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