effervescence, leaving the insoluble impurities, which were
disseminated through it, at the bottom of the tube as a little
clay.

We can now understand the changes in the upper layers of the
quarry. At the surface of the rock the limestone has completely
dissolved, leaving the insoluble residue as a layer of reddish
clay. Immediately below the clay the rock has disintegrated into
meal where the cement between the limestone grains has been
removed, while beneath this the laminae are split apart where the
cement has been dissolved only along the planes of lamination
where the stone is more porous. As these changes in the rock are
greatest at the surface and diminish downward, we infer that they
have been caused by agents working downward from the surface.

At certain points these agencies have been more effective than
elsewhere. The upper rock surface is pitted. Joints are widened as
they approach the surface, and along these seams we may find that
the rock is altered even down to the quarry floor.

A SHALE PIT. Let us now visit some pit where shale--a laminated
and somewhat hardened clay--is quarried for the manufacture of
brick. The laminae of this fine-grained rock may be as thin as
cardboard in places, and close joints may break the rock into
small rhombic blocks. On the upper surface we note that the shale
has weathered to a clayey soil in which all traces of structure
have been destroyed. The clay and the upper layers of the shale
beneath it are reddish or yellow, while in many cases the color of
the unaltered rock beneath is blue.