The Public Improvement Commission of Baltimore City is constructing a dam on the Gunpowder Falls in Baltimore County about 20 miles north of the city. The rock on which the dam is being built is the Wissahickon schist. The character of the rock proved to be such that unexpected difficulties were encountered in preparing a safe and sound foundation. The ease with which the rock broke along the planes of schistosity in shooting necessitated the excavation of an excessive amount of rock. This difficulty was overcome by substituting wire saws for blasting. At the west end of the dam site, weathered rock extended much deeper than it had been planned to excavate. This area was prospected by core drill holes 3 feet in diameter which the geologist descended in order to obtain a complete log of the character and structure of the rock. Microscopic examination of thin sections of . . .