The Elements of Geology | Page 8

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.
THE SEDIMENTARY ROCKS. The three kinds of layered rocks
whose acquaintance we have made--sandstone, limestone, and
shale--are the leading types of the great group of stratified, or
sedimentary, rocks. This group includes all rocks made of sediments,
their materials having settled either in water upon the bottoms of rivers,
lakes, or seas, or on dry land, as in the case of deposits made by the
wind and by glaciers. Sedimentary rocks are divided into the
fragmental rocks--which are made of fragments, either coarse or

fine--and the far less common rocks which are constituted of chemical
precipitates.
The sedimentary rocks are divided according to their composition into
the following classes:
1. The arenaceous, or quartz rocks, including beds of loose sand and
gravel, sandstone, quartzite, and conglomerate (a rock made of
cemented rounded gravel or pebbles).
2. The calcareous, or lime rocks, including limestone and a soft white
rock formed of calcareous powder known as chalk.
3. The argillaceous, or clay rocks, including muds, clays, and shales.
These three classes pass by mixture into one another. Thus there are
limy and clayey sandstones, sandy and clayey limestones, and sandy
and limy shales.
GRANITE. This familiar rock may be studied as an example of the
second great group of rocks,--the unstratified, or igneous rocks. These
are not made of cemented sedimentary grains, but of interlocking
crystals which have crystallized from a molten mass. Examining a
piece of granite, the most conspicuous crystals which meet the eye are
those of feldspar. They are commonly pink, white, or yellow, and break
along smooth cleavage planes which reflect the light like tiny panes of
glass. Mica may be recognized by its glittering plates, which split into
thin elastic scales. A third mineral, harder than steel, breaking along
irregular surfaces like broken glass, we identify as quartz.
How granite alters under the action of the weather may be seen in
outcrops where it forms the bed rock, or country rock, underlying the
loose formations of the surface, and in many parts of the northern states
where granite bowlders and pebbles more or less decayed may be found
in a surface sheet of stony clay called the drift. Of the different
minerals composing granite, quartz alone remains unaltered. Mica
weathers to detached flakes which have lost their elasticity. The
feldspar crystals have lost their luster and hardness, and even have
decayed to clay. Where long- weathered granite forms the country rock,

it often may be cut with spade or trowel for several feet from the
surface, so rotten is the feldspar, and here the rock is seen to break
down to a clayey soil containing grains of quartz and flakes of mica.
These are a few simple illustrations of the surface changes which some
of the common kinds of rocks undergo. The agencies by which these
changes are brought about we will now take up under two
divisions,--CHEMICAL AGENCIES producing rock decay and
MECHANICAL AGENCIES producing rock disintegration.
THE CHEMICAL WORK OF WATER
As water falls on the earth in rain it has already absorbed from the air
carbon dioxide (carbonic acid gas) and oxygen. As it sinks into the
ground and becomes what is termed ground water, it takes into solution
from the soil