484 PROFESSOR GEIKIE ON THE 
was eventually arrested first by the deposit of a dark mud, now to be seen in 
the form of three or four inches of a black pyritous shale (E), and next by 
the inroad of a large quantity of dark sandy mud 
ee’: and drift vegetation, which has been preserved as a 
‘sandy shale (F), containing Calamites, Producti, ganoid 
scales, and other traces of the life of the time. Finally, 
a great sheet of lava, represented by the uppermost 
amygdaloid (G), overspread the area, and sealed up 
these records of Palaeozoic history.* 
A final example may be cited of the regular alter- 
nation of lavas and tuffs with each other, and with — 
ordinary sedimentary accumulations. The well-known — 
Calton Hill of Edinburgh consists of the succession 
of rocks shown in the subjoined section (fig. 24). The 
great mass of the hill is made up of beds of porphyrite, 
Fig, 23.—Section in Wardlaw representing true superficial lava-currents (Nos. 1, 
Quarry, Linlithgowshire, § 5, 7, 9, 11, 18, 15). With these are intercalated bands © 
of nodular tuff, and occasional seams of shale and sandstone, more or less 
charged with volcanic detritus (Nos. 2, 4, 6, 6, 8, 10, 12, 14). The whole 
Fig. 24.—Section of Calton Hill, Edinburgh. 
of this thoroughly volcanic series of rocks passes conformably under the 
Calciferous sandstones and shales shown at the right hand of the diagram. 
As the interstratified lavas and tuffs were laid down in sheets at the surface, 
they necessarily behave like the ordinary sedimentary strata, and have under- 
gone with them the curvatures and fractures which have affected this region 
since Carboniferous times. Notwithstanding their volcanic nature, they can be 
traced and mapped precisely as if they had been limestones or sandstones, 
This perfect conformability with the associated stratified rocks is strikingly 
seen in the case of the great sheets of lava which, as I have already said, lie 
imbedded in the heart of the Borrowstounness coal-field. The overlying strata 
having been removed from their surface for some distance, and the ground 
having been broken by faults, these volcanic rocks might at first be taken for 
irregular intrusive bosses, but their true character is that shown in fig 25, 
* See “ Geol. Surv. Mem., Geology of Edinburgh,” p. 58. 
