CHAP. XXV 
LAVAS AND TUFFS OF THE PLATEAUX 
389 
between them the deposition of sand and mud went on as before. Thus 
the lower 400 feet of the Campsie Tells are built up of slaggy andesites and 
thick heds of fine-grained stratified tuff, with bands of red, green and grey 
clays and cement-stone and a zone of white sandstone. The Calton Hill at 
Edinburgh (Fig. 118) affords an excellent illustration of the interstratifica- 
Kig. 118. — Section of Calton Hill, Edinbnrgli, 
1 . JwOwer Carboniferous sandstones ; 2. Basic lava at tho bottom of the volcanic series ; 3. Tuff often interstratilied 
with sandstones and slnales ; 4. Sheets of andesite-lava freiiiiently separated by layers of tuft ; 0. Shale passing 
into tulf; 0. White sandstone and black carbonaceous shales overlying the volcanic series. 
tion both of tufts and ordinaiy sediments among tho successive outflows of 
lava. In the total thickness of about 1100 feet of volcanic material in 
this hill, at least eight intervals in the discharge of the lavas are marked 
by the intercalation of as many bands of nodular tuff, together with seams 
of shale and sandstone more or less charged with volcanic detritus. The 
highest lava is immediately covered by the white sandstones and black 
shales of the Calciferoiis Sandstone series. 
The tuffs, as might be expected, are coarsest in texture and thickest in 
mass where they approach most nearly to some of the vents of eruption, 
and, on the other hand, become finer as they recede from these. As a rule, 
they are distinctly stratified, and consist of layers varying in the size of 
their component lapilli. Here and there, near the centres of discharge, the 
bedding becomes liardly traceable or disappears, and the fragmentary 
materials take the form of agglomerate. 
Til the admirable range of coast-cliffs which extend from North Berwick 
to Dunbar, we learn that above the red sandstones at the base of the 
Carboniferous system, a thick pile of volcanic ashes was accumulated by 
numerous discharges from vents in the immediate, neighbourhood. Some of 
the explosions were so vigorous that blocks of different lavas, sometimes a 
yard or more in length, were thrown out and heaped up in irregular mounds 
and liollows. Others discharged exceedingly fine dust, and between these 
two extremes every degree of coarseness of material may be recognized. 
As an illustration of the remarkable rdteniation of coarse and fine 
materials, according to tlie varying intensity of the volcanic paroxysm. Tig. 
119 is here introduced. It represents a portion of the tuff-cliffs east of 
Tantallon Castle, and shows at the bottom fine well-stratified tuff, over which 
a shower of large blocks of lava lias fallen. Fine detritus is seen to cover 
the deposits of this shower, and successive discharges of large stones may be 
noticed higher up on more or less well-defined horizons. 
The space over winch this pyroclastic material can Jiow be traced, large 
