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 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.