The granite shaft is made out of the original substance of the earth's crust. Its minerals are the elements out of which all of the rock masses of the earth are formed, no matter how different they look from granite. Sandstone is made of particles of quartz. Clay and slate are made out of feldspar and mica. Iron ore comes from the hornblende in granite. The mineral particles, reassembled in different proportions, form all of the different rocks that are known.

Here in my hand is a piece of pudding-stone. It is made of pebbles of different sizes, each made of different coloured minerals. The pebbles are cemented together with a paste that has hardened into stone. This kind of rock the geologists call conglomerate. Pudding-stone is the common name, for the pebbles in the pasty matrix certainly do suggest the currants and the raisins that are sprinkled through a Christmas pudding.

Under the seashores there are forming to-day thick beds of sand. The rivers bring the rock material down from the hills, and it is sorted and laid down. The moving water drops the heaviest particles near shore, and carries the finer ones farther out before letting them fall.

The hard water, that comes through limestone rocks, adds lime in solution to the ocean water. All the shellfish of the sea, and the creatures with bony skeletons, take in the bone-building, shell-making lime with their food. Generations of these inhabitants of the sea have died, and their shells and bones have accumulated and been transformed into thick beds of limestone on the ocean floor. This is going on to-day; but the limestone does not accumulate as rapidly as when the ocean teemed with shell-bearing creatures of gigantic size. Of these we shall speak in another chapter.

The fine dust that is blown into the ocean from the land, and that makes river water muddy, accumulates on the sea bottom as banks of mud, which by the burden of later deposits is converted into clay. Sandstone is but the compressed sand-bank.

In the study of mountains, geologists have discovered that old seashores were thrown up into the first great ridges that form the backbone of a mountain system. The Rocky Mountains, and the Appalachian system on the east, were made out of thick strata of rocks that had been formed by accumulations of mud and sand—the washings of the land—on the opposite shores of a great mid-continental sea, that stretched from the crest of one great mountain system across to the other, and north and south from the Laurentian Hills to the Gulf of Mexico. The great weight of the accumulating layers of rock materials on one side, and the wasted land surfaces on the other, made the sea border a line of greatest weakness in the crust of the earth. The shrinking of the globe underneath caused the break; mashing and folding followed, throwing the ridge above sea-level, and making dry land out of rock waste which had been accumulating, perhaps for millions of years, under the sea. The wrinkling of the earth's crust was the result of crushing forces which produced tremendous heat.

Streams of lava sprang out through the fissures and poured streams of melted rock down the sides of the fold, quite burying, in many places, the layers of limestone, sandstone, and clay. Between the strata of water-formed rocks there were often created chimney-like openings, into which molten rock from below was forced, forming, when cool, veins and dikes of rock material, specimens of the substance of the earth's interior.

Tremendous pressure and heat, acting upon stratified rocks saturated with water transform them into very different kinds of rock. Limestone, subjected to these forces, is changed into marble. Clays are transformed into slates. Sandstone is changed into quartzite, the sand grains being melted so as to become no longer visible to the naked eye. The anthracite coal of the Pennsylvania mountains is the result of heat and pressure acting upon soft coal. Associated with these beds of hard coal are beds of black lead, or graphite, the substance used in making "lead" pencils. We believe that the same forces that operated to transform clay rocks into slate, and limestone into marble, transformed soft coal into hard, and hard coal into graphite, in the days when the earth was young.

The word sedimentary is applied to rocks which were originally laid down under water, as sediment, brought by running water, or by wind, or by the decay of organic substances. Stratified rocks are those which are arranged in layers. Sedimentary rocks will fall into this class. Aqueous rocks are those which are formed under water. Most of the stratified and sedimentary rocks, but not all, may be included under this term. Rocks that are made out of fragments of other rocks torn down by the agencies of erosion are called fragmental. Wind, water, and ice are the three great agencies that wear away the land, bring rock fragments long distances, and deposit them where aqueous rocks are being formed. Volcanic eruptions bring material from the earth's interior. This material ranges all the way from huge boulders to the finest impalpable dust, called volcanic ashes. Rivers of ice called glaciers crowd against their banks, loosening rock masses and carrying away fragments of all sizes, in their progress down the valley. Brooks and rivers carry the pebbles and the larger rock masses they are able to loosen from their walls and beds, and grind them smooth as they move along toward lower levels.

The air itself causes rocks to crumble; percolating water robs them of their soluble salts, reducing even solid granite to a loose mass of quartz grains and clay. Plants and animals absorb as food the mineral substances of rocks, when they are dissolved in water. They transform these food elements into their own body substance, and finally give back their dead bodies, the mineral substances of which are freed by decay to return to the earth, and become elements of rock again.

The decay of rock is well shown by the materials that accumulate at the base of a cliff. Angular fragments of all sizes, but all more or less flattened, come from strata of shaly rock, that can be seen jutting out far above. A great deal of this sort of material is found mingled with the soil of the Northeastern States. Round pebbles in pudding-stone have been formed in brook beds and deposited on beaches where they have become caked in mud and finally consolidated into rock. If the beach chanced to be sandy instead of muddy, a matrix of sandy paste holds the larger pebbles in place. Limestone paste cements together the pebbles of limestone conglomerates.

In St. Augustine many of the houses are built of coquina rock, a mass of broken shells which have become cemented together by lime mud, derived from their own decay. On the slopes of volcanoes, rock fragments of all kinds are cemented together by the flowing lava. So we see that there are pudding-stones of many kinds to be found. If some solvent acid is present in the water that percolates through these rocks it may soften the cement and thus free the pebbles, reducing the conglomerate again to a mere heap of shell fragments, or gravel, or rounded pebbles.

The story of rock formation tells how fire and water, and the two combined, have made, and made over, again and again, the substance of the earth's crust. Chemical and physical changes constantly tear down some portions of the earth to build up others. The constant, combined effort of wind and water is to level the earth and fill up the ocean bed. Rocks are constantly being formed; the changes that have been going on since the world began are still in progress. We can see them all about us on any and every day of our lives.