CONCLUDING OBSERVATIONS. . 305 



And if they had built upon it, some slight traces of their works would yet be visi- 

 ble, however much influence we may assign to disturbing causes, — overflows, and 

 shifting channels. Assuming, then, that the lowest terrace, on the Scioto river for 

 example, has been formed since the era of the mounds, we must next consider that 

 the excavating power of the Western rivers diminishes yearly, in proportion as 

 they approximate towards a general level. On the lower Mississippi, — where 

 alone the ancient monuments are sometimes invaded by the water, — the bed of the 

 stream is rising, from the deposition of the materials brought down from the upper 

 tributaries, where the excavating process is going on. This excavating power, it 

 is calculated, is in an inverse ratio to the square of the depth, that is to say, dimi- 

 nishes as the square of the depth increases. Taken to be approximately correct, 

 this rule establishes that the formation of the latest terrace, by the operation of 

 the same causes, must have occupied much more time than the formation of any 

 of the preceding three. Upon these premises, the time, since the streams have 

 flowed in their present courses, may be divided into four periods, of different 

 lengths, — of which the latest, supposed to have elapsed since the race of the 

 mounds flourished, is much the longest. 



The fact that the rivers, in shifting their channels, have in some instances 

 encroached upon the superior terraces, so as in part to destroy woi-ks situated 

 upon them, and afterwards receded to long distances of a fourth or half a mile or 

 upwards, is one which should not be overlooked in this connection. (See pages 

 .50, 60, and 89.) In the case of the " High Bank Works," Plate XVI, the recession 

 has been nearly three fourths of a mile, and the intervening terrace or " bottom " 

 was, at the period of the early settlement, covered with a. dense forest. This 

 recession, and subsequent forest growth, must of necessity have taken place since 

 the riv^r encroached upon the ancient works here alluded to. 



Without doing more than to allude to the circumstance of the exceedingly decayed 

 state of the skeletons fouiid in the mounds, (see page 168,) and to the amount of vege-. 

 table accumulations in the ancient excavations, and around the ancient works, (see 

 pages 55 and 90,) we pass to another fact, perhaps more important in its bearing 

 upon the question of the antiquity of these works than any of those presented above. 

 It is that they are covered with primitive forests, in no way distinguishable 

 from those which surround them, in places where it is probable no clearings 

 were ever made. Some of the trees of these forests have a positive antiquity of 

 from six to eight hundred years (see pages 14 and 16). They are found sur- 

 rounded with the mouldering remains of others, undoubtedly of equal original 

 dimensions, but now fallen and almost incorporated with the soil. Allow a 

 reasonable time for the encroachment of the forest, after the works were aban- 

 doned by their builders, and for the period intervening between that event and the 

 date of their construction, and we are compelled to assign them no inconsiderable; 

 antiquity. But, as already observed, the forests covering these works correspond 

 in all respects with the surrounding forests ; the same varieties of trees are found, 

 in the same proportions, and they have a like primitive aspect. This fact was 

 remarked by the late President Harrison, and was put forward by him as one of 



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