368 J. E. TODD — COXCRETIOXS AND THEIR GEOLOGICAL EFFECTS 



Plate 50. — Various Concretions 



Figures I and 2. — Concretions from river sand. 



Figure 3. — Concretion from shale. 



Figures 6 and 12. — Concretions of various forms and composition, all accretions. 



Figures 7 and 10. — Barite, with radiate structure. 



Figures 6 and 9. — From shale with fern leaf nucleus. 



Figure 8. — A compound concretion from sandstone. 



Figure 13. — An "excretion" showing the siderite nucleus (compare numbers 13 

 and 14, plate 49). 



Figures 14 and 15. — Pyrite and calcareous concretions formed around ammonite 

 shells. 



Figure 16. — An intercretion showing cracked interior with cracks partly filled 

 with calcite. 



Figure 17. — A compound intercretion, opened to show different members. 



Figure 18. — Reptilian vertebra serving as a nucleus for calcareous concretion. 



Figures 19, 20 and 21. — Quartz geodes. 



Figure 22. ^Number 17, with the different members in position. 



Figure 23. — Another intercretion, like 16; both are from Benton shales. 



Plate 51. — Log-UJ:e Concretions 



Figure 1. — Log-like concretions in Laramie beds, near Camp Crook, South Dakota. 



Figure 2. Similar concretions coalescing laterally on Xorth fork of Grand river, 

 South Dakota. 



Plate 52. — Concretion Horizon and Natural Revetment 



Figure 1. — Concretion horizon in the Laramie, near the junction of Flint Rock 

 creek and Moreau river. 



Figure 2. — Natural revetment formed of concretions from the Laramie, on shore 

 of lake in northern Dewey county, South Dakota. 



Plate 53. — Concretions in Wyoming 



Figure 1. — Concretions in Fox Hills sand, near Cooper creek, Wyoming. 



Figure 2. — Lenticular concretion showing sand erosion, from the same formation, 

 near Rock creek, "Wyoming. 



