470 PETROLOGICAL: ABSTRACTS AND REVIEWS 
gneisses, metamorphosed sediments in part, with great masses of igneous 
intrusions and bosses, and still farther south, in Westchester Co., are 
gently rolling parallel ridges, formed by a succession of limestone and 
schist belts. Near the city the underlying Fordham gneiss is frequently 
exposed. Several igneous masses are also penetrated, and a number of 
other formations have been barely avoided. The whole area is thickly 
drift covered, and this feature of course greatly obscures the geology. 
Recent greater elevation of the continent must also be taken into con- 
sideration, since it has resulted in the cutting of inner gorges in most of 
the valleys (some of which have since been obscured by glaciation), and 
in the circulation of ground water at depths greater than the present, 
producing frequent rotten zones and occasional caves below the present 
ground water level. A wide variety of problems are thus presented, 
covering the fields of physiographic, glacial, petrographic, and structural 
geology. 
To supplement unusually close field study, the author commanded 
the resources of an extensive drilling equipment. Wash rigs were used 
where possible, but the chop and oil-well rigs were usually necessary, and 
the shot and diamond drills were frequently resorted to. The cores of 
the latter were preserved, and in many cases subjected to microscopic 
study; and careful records were always kept of the percentage and con- 
dition of the core saved, the rate of progress of the drill, its behavior, the 
loss of water in the hole, etc. Special pumping tests were made in some 
of the holes to determine the porosity and perviousness of the rock, this 
being of course a vital feature in the construction of pressure tunnels. 
After a résumé of the geology and physiography of the district, and 
the principles involved in their interpretation, the author proceeds to a 
discussion of a number of type problems. Passing briefly over the 
considerations—chiefly physiographic—which led to the selection of the 
present aqueduct line, he takes up rather fully the problem of selecting a 
suitable crossing under the Hudson River. Detailed exploration of a 
number of points was made, and the Storm King locality finally selected 
as the most advantageous, chiefly because of the character of the rock, 
which is a slightly gneissoid granite, in striking contrast to the slate and 
limestone belts over which the river flows at the other points explored. 
Drilling at this locality showed bedrock at the extraordinary depth of 
751 feet, the assumption being that the gorge was here glacially over- 
steepened several hundred feet. The geological features involved in 
selecting the great Ashokan dam site are then considered; they deal 
chiefly with the character of the bedrock, the assortment and impervious- 
ness of the overlying till, and the general glacial history of this locality. 
