374 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
the Geological Survey, and have been described in the Survey Memoirs. 
The rest of the plateau to the south-west is much less familiar. 
In Fig. 112 the great escarpment which descends from the right 
towards the centre is the sill of Salisbury Crags. The long dark crag 
(Long How) rising between the two valleys is the lowest of the interstrati- 
fied lavas. The slope that rises above it has been cut out of well-bedded 
tuffs, on which lie the basalts and andesites in successive sheets that form all 
the eastern or left side of the hill. The rocks around the summit belong to 
a much later period of volcanic eruption, and are referred to in Chapter xxxi. 
The rocks of this plateau are comparatively limited in thickness, and 
have a much more restricted vertical range than those of other districts. 
At Arthur Scat and Coi’ston Hill the}' begin above the cement-stones and 
cease in a low part of the great group of white sandstones and dark shales 
which form the upper half of the Caleiferous Sandstones of Midlothian. 
They do not ascend as high as the Ilurdiehouse Limestone, which to the 
west of Corston Hill is seen to come on above them. One of their most 
iTG. 112. — View of Avtliur Seat iVoni Caltoii Hill to the nortli. 
remarkable features is the manner in which they diminish to a single thin 
bed and then die out altogether, reaijpearing again in a similar attenuated 
form on the same horizon. This impel’sistence is well seen in the south- 
western part of the area, between Tluteland, in the parish of Currie, and 
CTosswood, in the parish of Mid-Calder. The lowest more basic band may 
there be traced at intervals for many miles without the overlying andesitic 
group. Yet that andesites followed the basalts, as in other plateaux, is well 
shown by large remnants of these less basic lavas left in Arthur Seat and 
Calton Hill. On the extreme southern margin of the area also a thin band 
of porphyrite with a group of overlying tuffs is seen above the red sand- 
stones near Dunsyre.^ The eruptions over the site of this plateau seem to 
have been much more local and limited than in the other plateaux. They 
appear to have gathered chiefly around two centres of activity, one of which 
lay about the position of Kdiuburgh, the other in the neighbourhood of 
Corston Hill. It is worthy of remark that this tract of volcanic material 
flanks the much older range of lavas and tulls of the Pentland Hills and 
b:iyiila nation j <Jeol. tSuri\ Scotland, Slieet 24, p. 13 (1869). 
