44 



PHYSICAL GEOLOGY 



Marble is metamorphic limestone. It represents an ad- 

 vanced stage in the crystallization of calcareous sediments 

 (p. 37). In some marbles the fine grains of calcite have re- 

 formed as interlocking crys- 

 tals larger than those in most 

 granites, while in other cases 

 the texture is so fine that in- 

 dividual grains cannot be 

 distinguished. Marble is 

 white if formed from pure 

 limestone, but because of im- 

 purities may be of any color. 

 Carbon and other impurities 

 often form streaks or bands of 

 varying color, producing beau- 

 tiful and odd effects on pol- 

 ished surfaces. Marble is 

 much used as an ornamental 

 building stone. There are 

 extensive quarries at various 

 points in the East, particu- 

 larly in Vermont. Unlike most metamorphic rocks, pure mar- 

 ble is without cleavage. It may be scratched easily with a 

 knife, and thus distinguished readily from sandstone and 

 quartzite, which it may resemble in appearance. 



FIG. 19. Fossiliferous slate near 

 Townsead, Mont. (Walcott, U.S. 

 Geol. Surv.) 



A. META-SEDIMENTARY SERIES 



