Savage and Ross — Age of Iron Ore. 191 



From the bed of iron ore at Cascade Falls the following 

 fossils were collected : 



Cf. Stenaster sp., 

 Cf. Eurydictya montifera, 

 Lingula cf. cobourgensis, 

 Strophomena wisconsinensis, 

 Dalmanella tersa, 

 Byssonychia intermedia, 

 Byssonychia cf. radiata, 

 Pterinea cf. demissa, 

 Liospira sp. 



The fossils listed above, taken from the iron ore bed at Cas- 

 cade Falls, are Maquoketa species. They came from an undis- 

 turbed zone one to one and one-half feet above the base of the 

 iron ore bed, where they could neither have been squeezed up 

 from the underlying Maquoketa shale, nor have been carried 

 up into the iron ore by the movement of glacier ice. The 

 occurrence of Maquoketa fossils in the iron ore is interpreted 

 as indicating the Maquoketa age of the bed. A possible 

 alternative interpretation might assume that the fossils were 

 washed by the waves out of the Maquoketa shale, and 

 redeposited with the material of the oolite as the sea in which 

 the latter accumulated advanced over the region. The fact 

 that the surface of the shells present in the iron ore show little 

 evidences of wear, and that no fossils of Silurian age are found 

 in the deposit support the former interpretation. 



The middle and lower parts of the iron ore bed contain peb- 

 bles of shale or slab-like iron ore, which in places form a con- 

 glomerate. Many of these pebbles also contain grains of oolitic 

 iron, and are of the nature of iron concretions of irregular shape, 

 the iron not only having replaced the calcareous constituents, 

 but also having been deposited in the spaces between the grains, 

 forming irregular concretionary structures. Another part of 

 the pebbles appear to consist of fragments of Maquoketa shale 

 that were worked over and deposited by the waves as the sea 

 in which the oolite accumulated advanced upon the area, and 

 later became thoroughly impregnated with iron. Still another 

 part of the pebbles may have been formed by wave action 

 upon the more clayey portions of the sediments during the 

 progress of deposition of the material, the calcareous portion 

 subsequently having been replaced by iron oxide. The fossil 

 shells present in the iron ore bed were originally calcareous 

 and have all been replaced by iron. 



The surface of the pebbles, and also of the grains of oolite, 

 has a shiny, varnished appearance. Grabau* has suggested 



* A. W. Grabau, Principles of Stratigraphy, p. 57, 1913. 



