396 
GEOLOGY OF THE FOURTH DISTRICT. 
a portion of a once much more extensive deposit. Upon the southern side of this island, 
where there is an escarpment, the thickness of the superficial deposit is about twenty-five 
feet. The upper half consists of coarse gravel and sand, with abundance of fresh water shells 
of the genera Unio, Cyclas, Limnea, Planorbis, Valvata, and Melania; the same, both in 
genera and species, as those now inhabiting the river and lakes. The occurrence of these 
shells, in this situation, about forty feet higher than the top of the fall, proves the existence 
of a river or lake at an elevation sufficient to allow of such a deposition, for this accumula¬ 
tion of shells and gravel bears all the evidence of a fluviatile deposit. It is equally evident 
that this deposit could not have been made while the falls were in their present position. 
During a reexamination of this place in 1841, in company with Mr. Lyell, we discovered 
the Cyclas, Valvata, &c. in a terrace, upon the east side of the river, of the same elevation as 
Goat island. It was at this place, and in the same deposit, that a Mastodon’s tooth was 
found eleven feet below the surface. Farther northward, and more than half a mile north of 
Goat island, in another excavation, we discovered similar shells. At the same time we noticed 
the continuation of this terrace as far as the whirlpool. Since that time I have levelled the 
whole distance from the falls to the whirlpool, and find the elevations at the two places to 
correspond. Shells are even more abundant in this terrace at the whirlpool than at the falls.* 
Farther north than this point, I have made no examinations for freshwater shells. 
Upon the west side of the river there is a similar terrace, which is mentioned by Mr. 
Hayes as also containing freshwater shells,! in common with that on the east side. Now, in 
order to find a barrier on the north sufficient to elevate the water so as to make this deposit, it 
would require to extend it far towards Lewiston. Furthermore, it is evident that the deposits 
forming these banks or terraces, on either side of the river, and that of Goat island, were not 
made in this form, but are parts of a once much more extensive formation, which has been 
removed by the waters of the river. During the time of its deposition the river must have 
occupied this ancient valley, which extended toward Lewiston, finding a barrier much nearer 
the edge of the terrace than the whirlpool. In the bed of this expanded river or lake, much 
as it now is above the rapids, this deposit was made, probably covering the whole extent. 
From the subsequent recession and drainage, the margins only have remained, together with 
Goat island, indicating its former extent. The position of this fluviatile deposit, which is 
subsequent to the ancient drift, may be illustrated in the following diagram: 
* See accompanying map and elevations. Mr. Lyell informs me that in a second visit to the falls in the spring of 1842, he 
found freshwater shells at the Whirlpool. 
t Am. Journal of Science and Arts, Vol. XXXY. No. 1. 
