The Elements of Geology | Page 7

wind, and the sea. In

Part II we shall treat of those agencies whose
sources are within the
earth, and among whose manifestations are volcanoes and earthquakes
and the various movements of the earth's crust. As we study each
agency we shall notice not only how it does its work, but also the
records which it leaves in the rock structures and the land forms which
it produces. With this preparation we shall be able in

Part III to read in the records of the rocks
the
history of our planet and the successive forms of life which have dwelt
upon it.



PART I

EXTERNAL GEOLOGICAL AGENCIES


CHAPTER I
THE WORK OF THE WEATHER
In our excursion to the valley with sandstone ledges we witnessed a
process which is going forward in all lands. Everywhere the rocks are
crumbling away; their fragments are creeping down hillsides to the
stream ways and are carried by the streams to the sea, where they are
rebuilt into rocky layers. When again the rocks are lifted to form land
the process will begin anew; again they will crumble and creep down
slopes and be washed by streams to the sea. Let us begin our study of
this long cycle of change at the point where rocks disintegrate and
decay under the action of the weather. In studying now a few outcrops
and quarries we shall learn a little of some common rocks and how they
weather away.
STRATIFICATION AND JOINTING. At the sandstone ledges we saw
that the rock was divided into parallel layers. The thicker layers are
known as STRATA, and the thin leaves into which each stratum may
sometimes be split are termed LAMINAE. To a greater or less degree
these layers differ from each other in fineness of grain, showing that the
material has been sorted. The planes which divide them are called
BEDDING PLANES.
Besides the bedding planes there are other division planes, which cut
across the strata from top to bottom. These are found in all rocks and
are known as joints. Two sets of joints, running at about right angles to
each other, together with the bedding planes, divide the sandstone into
quadrangular blocks.
SANDSTONE. Examining a piece of sandstone we find it composed of
grains quite like those of river sand or of sea beaches. Most of the

grains are of a clear glassy mineral called quartz. These quartz grains
are very hard and will scratch the steel of a knife blade. They are not
affected by acid, and their broken surfaces are irregular like those of
broken glass.
The grains of sandstone are held together by some cement. This may be
calcareous, consisting of soluble carbonate of lime. In brown
sandstones the cement is commonly ferruginous,--hydrated iron oxide,
or iron rust, forming the bond, somewhat as in the case of iron nails
which have rusted together. The strongest and most lasting cement is
siliceous, and sand rocks whose grains are closely cemented by silica,
the chemical substance of which quartz is made, are known as
quartzites.
We are now prepared to understand how sandstone is affected by the
action of the weather. On ledges where the rock is exposed to view its
surface is more or less discolored and the grains are loose and may be
rubbed off with the finger. On gentle slopes the rock is covered with a
soil composed of sand, which evidently is crumbled sandstone, and
dark carbonaceous matter derived from the decay of vegetation. Clearly
it is by the dissolving of the cement that the rock thus breaks down to
loose sand. A piece of sandstone with calcareous cement, or a bit of old
mortar, which is really an artificial stone also made of sand cemented
by lime, may be treated in a test tube with hydrochloric acid to
illustrate the process.
A LIMESTONE QUARRY. Here also we find the rock stratified and
jointed (Fig. 2). On the quarry face the rock is distinctly seen to be
altered for some distance from its upper surface. Below the altered zone
the rock is sound and is quarried for building; but the altered upper
layers are too soft and broken to be used for this purpose. If the
limestone is laminated, the laminae here have split apart, although
below they hold fast together. Near the surface the stone has become
rotten and crumbles at the touch, while on the top it has completely
broken down to a thin layer of limestone meal, on which rests a fine
reddish clay.
Limestone is made of minute grains of carbonate of lime all firmly held

together by a calcareous cement. A piece of the stone placed in a test
tube with hydrochloric acid dissolves with brisk effervescence, leaving
the insoluble impurities, which