DYNAMICAL SERIES. 



13 



Among the forces which produce perplexing and curious Gallery XI. 

 forms and markings in the rocks are those to which the 

 general name of concretionary action is frequently applied. 

 The term is a convenient one, since, as it implies nothing, 

 it cannot well be incorrect. A beautiful example of it is Wall-ease 

 furnished by the dendritic markings seen on many flat 

 surfaces, and often resembling mosses or sea-weeds. The 

 manner of their production is illustrated in some artificial 

 preparations by Professor William Watson. The iron 

 pyrites (sulphide of iron) so common in the Chalk constantly 

 assumes shapes which may be mistaken for fossils, and the 

 same substance often does replace the original constituents 

 of shells and similar objects, which are then said to be 

 pyritised. The passage of water through the rocks, often 

 carrying minerals in solution, is a frequent cause of spotting 

 and of banded structure. The banded flints, of which some 

 fine specimens are exhibited, are exceedingly common 

 examples of such a process. 



Concretions are frequently formed around some organic Wall-case 

 nucleus, as a plant or a fish, in consequence of the chemical 

 action produced by the decaying matter. Some of the larger 

 concretions often become cracked, and the cracks again filled 

 up by the infiltration of carbonate of lime or a similar 

 substance, which forms numerous partitions or septa through 

 the concretion, which is then known as a septarium. In 

 addition to the examples of septaria shown in Wall-case 6, 

 there is a polished section of a fine one from the Oxford 

 Clay of Weymouth, fixed on the end wall of the Gallery. 

 The curious forms assumed by flint in the Chalk should by 

 this time be well known, and yet there are many people 

 who still offer curiously shaped flints to the British Museum 

 under the impression that they are rare fossils. A few 

 specimens obtained in this way are here shown as a warning 

 to others. It will, however, be noticed that many flints do 

 actually contain fossil animals. Among the extraordinary 

 forms assumed by flints, particular mention may be made 

 of the pot-stones or Paramoudras, common in Norfolk. Between 

 These huge, roughly cylindrical masses, usually with a Wall^cases 

 central cavity, are arranged in vertical columns in the Chalk, 

 and often pass through it for long distances. Specimens 

 and illustrations of these are exhibited. 



We come next to a fine series of concretions collected. Wall-ease 

 chiefly by Mr. G. Abbott, from the Magnesian Limestone of '^"^» 

 Upper Permian Age in Sunderland and Marsden in Durham. 



