correlative evidence 381 



Correlative Evidence of syngenetic Origin 

 f08silifer0u8 concjtetions 



Last year the writer read a short paper to the Geologic Section of the 

 American Association for the Advancement of Science on the syngenetic 

 origin of the concretions in the Pennsylvanian shales in Missouri, and in 

 the short discussion which followed the point was made by David White 

 and others that the beautifully preserved fossils, both plant and animal, 

 so frequently found in concretions were certainly favorable to their syn- 

 genetic origin. 



The concretions which are so abundant in the sandy shales along Mazon 

 Creek, Grundy County, Illinois, furnish excellent examples of the won- 

 derful preservation of organic remains in concretions. The fossiliferous 

 concretions are especially abundant in a nearly pure clay shale. Leo 

 Lesquereux^^ makes the following statement regarding them: 



"They appear to have been formed by superposition of concentric layers of 

 slowly deposited carbonate of iron or ferruginous clay around central nuclei, 

 which are most commonly parts of plants, bones of fishes, or the remains of 

 insects and Crustacea. Their size and form vary according to that of the body 

 around which the deposit has been made." 



He states further : 



"It is not clear whether the flattening of some of the specimens is the result 

 of compression." 



Cylindrical stems preserve their original shape. Discussing their ori- 

 gin, this interesting statement, especially in the light of Harder's val- 

 uable studies of iron bacteria, is made : 



. . . "the nodules of Mazon Creek rather seem to be the work of infusoria 

 or Bacillaria concentrating molecules of iron around some centers, as it now 

 happens in the formation of bog iron ore." . . . 



Lesquereux makes the following remarks about the perfect preservation 

 of even the minutest detail of the plants : 



"The pinme or leaflets of ferns are always found in them in a flattened posi- 

 tion, their axis or rachis extending through the center of the elongated nodule, 

 with the divisions on both sides ; the surface of the pinnules, slightly swollen, 

 as when in their living state, is marked by recognizable hairs or fruit dots, 

 with distinct veins and veinlets, and their appendages, like the scales, are seen 

 in the various modifications which they present in living specimens. . . . 

 The smaller organs of plants appear, therefore, in a better state of preserva- 



" Leo Lesquereux: Geological Survey of Illinois, vol. iv, pp. 4S1-48H. 



