52 ELEMENTARY SCIENCE 



whose material once was sediment; that is, it was laid 

 down in water. Limestone and sandstone are sedimentary 

 rocks (see Fig. 7). 



6. The strata or formations of the earth's crust are 

 very different with respect to their relations to water. 

 Some are much more soluble in water than others. Lime- 

 stone is especially soluble in water. Sandstone is com- 

 posed of grains of sand held firmly together by a sort of 

 cement. Water may act on the cement and leave loose 

 sand. Large caves occur in limestone regions and are due 

 to underground water having dissolved and carried away 

 particles of the rock. The stalactites are due to dripping 

 water which, at the point of drip, deposited certain solutes 

 they had been carrying (see Fig. 18). 



7. Clay, gravel, and sand, and mixtures of these with 

 each other and with organic substances, form what we 

 call 50*7 and subsoil. They are the latest geological forma- 

 tions. Gravel and sand, being loose formations, can hold 

 a good deal of water. Clay, whose particles are very 

 small, packs down into a firm formation which water pene- 

 trates very slowly. Some of the best wells are made by 

 boring into formations of wet sand or gravel down below 

 layers of clay. The water may have entered the strata 

 of sand or gravel many miles away where they outcrop at 

 the surface (see Fig. u). It may have soaked through 

 them, being hindered in working upward by the dense 

 layers of clay above. So, in northern Illinois, many of the 

 best wells tap a layer of sand whose outcrop is far away in 

 Wisconsin. Springs are caused by the outcropping of 

 layers which have received water somewhere else. A rtesian 

 wells (so called because first found in Artois, France) are 

 those whose water rises to the surface without pumping, 



