A very large proportion, however, of this rock is totally unfit for building purposes. The surface soil is generally a sandy loam, containing a very small proportion of argillaceous earth. Below the surface soil, gravel, sand, boulders, or detached rock, have in most cases been found, and also hard pan to a considerable extent. A large proportion of the open cutting, and nearly the whole tunnel cutting, ha.* been through rock. More than 400,000 cubic yards of rock have been excavated.

204 MEMOIR OF THE The general formation of the country is extremely irregular, and unfavorable for the economical construction of such a work. Commencing at the Croton dam, on a level 40 feet above the river, which descends from this point to tide-water, at the average rate of 25 feet to the mile, the grade of the aqueduct was compelled to encounter great irregularities of surface. Very little regular tableland could could be found for its location. While the main ridge of high land, that lay on the left of the line in Westchester county, controlled the general location, numerous spurs of ridges, of various forms and extent, bounded by deep ravines, rendered it impossible to avoid deep cutting and frequent tunnelling, through ridges, and heavy rilling in the valleys; leaving a very limited proportion of the line in favorable cutting. On the island of New York, with the exception of about one mile from Harlem river, there was no leading ridge to control or guide the location ; which rendered it more difficult to decide on the conflicting claims of different routes.