SOME CHEMISTRY OF EVERYDAY LIFE 193 



deposits known as marl are an admixture of clay and carbon- 

 ate of lime. They all give evidences of an origin in common 

 with the material brought up by dredges from the bottom 

 of the ocean and found to be rich in shells. 



It would seem that carbonate of lime in solution in sea 

 water is appropriated by varied forms of marine life in the 

 building of shells as external skeletons. These shells more or 

 less broken are in time deposited along with other material 

 on the ocean floor. As result of pressure from the material 



FIG. 62. Concrete construction. (Courtesy of the East Florida Railway 

 ^ Company.) 



accumulating above, aided by the cementing effect of mate- 

 rial from the water, the whole becomes a compact mass in the 

 course of time. By reason of changes in the earth's crust 

 these sedimentary rocks, more or less distinctly arranged 

 in layers, may become elevated from the ocean depths even 

 as we now find them as limestone strata of enormous extent 

 and of great thickness. 



Marble is limestone rock which is somewhat crystalline, 

 and sufficiently compact to take and retain a high polish. 

 It is valued highly for building purposes, and in art. Marble 

 varies greatly in color, grain, and durability by reason, 

 possibly, of other earthy material with which the calcium 

 carbonate is often mixed, and because of a variation in the 

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