176 
OLD"ER PLIOCENE PERIOD. 
£Ch. XIII. 
When we inquire into the causes of such a disposition of the 
materials of each bed or group of lay- 
ers, we may, in the first place, remark, 
that however numerous may be the suc- 
cessive layers a, b, c, the layer a must 
have been deposited before b, b before c, and so of the rest. 
We must suppose that each thin seam was thrown down on 
a slope, and that it conformed itself to the side of the steep 
bank, just as we see the materials of a talus arrange themselves 
at the foot of a cliff when they have been cast down successively 
from above. If the transverse layers are cut off by a nearly 
horizontal line, as in many of the above sections, it may arise 
from the denuding action of a wave which has carried away the 
upper portion of a submarine bank and truncated the layers of 
which it was composed. But I do not conceive this hypothesis 
to be necessary ; for if a bank have a steep side, it may grow by 
the successive apposition of thin strata thrown down upon its 
slanting side, and the removal of matter from the top may pro- 
ceed simultaneously with its lateral extension. The same current 
may borrow from the top what it gives to the sides, a mode of 
formation which I had lately an opportunity of observing on the 
rippled surface of the hills of blown sand near Calais. The un- 
dulating ridges and intervening furrows on the dunes of blown 
sand resembled exactly in form those caused by the waves on 
a sea-beach, and were always at right angles to the direction 
of the wind which had produced them. Each ridge had one 
No. 36. 
b d 
side slightly inclined and the other steep, the lee side being 
always steep, as b c, d e, the windward side a gentle slope, as 
a b, c d. When a gust of wind blew with sufficient force to 
drive along a cloud of sand, all the ridges were seen to be in 
motion at once, each encroaching on the furrow before it, and, 
in the course of a few minutes, filling the place which the fur- 
