THE STUDY OF HUMAN PREHISTORY 
If it be asked how we can tell the relative age of dif- 
ferent kinds of remains found in the soil, how we know 
that one type of human culture, for instance, is older than 
another, a simple illustration may answer. Most of us 
cherish early memories of the “‘old swimming hole” and 
its sometimes forbidden delights. Very often, we recall, 
the creek curved around, with a high bank on the outer 
edge of the bed, where the water was deep and safe for 
diving, and with a low, shelving beach on the inner side. 
The steep outer bank tended always to be undercut by 
the current, so that portions of it occasionally slipped 
down into the water, leaving exposed a fresh surface of 
clay. Near the top of this we should perhaps find sticking 
out of the earth objects that had been left there in recent 
times since white people inhabited the country: a rusty 
piece of iron, some baked bricks, a few fragments of broken 
chinaware, or a decaying log bearing marks of the pioneer’s 
steel ax. There might, too, be bones of horses or oxen, 
pigs or sheep—animals which we know the early settlers 
brought with them from Europe. Lower down in the 
freshly exposed face of the bank we might find stone 
arrowheads, fragments of coarse, unglazed pottery, or 
bones and antlers of deer—animals which shared the 
country with the Indians. 
Sometimes, of course, we should find things mixed up— 
jumbled together by the plow of the farmer, by burrow- 
ing animals, spring freshets, or the caving in of the banks. 
In general, however, we should see that traces of the later 
comers in the country lie above those of the earlier in- 
habitants. In a brick wall, the lowest course is bound 
to be the earliest, while the top one is laid down last. 
That, in essence, is the principle on which archeology 
depends (Fig. 4). 
As our knowledge accumulates, the easier it is to apply 
it. If an archeologist from Mars were to come upon a 
chipped Indian arrowhead of stone sticking out of our 
clay bank above a rusty old iron hoe instead of de/ow it, 
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