daeton.] DISTRICT OF COLUMBIA. 161 
depth of 360 feet, and at tlie Mount Vernon Apartment House, at 133 
feet. On the other hand, the St. Elizabeth, Metropolitan Railroad 
power house, Palais Eoyal, Eckington, Washington Brewery, and 
National Capitol Brewery wells obtain large supplies from that horizon. 
In surface outcrops the basal Potomac beds are usually coarse sands and 
gravels, which are filled with water, but in some few instances there 
are local areas in which there is a clay matrix, or even quite pure clay 
extending down to the crystalline floor. This latter was found to be 
the case at the Mount Vernon Apartment House, while at the Palais 
Eoyal, three squares southwest, the basal beds were coarse sands and 
gravels containing much water. In the 380-foot well at the ice works, 
coarse basal beds were found, but they contain no large supply of 
water; which leads to the conclusion that the waters are in part choked 
off by a local area of clay admixture to the northward, but which does 
not influence the wells at the breweries near by. 
These considerations must everywhere qualify a judgment as to the 
prospects for basal Potomac waters at any given point in the eastern 
portion of the District, but the general prospects are so good that I 
should never hesitate to sink wells to these basal beds, particularly as 
there is always a good chance also of finding water in overlying beds, 
as at the ice works. The area to which this prognostication applies 
lies east of the zero line on the map, PI. XIII. Along this line the 
basal beds of the Potomac formation pass beneath tide level, but to 
the west and north they rise, and the waters are in greater or less 
measure free to flow out in surface springs. The heavy blue contour 
lines east of the zero line indicate the depths below tide level of the 
basal Potomac beds, for each 100 feet. To calculate the depth for a 
well, it is necessary to add the elevation of the ground above tide level, 
which is indicated for each 20 feet by the brown contour lines on the 
map. The slope and depths of the basal beds are also indicated in the 
sections on PI. XIV. 
To the east of the 100-foot contour line (in blue on map, PI. XIII), 
the higher Potomac waters may be expected, as indicated by the expe- 
rience of the Reform School, Highlands Hotel, Ice Works, Asylum, and 
power house wells, at from 120 to 1G0 feet above the basal Potomac beds 
or crystalline rock floor. This horizon appears to be continuous, and 
to be due to the wide extension of a bed of sand among the clays. 
As to waters in the crystalline rocks, I can make no prediction from 
present knowledge. They occur in steeply inclined fissures and in 
decomposed belts in the rocks, of which the extent and course are diffi- 
cult to determine and impossible to predict without many additional 
data. The wells given in the list on page 156 which draw their waters 
from the crystalline rocks, obtain only moderate supplies, with the 
exception of the well at the Mount Vernon Apartment House, where 
the supply is claimed to amount to 40 gallons a minute. 
Bull. 138 11 
