170 OLDER PLIOCENE PERIOD. 



[Ch. XIII. 



When we inquire into the causes of such a disposition of the 

 - 3 ' J - 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. 3G. 



side slightly inclined and the other steep, the lee side being 

 always steep, as b c, d <?, 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- 



