Building Materials, Their Use and Origin 411 



sea and lake bottoms have changed their relative positions, the 

 land surfaces of to-day having been the bottoms of lakes or seas 

 in former ages of the earth's history. 



Stratification in the structure of rocks, such as limerock, sand- 

 rock, and slate, is advantageous in quarrying. The strata are 

 separated slightly by what are called planes of cleavage and they 

 are also broken crosswise by cracks called joints. These fea- 

 tures are decidedly useful in quarrying and preparing the rock 

 for use. 



Limerock is soluble in practically all natural water which 

 contains carbon dioxid, but not so freely in pure water. Parts 

 of many kinds of rocks are easily soluble, but the relatively 

 insoluble parts, making up by far the greater bulk, remain as 

 soil. Limestone is almost wholly soluble. This helps in under- 

 standing the rapid weathering of limestone when exposed as 

 in buildings, and also the formation of caves in limestone regions. 

 Water charged with carbon dioxid makes its way through 

 cracks or fissures of limerock, dissolving the limerock and 

 widening the fissures into caverns. The Mammoth Caves in 

 Kentucky and the Luray Cavern in Virginia are familiar 

 examples of the solvent power of water in limerock. In such 

 caverns, moreover, the water, saturated with lime carbonate, 

 dripping through the roofs and falling upon the floors, is evapo- 

 rated, leaving deposits of limestone. These deposits when 

 suspended from the roof are called stalactites, and when built 

 upon the floor, stalagmites ; when these join, as they sometimes 

 do, they form limestone pillars. 



Many carbonated springs deposit quantities of limestone as 

 they issue at the surface of the earth. Water, when subjected 

 to pressure, has a greater capacity for carbon dioxid, thus in- 

 creasing its solvent power and its capacity for holding dissolved 

 limerock. This occurs when water goes down through the 

 decayed vegetable matter in the soil and the vegetable matter 

 and other sources of carbon dioxid in the deeper parts of the 

 earth's crust. As water thus charged permeates limestone 



