CHAP. XXXVII 
THE PLA TEA U OF MULL 
21 I 
deposits. The hollow appears thereafter to have become a land-valley, 
" hence the Jurassic strata were to a large extent cleared out by denudation 
before its subsequent submergence under the sea in which the upper Creta- 
ceous deposits accumulated. Professor Judd has shown that relics of these 
Cretaceous strata appear on both sides of the plateau from under the 
protecting cover of basalt-sheets. But, before the volcanic eruptions began, 
the area had once again been raised into land, and the youngest Secondary 
formations had been extensively eroded. 
In their general aspect the basalts of Mull agree with those of Antrim, 
and the circumstances under which they were erupted were no doubt 
essentially the same. But considerable differences in detail are observable 
between the succession of rocks in the two areas. When I first visited the 
island in 1866, the only available maps, with any pretensions to accuracy, 
were the Admiralty charts ; but, as these do not give the interior except in 
a generalized way, it was difficult to plot sections from them, and to arrive 
at satisfactory conclusions as to the thickness of different groups of rock. 
Accordingly, as the successive nearly flat flows of basalt can be traced from 
the sea-level up to the top of Ben More, I contented myself with the fact 
that the total depth of lava-beds in Mull was at least equal to the height of 
that mountain, or 3169 feet. The publication of the Ordnance Survey 
Maps now enables us to make a nearer approximation to the truth. From 
tlie western base of the magnificent headland of Clribon, the basalts in 
almost horizontal beds rise in one vast sweep of precipice and terraced slope 
to a height of over 1600 feet, and then stretch eastwards to pass under the 
higher part of Ben More, at a distance of some eight miles. They have a. 
flight easterly inclination, so that the basement sheets seen at the sea-level, 
a t the mouth of Loch Scridain, gradually sink below that level as they go 
eastward. It is not easy to get a measurement of dip among these basalts, 
except from a distance. If we take the inclination at only 1°, the beds 
which are at the base of the cliff on the west, must be about 700 feet below 
Hie sea on the line of Ben More, which would give a total thickness of 
nearly 3900 feet of bedded lava below the top of that mountain. We shall 
not probably overestimate the thickness of the Mull plateau if we put it at 
■' 500 feet. 
'The base of the volcanic series of Mull can best be seen on the south 
-‘oast at Garsaig, and at the foot of the precipices of Gribon. As already 
stated, it is there found resting above Cretaceous and Jurassic rocks. The 
"West beds are basalt-tuffs, of the usual dull green colour. They are in 
1 'laces much intermingled with sandy and gravelly sediment, as if the 
ydcanic debris had fallen into water where such sediment was in course of 
1 "position. One of the most interesting features, indeed, in this basement 
! >ai 't of the series, is the occurrence of bands of non-voleanic material which 
""cumulated after the tuffs and some of the lavas had been erupted, but 
'Gore the main mass of basalts. Those at Carsaig include a lenticular bed, 
i feet thick, of rolled flints, which, with some associated sandy bauds, lies 
’"tween sheets of basalt On the opposite side of the promontory is the 
