The composition of different land areas varies greatly. Many different kinds of rocks are often found crowded together, or it may happen that the same kind of rock covers a large area. There is noGRANITE.
Igneous rock formed deep below the surface of the earth.
uniformity. The soil on top of the rock is also variable. In some places it contains the minerals which are in the rock below and in other places its composition is not at all dependent upon the bed rock.
The great variety of rocks of which the crust of the earth is composed has been divided into three great groups inaccordance with the manner in which they were formed. These groups are igneous, sedimentary and metamorphic.
FOSSIL-BEARING LIMESTONE.
A sedimentary rock formed from sea shells.
The igneous rocks are those which have solidified from a melted condition. They may have solidified deep down within the crust, or on the surface, or somewhere between the depths and the surface. If these rocks cooled slowly, they will have a crystalline structure, as in granite, and if very rapidly, a glassy structure, as in obsidian. Their structure can vary anywhere between these two extremes. A common dark colored variety of this kind of rock is called basalt. There are many varieties of igneous rocks, but they need not be considered here.
CONGLOMERATE.
A sedimentary rock formed from old gravel beds.
The sedimentary rocks are those that are made by deposition in water. When rocks are worn away into fragments and these fragments are deposited in water they will, under certain conditions, harden into rocks. The shells and remains of sea animalsalso accumulate, and after a time consolidate into rock. The remains of plants may accumulate under such conditions that they will not rot but will solidify into rock which we call bituminous or soft coal. About four fifths of the land surface of the earth is composed of sedimentary rocks. They vary greatly in color, durability and usefulness to men.
The sandstones, which are composed of little grains of sand cemented together, are used for buildings and for many other purposes. The limestones, which are mostly made from the remains of sea animals, are the source of our lime and are also used for building and for other purposes. The shales are finely stratified mud depositsOIL WELLS.
Tapping the rock layers containing petroleum.
often having many layers in an inch of thickness. Bituminous coal, which is formed from plants of former ages, is the most useful and valuable of all mineral products. None of these rocks is crystalline. They are composed of fragments of other rocks or remains of plants or animals and usually occur in layers or strata.
Petroleum is probably a result of the accumulation in the sea of layers of animal and plant remains. These were covered by other layers and, during the ages since their formation, they have decomposed and changed into oil and gas.
The metamorphic rocks have a crystalline structure, often contain well-formed crystals imbedded in them and often bands of crystalline substances extending through them. These rocks are not in the condition in which they were originally laiddown, but are modified forms of either the igneous or sedimentaryGNEISS.
Probably metamorphosed granite.
rocks. The rocks originally laid down have been subjected to changes which have rearranged their mineral constituents and changed the structure.
These changes are generally due to heat and pressure. Marble is a crystallized limestone and gneiss generally a metamorphosed granite. Slate and mica-schist are greatly changed clay rocks and anthracite coal is a metamorphosed form of bituminous coal. The rocks of this group are often hard to distinguish from igneous rocks.
41.tructure of the Land Areas.
Experiment 40. -Take a copper ball having a ring just large enough to encircle it, the same apparatus as used in Experiment 19. (Fig. 22.) Place the ball within the ring and heat them both to a high temperature. Remove the ball from the ring and plunge it into a dish of water. Place the cooled ball again within the ring. The ring will be found too large to fit snugly upon it.
If the ring had been a cold hollow sphere fitting tightly to the surface of the hot ball and the ball had then been cooled until its temperature approached the temperature of the cold surrounding spherical surface, it would have shrunk away from this spherical surface. This would leave an unfilled space between the two into which the spherical shell must have shrunk if not strong enough to support itself. This shrinking would cause wrinkling in some parts of its surface.
Experiment 41. -When at home measure the greatest and least circumference of a large smooth apple by winding a string around it and then unwinding and measuring the length of the string. Bake the apple. Measure its circumferences again. Are they greater or less than before? Is the skin of the apple as smooth as it was before?
Not only do the land areas differ greatly in the kind of rocks of which they are composed, but also in the way in which these rocks are placed. Some of the rocks lie nearly in the condition in which they were originally formed while others have been folded and warped and twisted. Vast layers of rocks have been worn away by the forces which are continually wearing away and removing the rocks at the surface ofSTRATIFIED ROCK.
These layers have remained horizontal as originally formed.
the earth, and thus rocks which were once at great depths below the surface have been exposed. Even granite rocks which were originally formed at a depth of thousands of feet below the surface now appear at the surface and are being quarried in many places.