THE ORDOVICIAN PERIOD 517 



the transition from the Ordoviciaii period to the Silurian. The 

 duration of the interval of transition was probably long. 



Economic Products 



In Ohio and eastern Indiana the Trenton formation yields much 

 gas and oil. 1 Both these substances are believed to be products of 

 the decay or distillation of organic matter which was included in 

 the sediments at the time of their deposition. The oil is most 

 abundant under low anticlines, where it occurs in the pores and 

 openings of the rock, somewhat as ground-water does. 



The Galena and Trenton formations in Wisconsin 2 and in the 

 adjacent parts of Iowa and Illinois contain ores of lead and zinc, 

 mainly in the form of sulphides and carbonates. Lead ores are 

 also found in the Ordovician (or Cambro-Ordovician) formations 

 of southeastern Missouri, 3 and lead and zinc ores in the south- 

 central part of the same state. In all these regions the ores occur 

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

 or (3) in crevices. In these positions, the ore was concentrated by 

 ground-water. The metallic substances were doubtless derived 

 from the limestone itself, which, at the time of its deposition, is 

 thought to have contained trifling amounts of lead and zinc, 

 derived from sea-water by organic deposition. 



The Ordovician limestones of central Tennessee 4 locally yield 

 calcium phosphate, valuable as a fertilizer. The workable deposits 

 have, resulted from the leaching of phosphatic limestone, leaving 

 the less soluble phosphate concentrated at the surface (Fig. 383). 

 The Manganese ore of Arkansas had a similar origin. The meta- 

 morphic Ordovician limestones of New England and some parts of 

 the Appalachian Mountains have been extensively used for marble. 



Foreign Ordovician 

 The Ordovician formations appear at the surface in various 



, 8th Ann. Kept., U. S. Geol. Surv.; Phinney, llth Ann. Kept.; 

 also the reports of the State Geol. Surv. of Ohio and Indiana. 



2 Chamberlin, Geol. of Wis., Vol. IV, 1879, pp. 365-568; Calvin and Bain, 

 Iowa Geol. Surv., Vol. VI, and Grant, Bull. XIV, Wis. Geol. Surv., 1906. 



3 Winslow, Missouri Geol. Surv., Vols. VI and VII. 



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



