KINDS OF SOILS. 49 



or parts of crystals, thin scales, broken bits, and 

 ragged scraps of different materials, thrown together 

 in a confused mass. Look next at the sandstone. It 

 appears to be formed of small grains, each one rounded 

 and worn as if rolled in the water. It is made of sand 

 arranged in layers and lines and cemented together. 

 Under the glass, the grains of sand can be plainly 

 seen. By rubbing, the sand can be rubbed out of the 

 stone. Hold the piece of chalk over a tumbler of 

 water, and brush or nib it till the dust falls and makes 

 the water white and muddy. Let this settle ; and 

 then, after pouring the water off, spread some of the 

 soft sediment in the tumbler on a piece of glass, and 

 examine it under the microscope. The white powder 

 appears to be composed in part of minute shells, and 

 bits of broken shells. These three samples of rock 

 represent the three great classes into which all rocks 

 have been divided : the granite belongs to the igneous 

 or fire-formed rocks; the sandstone represents the 

 sedimentary or water-formed rocks ; and the chalk 

 came from the organic rocks, or rocks formed from 

 the remains of shellfish laid down as sediment on the 

 bottom of some old sea. The sedimentary rocks are 

 the most abundant in the world, because they are 

 composed of the remains of all kinds of rocks. They 

 are divided, in turn, into three classes or groups, the 

 conglomerates, or pudding-stones, composed of gravel 

 bound together into solid rock ; the sandstones, com- 

 posed of sand cemented together; and the shales, 

 made from fine silt or mud hardened into stone. 



