Building Materials, Their Use and Origin 413 



Marble. Sedimentary rocks, limerock, sandrock, and shale 

 have in some localities been subjected to conditions that 

 worked a complete change in their character. The conditions 

 which produce such changes are pressure, heat, and moisture. 

 Rock produced in this way from other rock is generally known 

 as metamorphic (transformed). Examples of rock strata have 

 been found which show the gradual transition from sedimentary 

 or other rock into metamorphic rock. These show both the 

 kinds of rock metamorphosed and the causes of the meta- 

 morphosis. 



In laboratory experiments limestone has been converted into 

 marble. Marble has been found in nature, which showed 

 clearly by the fossils with it its origin from limerock. Besides 

 the metamorphic change of limerock into marble, other sedi- 

 mentary rocks, as sandrock and shale, are known to have been 

 similarly metamorphosed, a sandrock composed mostly of 

 quartz grains into quartzite, and shale into slate. 



Marble is ordinarily hard, crystalline, and granular, as may 

 be seen by means of a microscope, and it takes a beautiful polish. 

 When produced from pure limerock, it is white ; from impure, 

 it is veined and variously colored, as blue, gray, blue-gray, 

 yellow, and reddish. Colored marbles are generally used for 

 ornamentation. The marble quarries of New England are well 

 known. The Greek marble called Parian, from the island where 

 it was quarried, is famous for its beauty and durability. 



III. GROUPS OF ROCKS 



197. Three groups. The rocks which have been considered 

 with special reference to building and construction have a 

 wider significance to which attention may be directed. The 

 kinds named are representative forms of the three great groups 

 or classes of rocks, called sedimentary, metamorphic, and 

 igneous. 



The sedimentary rocks include, as already stated, sandstone, 

 shale, and a familiar conglomerate rock made up of gravel 



