THE IRON SERIES (IN PART). 87 



characteristic of Carboniferous strata and differ from black-band 

 only in the absence of bituminous matter and in the consequent 

 drab color. They weather to liinonite, generally in concentric shells 

 with a core of unchanged carbonate within. Fossil leaves or shells 

 often furnish the nucleus for the original concretion, and are thus, 

 as at Mazon Creek, 111., beautifully preserved. When in beds the 

 ore is sometimes called flagstone ore; when broken into rectangular 

 masses by joints, it is called block ore. 



2.01.29. Example 3a. Slack-band. The name is applied to 

 beds consisting chiefly of carbonate of iron with more or less earthy 

 and bituminous matter. They are of varying thickness, though 

 rarely more than six feet, and are almost invariably associated 

 with coal seams. They are thus especially found in the Carbonif- 

 erous system, and to a far less degree in the eastern Jura-Trias. 

 They are also recorded with the Cretaceous coals of the West. It 

 is not possible to separate the two varieties in discussing their dis- 

 tribution. The various productive areas are taken up geograph- 

 ically, beginning with the Appalachian region. 



2.01.30. The carbonate ores are of great importance in the 

 Carboniferous of western Pennsylvania and in the adjacent parts of 

 Ohio, West Virginia, and Kentucky. In these States the system is 

 subdivided in connection with the coal, from above downward, as 

 follows : I. The Upper Barren Measures, Permo-Carboniferous, or 

 Dunkard's Creek Series ; II. The Upper Productive Coal Meas- 

 ures, or Monongahela River Series ; III. The Lower Barren Meas- 

 ures, or Elk River Series ; IV. The Lower Productive Coal 

 Measures, or Alleghany River Series ; V. The Great or Pottsville 

 Conglomerate. In the Upper Barren Measures of Pennsylvania, 

 according to McCreath, there is hardly a stratum of shale or sand- 

 stone without clay ironstone nodules, but no continuous beds are 

 known. 1 The deposits are not of great actual importance, and are 

 worthy of only passing mention. In the Upper Productive Coal 

 Measures some ore occurs associated with the Waynesburg Coal 

 seam, and again, just under the Pittsburg seam, there is consider- 

 able known as the Pittsburg Iron Ore Group. This latter ore be- 

 comes of great importance in Fayette County and extends through 

 several tfeds. 2 The Lower Barren Measures in Pennsylvania also 

 contain carbonate ore in a number of localities. The most per- 



1 Second Penn. Survey, Rep. K, p. 386 ; MM, p. 159. 



2 Rep. MM, p. 162 ; KK, p. Ill ; L, p. 98. 



