B. Smith—Ball or Pillow-form Rocks. 151 
Cause of Contortion. 
Anything more unlike concretionary action could hardly be 
imagined. To what, then, must these structures be attributed ? 
’ TIT can imagine no means that would avail to turn over, and break 
up, layers of sediment on this scale—sandwiched between normally 
bedded identical layers of identically similar sandstone‘—after the 
higher layers had been deposited. Movements during consolidation, 
due to differences in composition, cannot be invoked, because there is _ 
no difference in composition. Nor can we appeal to cleavage-stresses, 
although in some cases, discussed below, they may have achieved 
something of a secondary order. 
There remains to find some means whereby the sediment could have 
been disturbed before the higher undisturbed layers were deposited. 
Two agencies are possible: either (1) packing or sliding movements 
on gentle slopes owing to irregularity of deposition and gravitational 
-ereep, or (2) movements of overlying bodies of water as tidal or 
ocean currents. 
Nature of the Movements. 
The highly lenticular character of these beds has already been 
emphasized, and the examples figured show how the rock was drift- 
bedded and ripple-marked during deposition. Nothing is more obvious 
than that the sediment was transported by currents and laid down 
sporadically in fairly shallow water, layers of mud being frequently 
interspersed between layers of sand and silt. From these facts we 
may safely infer that the sea-bed was gently undulating. Again, 
the sediments were apparently formed off the shores of volcanic 
islands drained by rivers carrying ashy and other sediment to the sea, 
which would be agitated periodically by strong tidal currents. The 
large voleanic bombs and lumps of shale in the Bala ashes would seem 
to prove that the volcanoes rose above the water, for the bombs could 
not have been cast far from the vents had the latter been submarine. 
Most of the sediment had apparently reached a state of pasty semi- 
consolidation that allowed of overfolding and thrusting within its 
mass, but its upper layers were still so incoherent that they could 
be stirred up and easily rendered structureless. 
A lenticle of sandstone, deposited on a gentle slope, might find 
itself in a state of unstable equilibrium through gravitation alone, 
which would set up a tendency to creep and internal readjustment. 
Furthermore, the edges of a lenticle deposited on a slope of mud 
would tend to break up and have its detached fragments incorporated 
in the enveloping mud. 
It is well-known how Glacial and post-Glacial gravels, resting on 
gentle inclines, have become contorted by gravitational creep. 
A mass of laminated sandstone under water would be an even more 
likely subject for this operation, which might be started or accentuated 
by tidal movements, or the sweep of changeable ocean currents. 
1 Contrast ‘ mud-lumps’, where mud is squeezed out from between sand- 
stones that are of different composition and grade of material (see later, 
p. 155). 
