904 : STRATIGRAPHICAL GEOLOGY. [Book VI. 
period, and be included in the Paleolithic series. A satisfactory 
chronological classification of the deposits containing the first relics 
of man is perhaps unattainable, for these deposits occur in detached 
areas with no means of determining their physical sequence. To 
assert that a brick-earth is older than a cayvern-breccia, because it 
contains some bones which the latter does not, or fails to show some 
which the latter does yield, is too often a conclusion drawn because 
it agrees with preconceptions. 
River-Alluvia—Above the present levels of the rivers there 
‘lie platforms or terraces of alluvium, sometimes to a height of 80 or 
100 feet. These deposits are fragments of the river-gravels and 
loams laid down when the streams flowed at that elevation, and 
therefore before the valleys were widened and deepened to their 
present form. Miver-action is at the best but slow. ‘To erode the 
valleys to so great a depth beneath the level of the upper alluvia, 
must have demanded a period of many centuries. There can there- 
fore be no doubt of the high antiquity of these deposits. They 
have yielded the remains of many mammals, some of them extinct, 
together with the flint flakes made by man. From the nature and 
structure of some of the high-lying gravels there can be little doubt 
that they were formed at a time when the rivers, then larger than 
now, were liable to be frozen and to be obstructed by large accu- 
mulations of ice, We are thus able to connect the deposits of the 
Human Period with some of the later phases of the Ice Age in the 
west of Europe. 
Brick Harths.—In some regions that have not been below the 
sea for a long period a variable accumulation of loam has formed on 
the surface from the decomposition of the rocks im situ aided by 
the drifting of fine particles by wind and the gentle washing action 
of rain and occasionally of streams. Some of these brick-earths or 
loams are of high antiquity, for they have been buried under fluvi- 
atile deposits, which must have been laid down when the rivers 
flowed far above their present levels. They have yielded traces of 
man associated with bones of extinct mammals. 
Cavern Deposits.—Most calcareous districts abound in 
underground tunnels and caverns which have been dissolved by the 
passage of water from the surface (p. 855). Where these cavities 
have communicated with the outer surface, terrestrial animals, in- 
cluding man himself, have made use of them as places of retreat, or 
have fallen or been washed into them. The floors of some of them 
are covered with a reddish or brownish loam or cave-earth, resulting 
either from the insoluble residue of the rock left behind by the 
water that dissolved out the caverns, or from the deposit of the silt 
carried in the water which in some cases has certainly flowed through 
them. Very commonly a deposit of stalagmite has formed from the 
drip of the roof above the cave-earth. Hence any organic remains 
which may have found their way to these floors have been sealed up 
and admirably preserved, 
