374 ORDOVICIAN PERIOD 



as occurring at the close of the Ordovician. It would perhaps be 

 more accurate to say that their beginning marks the beginning of 

 the transition from the Ordovician 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 are commonly held to be products of the decay 

 or distillation of organic matter included in the sediments when they 

 were deposited. 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 

 (i) 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, perhaps 

 extracted from sea-water by organisms. 



The Ordovician limestones of central Tennessee 4 locally yield 



P h 



Fig. 333. Shows modes of occurrence of the phosphates (the shaded surface 

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

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



1 Orton, 8th Ann. Kept., U. S. Geol. Surv.; Phinney, nth 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, and Buckley, Vol. IX. 



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



