Calciferous and Trenton beds, which appear in succession in
parallel bands through the central part of the county. These
are covered in the southern half of the county by the Utica and
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.