IOWA ACADEMY OF SCIENCES. 55 



three by three feet the deposition of calc spar had gone on to such an extent 

 that there was a deposit several feet thick on the lloor, while hanging from 

 above were numerous stalactites. These were arranged mostlj' in two rows 

 along the sides of the cavern and touching the bottom or joining the stal- 

 agmites below they formed columns. The passage-way thus made resembles 

 a miniature colonade. 



OCCURRENCE IN IOWA OF FOSSILIFEROUS CONCRETIONS SIMILAR 

 TO THOSE OF MAZON CREEK. 



BY ARTHUR C. SPENCER,] 



The wide celebrity of the fern-bearing concretions from the Carboniferous beds 

 of Mazon creek, Illinois, attaches more than passing interest to the occurrence of 

 similiar structures in the Coal Measures of Iowa. 



These concretions are found in a small ravine near the Des Moines river, north 

 of Dunreath, in Marion county. Careful search for similar concretions in the 

 gullies of the neighboring streams has not been successful, from which it 

 seems that the strata, which are cut by the streams in question, lie above their 

 general level on a slight anticline. The other alternative is that the concretions 

 are limited to a very small area, but from the relations of the overlying beds the 

 first explanation seems to be correct. 



The plant remains are found in nodules or concretions, scattered through beds 

 of drab shale. These, when broken open, often display very perfect formt?. Plant 

 remains are not, however, present in all the concretions. Others are like small 

 septarial masses and are filled with zinc blende. 



The nodule-bearing shale is from three to perhaps ten feet in thickness, and 

 of a light drab color. It rests upon an irregular layer of large septarial masses 

 which, exposed in the dry bed of the stream, resemble roche-moutenees on a small 

 scale. Above are shales in part bituminous and in part areaceous. Four 

 inches of compact gray limestone, bearing fern impressions follows, above which 

 is more sandy shale and a thin seam of coal which has been mined near by. The 

 coal is about fifteen feet above the concretionary bed. 



Many of the concretions have been washed out and are found already 

 opened, but the best specimens are those recently exposed, which afford very per- 

 fect leaflets of several ferns. Among the forms identified were: Neuropteris 

 hirsuta, Xeuvopteris angustifolia and Annularia longifolin. Others will 

 undoubtedly be found when more material is examined. 



