How long the Mound-builders occupied the country north of the Gulf of Mexico it is impossible in the present state of science to determine. Some authors conjecture that they were here two thousand years; that we think would be time enough, though after all it is but conjecture. It seems to us, however, that the time of the abandonment of their works may be more closely approximated. A thousand or two years may have elapsed since they vacated the Ohio Valley, and a period embracing seven or eight centuries may have passed since they retired from the Gulf coast. As an evidence of a large population having existed in this country at a former period, we have immense shell-heaps artificially collected, extending along the Atlantic coast from Nova Scotia to Florida, on the Gulf coast and up the river valleys through nearly all of the Southern States. It is difficult to assign the formation of these vast remains to any definite period or to any particular people. Though of the same character as the _Kjökken-Möddings_ (Kitchen-Middens) of the Danish, they furnish no indications of so great an antiquity. This has been shown by Dr. Jeffries Wyman in his researches in Maine and Massachusetts.[94] Sir Charles Lyell made an examination of a shell-bank on St. Simon’s Island, near the mouth of the Allamaha River, Georgia, so extensive that it covers ten acres to a depth varying from five to ten feet.[95] Dr. Brinton has described immense accumulations in Florida. On Amelia Island, shells exist to the depth of three feet over an area 150 yards wide and a quarter of a mile long. Notable instances of a similar kind are Turtle Mound near Smyrna—a mass of oyster shells thirty feet thick—and a shell-bank on Crystal River four miles from its mouth, reaching a height of forty feet.[96] Dr. Wyman carefully examined many of the fresh-water shell-heaps of Florida and obtained pretty satisfactory results.[97] Near the Silver spring upon a shell-heap covering nearly twenty acres, stand several live-oaks of immense size, the largest of which measured between twenty-six and twenty-seven feet in circumference. Excavations under this monster, taken together with its position on the side of the shell-bank, proved it to be of more recent origin than the latter. Prof. Wyman, by allowing twelve rings to the inch and granting it a semi-diameter of fifty inches, estimated that it was not less than six hundred years old. Of course the shell-bank may have existed a long time before any vegetation appeared upon it. The crania of the shell-banks of Florida differ from those of the Mound-builders in greater thickness as well as greater mean capacity.[98] In his _Fresh-water Shell-Mounds of the St. John’s River_, and in his memoir on _Human Remains in the Shell-heaps of the St. John’s River_ (_Seventh Annual Report of Peabody Museum_, pp. 26 _et seq._), Dr. Wyman reports having discovered the startling fact that cannibalism prevailed among the barbarous people of the shell-banks.