PERMANENCE OF SOILS.–This last material experimented with suggests the subject of _permanence_ of soils. This was reported upon separately and independently by Mr. Allen Hazen and Prof. W. O. Crosby. These experts agreed in their conclusion, stating that the process of oxidation below the line of saturation would be extremely slow, requiring many thousands of years for the complete removal of all the organic matter, and that the tightness of the bank would not be materially affected by any changes which are likely to occur. It has been remarked, that of all the materials used in the construction of dams, _earth_ is physically the least destructible of any. The other materials are all subject to more or less disintegration, or change in one form or another, and in earth they reach their ultimate and most lasting form. In speaking of the North Dike of the Wachusett Reservoir, Mr. Stearns remarked that, it was evident by the application of Mr. Hazen’s formula for the flow of water through sands and gravels, that the very fine sands found at a considerable depth below the surface would not permit enough water to pass through them if a dike of great width were constructed, to cause a serious loss of water, and it was also found that the soil, which contained not only the fine particles or organic matter, but also a very considerable amount of fine comminuted particles, which the geologist has termed “rock flour,” would be sufficiently impermeable to be used as a substitute for clay puddle. Fig. 25 shows the maximum section of the North Dike with its cut-off trench. The quantities and estimated cost of the completed structure are given in the table herewith: |––– Cost –––| Per cent. Work. Quantities. Unit Actual. total. (cu. yds.) Price. Soil 5,250,000 $0.05 $262,500 34.7 Cut-off trench 542,000 .20 108,400 19.3 Borrowed earth and gravel 200,000 .20 40,000 Slope paving 50,000 2.20 110,000 14.6 Sheet-piling, pumping, etc. 117,000 15.5 Engineering and preliminary investigations 120,000 15.9 ––––––– ––––– Total cost $757,900 100.0 Druid Lake Dam, Baltimore, Md. Another very interesting and instructive example of high earth dam construction is that of the Druid Lake Reservoir embankment, Baltimore, Md.