342 



HISTORICAL GEOLOGY 





Lead and zinc deposits. In parts of the Mississippi 

 valley ores of lead and zinc are now found abundantly in the 

 Ordovician limestone. Apparently minute particles of lead 

 and zinc minerals were deposited sparsely through the sedi- 

 ments while they were accumulating, and, at a later time, these 

 scattered particles were dissolved out by the waters which 

 saturate the rocks, and were redeposited along joints and 

 ...,-. .^.^- bedding planes in the lime- 



stone (Fig. 340). Thus 

 concentrated in veins, the 

 minerals may be profitably 

 mined. 



Wide distribution of the 

 sea life. The broad, shal- 

 low seas of the Ordovician 

 period afforded a congenial 

 home for many species of 

 marine organisms, and, al- 

 though it is certain that the 

 majority of the forms which 

 existed then have left no 

 traces in the rocks, yet 

 enough have been preserved 

 to show us the variety and 

 advancement of the ani- 

 mals of the time. The wide 

 expansion of the seas, and 

 the free communication which seems to have prevailed be- 

 tween them, permitted the individual species to migrate 

 readily from one part of the globe to another. This was par- 

 ticularly true of animals which floated in the water, such as 

 graptolites and young corals (p. 296). Hence some of the 

 Ordovician fossils of the United States are much like those 

 of Europe and even Asia and Australia. Such a widespread 

 assemblage of animals is called a cosmopolitan fauna. In 

 any one place the animals of Ordovician time were in part 



FIG. 340. Vertical section of a zinc- 

 and-lead ore deposit in southwestern 

 Wisconsin. (After Chamberlin.) 



