410 Introduction to the Study of Science 



erally described as igneous (from the Latin term ignis t mean- 

 ing fire). 



196. Limestone. Forty per cent of all stone quarried in 

 the United States for building and construction purposes is 

 limestone. The annual output is valued at about $40,000,000. 

 But limerock is quarried for other purposes, still larger quan- 

 tities being utilized in the preparation of building materials, as 

 lime and cement, or lime for fertilizer, and in the smelting of 

 iron and certain other metals from their ores. Great quantities 

 are moreover used as ballast on railroads and in making high- 

 ways. 



Limerock is widely distributed in the United States. It is 

 generally more accessible in regions east of the Rocky Moun- 

 tains. In quality, and therefore in adaptability for different 

 purposes, it varies greatly. The best limerock for building is 

 found in Indiana; it is excellent on account of its good color, 

 evenness of grain, freedom from cracks, and the ease with 

 which it can be cut and carved with tools. The vast bulk 

 of rock quarried elsewhere is used for making lime and cement 

 and for reducing ores (pages 438-9) . 



Limerock, like sandstone, appears in layers or strata. The 

 stratified character of the rock is significant in at least two ways. 

 First, it indicates with certainty the relative age of the different 

 strata where these have not been greatly disturbed by up- 

 heaval, crumpling, erosion, or other geological processes. 

 The lower strata are necessarily older than those higher or 

 nearer the present surface of the earth. Second, the stratifica- 

 tion shows clearly that the materials of which such rock is 

 composed were deposited by sedimentation. 



Sedimentary deposits are made almost always under water. 

 Well-stratified deposits occur only in bodies of still water*, such 

 as lakes or seas where the materials deposited are not regularly 

 and continually supplied, but only intermittently as when 

 streams swollen by rain or flood carry large quantities of sus- 

 pended material. This also suggests that land surfaces and 



