THE ORDOVICIAN PERIOD. 



337 



jacent parts of Iowa and Illinois contain ores of lead and zinc, 1 mainly 

 in the form of sulphides or carbonates. Lead ores are also found in 

 the Ordovician (or Cambro-Ordovician) formations of southeastern 

 Missouri, 2 and lead and zinc ores in the south-central part of the 

 same State. In all these regions the lead and zinc ores occur (1) in 

 cavities formed by solution, (2) as replacements of limestone, or (3) in 

 veins. 



The manganese ore of Arkansas is associated with the Ordovician 

 formations. 3 



The Ordovician limestones of central Tennessee locally yield phos- 

 phates, much used as fertilizers. The workable deposits have resulted 



Fig 154. — Shows the modes of occurrence of the phosphates (the shaded surface 

 parts of the limestone) in central Tennessee. (Haves and Ulrich, Columbia 

 (Tenn.) folio, U. S. Geol. Surv.) 



from the leaching of phosphatic limestone, and the concentration of 

 phosphatic material 4 (Fig. 154.) The metamorphic Ordovician lime- 

 stones of New England and some parts of the Appalachian mountains 

 have been extensively used for marble. 



1 See Chamberlin, Geology of Wisconsin, Vol. IV, pp. 365-568; and Calvin and 

 Bain, Iowa Geol. Surv., Vol. "VI. 



2 Winslow, Missouri Geol. Surv., Vol. VI and VII. 



3 Penrose, Ann. Rept. Arkansas Geol. Surv., 1890, Vol I. 



4 Hayes, Columbia (Tenn.) folio, U. S. Geol. Surv. 



