Hudson river slates and shales.

[Illustration: GEOLOGICAL SECTION AT SARATOGA SPRINGS.]

"The most remarkable feature is, however, the break, or
vertical fissure, which occurs in the Saratoga valley, which
you see indicated in the cut. Notice, especially, the fact that
the strata on one side of the fissure have been elevated above
their original position, so that the Potsdam sandstone on the
left meets the edges of the calciferous sand rock, and even the
Trenton limestone on the right. It is in the line of this
fissure, or _fault_, in the towns of Saratoga and Ballston that
the springs occur.

"The Laurentian rocks, consisting of highly crystalline gneiss,
granite and syenite, are almost impervious, while the overlying
Potsdam sandstone is very porous, and capable of holding large
quantities of water. In this rock the mineral springs of
Saratoga probably have their origin. The surface waters of the
Laurentian hills, flowing down over the exposed edges of the
Potsdam beds, penetrate the porous sandstones, become saturated
with mineral matter, partly derived, perhaps, from the
limestones above, and are forced to the surface at a lower
level, by hydrostatic pressure. The valley in which the springs
all occur indicates the line of a fault or fracture in the
rocky crust, the strata on the west side of which are hundreds
of feet above the corresponding strata on the east.

"The mineral waters probably underlie the southern half of the
entire county, many hundred feet below the surface; the
accident of the fault determining their appearance as springs
in the valley of Saratoga Springs, where, by virtue of the
greater elevation of their distant source, they reach the
surface through crevices in the rocks produced by the fracture.

"It is probable that water can be obtained anywhere in the
southern portion of the county by tapping the underlying
Potsdam sandstone. In these wells the water usually rises to
and above the surface. Down in the rocky reservoir the water
is charged with gases under great pressure. As the water is
forced to the surface, the pressure diminishes, and a portion
of gas escapes with effervescence. The spouting wells deliver,
therefore, enormous volumes of gas with the water, a perfect
suds of water, carbonic a