CHAP. XXV 
LAVAS AND TUFFS OF THE PLATEAUX 
387 
Although tuffs play, on the whole, a comparatively unimportant part 
among the constituents of the plateaux, they attain in a few localities an 
exceptionally great development, and even where they occur only as thin 
partings between the successive lava-flows, they are always interesting 
memorials of the volcanic activity of a district. In many portions of the 
plateaux, the lowest members of the volcanic series are tuffs and agglomer- 
ates, showing that the eruptions often began with the discharge of frag- 
mentary materials. Thus in the Midlothian plateau at Arthur Seat, though 
the lowest interbedded volcanic sheet is a dolerite, it is immediately followed 
by a series of bedded tuffs, before the main mass of the lavas <jf that hill 
Fig. 115. — Section of Craiglockliart Hill, Ediubnigli. 
1. Red sandstone.s and clays ; 2. Green stratified tuffs ; 3. Columnar basalt ; 4. Dark shales, iron.stones 
and sandstones, witli plants. 
make their appearance. At Craiglockliart Hill, three miles distant (Fig. 115) 
this lowest lava is absent, and a group of tuffs about 300 feet thick rests 
immediately on the red Carboniferous sandstones and shales, and is overlain 
by sheets of columnar basalt. The scoriaceous bottom of the latter rock may 
here and there be seen to have cut out parts of the tuff as it rolled over the 
still unconsolidated material. 1 n the same district, a few miles further to the 
south-west, some interesting sections of tlie Midlothian plateau are laid bare in 
the streams which descend from the western slopes of the Pentland Hills. I 
4 - 
Fig. 116. — Section of the bottom of the Midlothian Plateau, Linnhouse Water above Mid-Calder Oihvorks. 
1. Shales and cement-stones ; 2. Sandstones; 3. Highly vesicular lava ; 4. Tiifls and sandstone bands. /, Fault. 
may cite, in particular, those expo.sed in the course of the Linnhouse Water. 
At the railway viaduct near the foot of Corston Hill, a good section is 
displayed of the Cement-stone group — thick reddish, purplisli, and greenish- 
blue marly shales or clays, witli thin ribs and bauds of cement-stone and 
grey compact cyjirid-limestone, as well as lenticular seams and thicker beds 
of grey shaly sandstone, sometimes full of ripple-marks and sun-cracks. 
These strata, which exactly reproduce the typical litliological characters of 
the Cement -stone group of Stirlingshire (Ballagan Beds), Ayrshire and 
Berwickshire, are surmounted by a group of reddish, yellow and brown 
sandstones, sometimes pebbly and containing a band of conglomerate. 
