LIME AND CEMENT INDUSTRIES 645 



Limestones may be divided into lime rock and cement rock. 

 The former when burned slakes, or falls to pieces in water with 

 the evolution of heat. The latter when burned does not slake 

 but forms a hard mass on the addition of water. The two kinds 

 grade into each other, and under each group several subdivisions 

 can be made, as below. 



( Limestone 

 f lime rocks *j 

 Limestones -! Magnesian limestone 



I Slightly argillaceous limestone 

 . Cement rocks "j Argillaceous limestone 



( Argillaceous magnesian limestone 



This classification is based on the composition and uses of the 

 rock. 



Whether dolomites have been derived by direct chemical pre- 

 cipitation or by secondary changes has been much discussed, some 

 arguing for the former method of origin ; but many for the latter, 

 admitting the first explanation to hold good in only a few cases. 1 



It is definitely known, however, that dolomite is at times formed 

 by the replacement of some of the lime carbonate of a pure lime- 

 stone by magnesium carbonate. This process of dolomitization 

 is accompanied by a shrinkage in the rock. 2 



1 Geikie, A. Textbook of geology, Ed. 3. p. 321. 



2Zirkel, F. Lehrbuch der petrographie, 3: 509. Orton, E. 8th an. rep't 

 U. S. geol. sur. pt 2, p. 641. 



