670 
when the surface at the bottom is composed of very fine sand, 
pebbles, whose size in proportion to that of the grains of 
sand is the same as in the case of the coarse sand just 
described, sink so deeply into it that a greater force would 
be requisite to move them forward ; whilst at the same time 
the motive force of the current is reduced by a smaller 
proportional part of their surface being exposed to its 
action. On this account a current just capable of drifting 
forward the very fine sand, could not transport the pebbles 
over its surface ; whereas, if the current were strong enough 
to wash along the pebbles it would be too strong to permit 
the sand to be finally accumulated along with them, when 
anything occurred to reduce its velocity. These principles 
completely explain why pebbles do not occur in the fine- 
grained beds, but are very constantly met with in the coarse- 
grained ; whilst at the same time they are seldom found as 
separate beds without sand. 
Very much of the millstone-grit is made up of that kind 
of oblique false bedding for which I have adopted the term 
drift-bedding, as described in my various papers in the 
Edinburgh New Philosophical Journal, (new series, vols, iii., 
p. 112; iv., p. 317 ; v., p. 275 ; vii., p. 226 ;) and also in 
the Reports of this Society, (1852, p. 232, and 1854, p. 372.) 
My experiments show that, when pebbles mixed with sand 
are carried forward by the current, and arrive at the part 
where they are thrown down on a slope, so as to form a bed 
of this drift-bedding, the pebbles usually roll to the bottom of 
the slope, whilst the sand rests higher up ; and thus a bed of 
sand is formed, having oblique stratula and a band of pebbles 
along the bottom. 
A superficial observer, not aware of the peculiarities 
in the laws governing the transport and acccumulation of 
sand and pebbles, might be led to think that such a band 
of pebbles was a separate bed, formed by the action of a 
