VIII THE ANTIQUITY OF MAN IN NORTH AMERICA 
435 
as well as from trained observers who are fully aware of the 
importance of every additional fact and the weight of each 
fresh scrap of evidence. Having by the kindness of Major 
Powell, the able Director of the United States Geological Sur- 
vey, been able to look into the evidence recently obtained 
bearing on this question in the North American continent, I 
believe that a condensed account of it will certainly prove of 
interest to English readers. 
The most certain tests of great antiquity, even though they 
afford us no accurate scale of measurement, are furnished by 
such natural changes as we know occur very slowly. Changes 
in the distribution of animals or plants, modifications of the 
earth’s surface, the extinction of some species and the intro- 
duction of others, are of this nature, and they are the more 
valuable because during the entire historical period changes 
of this character are either totally unknown or of very small 
amount. Let us then see what changes of this kind have 
occurred since man inhabited the North American continent. 
Ancient Shell Mounds 
The shell heaps of the Damariscotta River, in Maine, are re- 
markable for their number and extent. The largest of these 
stretches for about half a mile along the shore, and is often six 
or seven feet, and in one place twenty-five feet, in thickness. 
They consist almost exclusively of oyster shells of remarkable 
size, frequently having a length of eight or ten inches, and some- 
times reaching twelve or fourteen inches. They contain frag- 
ments of bones of edible animals, charcoal, bone implements, and 
some fragments of pottery. The surface is covered to a depth 
of several inches with vegetable mould, and large trees grow on 
them, some more than a century old. The special feature to 
which we now call attention is “that at the present time 
oysters are only found in very small numbers, too small to 
make it an object to gather them; and we were credibly in- 
formed that they have not been found in larger quantities 
since the settlement in the neighbourhood. It cannot be sup- 
posed that the immense accumulations now seen on the shores 
of Salt Bay could have been made unless oysters had existed 
in very large numbers in the adjoining waters .” 1 Here we 
1 Second Annual Report of Trustees of Peabody Museum, p. 18. 
