the north, and the Silurian and the Old Red 
Sandstone heights on the south side, continued for fully a half of the Carboni- 
ferous period to be the scene of extraordinary volcanic activity, the eruptions, 
so far as we can judge, were always confined to the valley. It might be 
contended that possibly many sheets of tuff or of lava may have been stripped 
off the bounding hills on either side. But the fact remains, that even were it 
so, these volcanic materials were erupted from orifices in the valley, and not on 
the hills. In no case have I ever met with true volcanic necks on the hills on 
either side of the great central valley.* Denudation could not have removed 
* In the valley of the Nith and its tributary the Carron Water, among the high grounds of 
Dumfrieshire, necks belonging to the Permian series of voleanoes occur, At the head of Lauderdale, 
Myr B, N. Pzacn has observed a small neck coming through the Upper Old Red conglomerate, and 
possibly connected with the volcanic action in which the Berwickshire and Roxburghshire porphyrites 
were erupted. But in these cases the orifices have been opened in deep valleys among the hills. 
[Since this was written, Mr Pracu has met with a number of volcanic necks of Lower Carboniferous 
age in valleys of the Silurian uplands of Roxburghshire, extending to a distance of at least 10 miles 
from the edge of the lava-sheets. ] 
