92 



CONCRETIONS, 



Upper 

 Gallery. 



Wall-case 4A. 



Wall-case 44. 



Nodules, Concretions, Geodes, Sept aria, &c. 



These bodies are of many forms and of different compost 

 tions, and occur in strata of all kinds and ages. Concretions 

 are sandy, calcareous, phosphatic, ferruginous, &c, or of 

 these and other substances variously intermixed. The 

 spherical grains of oolitic rocks are small concretions of 

 lime, and it frequently happens that in the centre of a con- 

 cretion or nodule, a grain of sand, a shell, or some other 

 body has served as a nucleus round which the matter form- 



Small oolitic concre- 



mg 



the concretion has 



gathered. 



tions may have been formed during the accumulation of the 

 substance forming the rock, but many of the larger concre- 

 tionary bodies, such as Nos. 42 and 46, have been formed 

 in the body of the rock subsequent to its accumulation, and 

 perhaps during the process of consolidation. Much remains 

 to be done on this subject, and of some of the forms in this 

 case no explanation has yet been given. — A. C. Ramsay. 



Specimens arranged and described by H. W. Bristow. 



1. — Nodular concretions, Permian, from Magnesian 



■ 



Limestone. 



2. — Renifobm nodular concretion of white sand. 



3. 



4. 



5. — Fibrous carbonate of lime, termed "beef* by the 



quarrymen in the Isle of Purbeck, and "horseflesh" in 

 the Isle of Portland. It occurs in beds or thin laminae, in 

 the middle Purbeck. (Map 16. See vertical section 

 No. 22.) — Durlston Bay, Dorset. 





" The crystals of this mineral are usually found shooting 



upward from a band of perished bivalves, and appear due to 



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