LEAD AND ZINC. 



163 



ing, or only a brecciated and crushed strip. Chamberlain cites 

 twelve varieties in all, some of which are based on rather fine dis- 

 tinctions. 



2.06.03. The cavities were referred by J. D. Whitney to 

 joints, formed either by the drying and consolidating of the rock 

 or by gentle oscillations of the inclosing beds. The later work 

 lias largely corroborated this, and they are generally thought to 

 be chiefly caused by the cracks and partings formed by the gentle 

 synclinal foldings. Such cavities have usually been enlarged by 

 subsequent alteration of the walls. Whitney also essentially out- 

 lined the explanation of origin, which has been more fully elabo- 





FIG. 42. Idealized section of "flats and pitches," forms of ore bodies in 

 Wisconsin. After T. C. Chamberlain, Geol. Wis., Vol. IV., p. 458. 



rated by Chamberlain. Both these writers have urged that the 

 ores could not have come from below, for the lower rocks are sub- 

 stantially barren of them. The conclusion therefore follows that 

 they were deposited in the limestones at the time of their forma- 

 tion. The source of the ores is placed in the early Silurian sea, 

 from which it is thought they were precipitated by sulphuretted 

 hydrogen, exhaled by decaying seaweeds, or similar dead organ- 

 isms on the bottom. In carrying the idea further, Chamberlain 

 has endeavored to reproduce the topography of the region in the 

 Lower Silurian times and to indicate the probable oceanic cur- 

 rents. These are conceived to have made an eddy in the lead 

 district and to have collected there masses of seaweed, etc., re- 

 sembling the Sargasso Sea. While interesting, this must be con- 

 sidered very hypothetical. When the sulphides became precipi- 











