The drill foreman followed popular terminology in classing the deep samples nearly all as "blue clay," but the material also partakes largely of the characteristics of an extremely fine sand, or "rock flour."

From the topographic sheet plate No. 19a it will be seen that the thickness at water level at narrowest point, on proposed flow line at elevation 1,210 is 1,800 feet. On the section shown, Plate 21, it is 2,200 feet. With water down 25 feet, the horizontal distance is 3,200 feet.

The rise of the natural ground above proposed high water in the reservoir is about 100 feet and the thickness of the natural dam, having this great super-elevation is about 500 feet at its thinnest point.

The ground water does not sink down, but the surface water comes out to edge of steep slopes above the proposed high water level on the downstream side at elevation 1,220, or comes out at elevation 1,215 on the reservoir side.

Substantially everywhere below the proposed high water level in the reservoir, the material was classified by the superintendent of borings as either "hard pan or "blue clay."

From examining the samples, I find that most of those listed as sand are so extremely fine, that as they naturally lie in large masses and including the fine particles lost in the process of wash-boring, these sands would be impervious in the sense in which the word is used by practical men, the grains being mostly 1/500-inch in diameter.

The general results of the borings are shown in plate No. 21 of the drawings, and so far as they go, they show the conditions to be entirely safe for the work proposed.

66

As a matter of fact, although this place may look thin to one who stands on the Erie Railroad grade, or stands in the highway a hundred feet higher than the proposed high water mark, but without having marks to show where the water level would intersect the bank, it is actually more than one-third of a mile thick at the high water level at its narrowest point. The word quicksand" has been so persistently repeated by one of the gentlemen who has written upon this subject, that I have taken pains to secure expert. opinion in the classification of the boring samples returned as clay, and in order that anyone technically familiar with problems of percolation might reassure himself concerning the imperviousness of the material, have had microscopic measurement made to show the quality of much of the material classified as sand.

It will be noted that the particles are extremely fine, and those familiar with such problems will recognize that the rate of percolation in such materiał in such great width of embankment would be practically negligible.

This portion of natural dam so far as yet explored appears abundantly safe in comparison with such precedents, for example, as is found at the north dike of the Boston Metropolitan Water Works, the proposed Isthmian Canal dam at Gatun, and in sundry other engineering works.

Since extremely fine sand sometimes looks very much like clay, and since expert fcremen of borings sometimes classify material as clay, which chemically is not clay, six representative samples from the Portage borings were sent for classification to experts familiar with the interpretation of borings. Their reports are as follows:

Portions of the same samples were also tested for size of grain, with results given below.