Dr. Dickeson found, in the yellow loam of the Mississippi at Natchez, a human pelvic bone along with the bones of the mastodon and megalonyx. They were found at a depth of thirty feet from the surface, and the human bone had the same black color which characterized the others. Sir Charles Lyell calculated that it required sixty-seven thousand years to form the delta of the Mississippi, but admits, if the conclusions arrived at by the United States engineers be correct, in respect to the annual amount of sediment discharged at the delta, the growth would be reduced to thirty-three thousand five hundred years. Taking either of these estimates, the same would give the number of years which have elapsed since these bones were deposited.[84] In an excavation made near New Orleans, at a depth of sixteen feet from the surface, beneath four cypress forests superimposed one upon the other, the workmen found a complete human skeleton, and some charcoal. The cranium is similar to the aboriginal type of the Indian race. This discovery furnished the data from which Dr. Bennet Dowler assigned to the human race an antiquity, in the delta of the Mississippi, of fifty-seven thousand years.[85] Count Pourtalis found some fossil human remains, consisting of jaws, teeth, and some bones of the foot, in a calcareous conglomerate forming a part of the series of reefs of Florida. The whole series of reefs is of post-tertiary origin, and, according to Professor Agassiz, has been one hundred and thirty-five thousand years in forming. If this calculation be correct, then these bones must have an antiquity of ten thousand years.[86] Dr. Lund, a Danish naturalist, explored eight hundred caverns in Brazil, belonging to different epochs, and exhumed in them a great number of unknown animal species. In a calcareous cave, near the lake of Semidouro, he found the bones of not less than thirty persons of different ages, and showing a similar state of decomposition to that of the bones of animals with which they were associated. From the discoveries there made, Lund was forced to the conclusion that man was cotemporaneous with the megatherium and the mylodon--animals belonging to the post-tertiary.[87] The shell-heaps of America are coeval with those of Denmark. Those at Damariscotta, Maine, have been examined by Professor W. D. Gunning. He estimates that within, an area of one hundred rods in length, eighty in width there are piled one hundred million bushels of oyster shells. One dome-shaped hillock is nearly one hundred feet in height. The only human relics found among the shells are stone gouges, arrow-heads, bone needles, pottery, and copper knives. These shells were probably deposited by but a few individuals at a time. When formed, the oyster was a native of that coast, but within the memory of man the oyster has not lived there.