EXPLANATION OF PLATES 367 



FiGUKE 9. — Such molecules arranged in a crystal like number 19. 



Figure 10. — Such molecules irregularly arranged, as in a concretion. 



Figure 11.— External view of a ccxicretion like number 12. 



Figure 12. — A globular concretion, showing concentric rhythmical structure. 



Figure 13. — An excretion, or centripetal concretion, in which iron oxide is trans- 

 ferred from the interior, s (siderite), to the exterior, I (limonite), 

 leaving behind impurity, i, as in number 13, plate 50. 



Figure 14. — A siderite intercretion, b, the same changing to c ; c, the same with 

 siderite dissolved, forming a "hollow pebble" broken on one side 

 so that the impurity is discharged from two chambers. 



Figure 15. — An incretion forming around a rootmark — a, vertical section ; b, a 

 horizontal. 



Figure 16. — Excretionary action forming ferruginous shells next the surface of a 

 ferruginous sandstone, a-a, and also embedded excretions, 6 and c. 



FiGURK 17. — A globular concretion showing radiate structure. 



Figure 18. — Similar, showing crystalline facets on the surface. 



Figures 19 and 20. — Cubic and octahedral crystals. 



Figure 21. — A cluster of octahedral crystals. 



Figure 22. — An ideal section showing forms of concretions, according to character 

 of embedding formation — a, loess kindchen, starting as incretions, 

 afterward becoming accretions along the water-table, some of them 

 compound ; b, irregular nodules in loamy clay ; c-c, concretions 

 merging into one another, as in many cases including those figured 

 in plate 51 ; a-d, lenticular form in shale ; e, globular forms in massive 

 sandstone; irregular chert, in limestone. 



Figure 23. — Concretions including Lucina occidentalis, causing a teepee butte — a; 

 b, the uncovered core. 



Figure 24. — Buttes caused by a stratum of sandstone. 



Figure 25. — Similar buttes formed by concretions locally abounding in a stratum 

 of sand. 



Figure 26.— Concretions forming a revetment by a lake or river. 



Figure 27. — A more comprehensive view of buttes passing into mesas where erosion 

 has not been so active or where concretions have become more 

 extensive or more numerous. 



Figure 28, — A knob caused by concretions from different strata. 



Figure 29. Formation of clay-iron-stone concretions in the sediment of a pond. 

 Eventually by long erosion this may form a knob. 



