DYNAMICAL SEKIES. 
13 
Among the forces which produce perplexing and curious 
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 
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 
Hue specimens are exhibited, are exceedingly common 
examples of sucli a process. 
Concretions are frequently formed round some organic 
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 cracketl, 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 north 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 Norlblk. 
These huge, roughly cylindrical masses, usually with a 
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, 
chiefly by Mr. G. Abbott, from the Magnesian Limestone of 
Upper Permian Age at Fulwell Hill and Marsden in Durham. 
Gallery XI. 
Wall-case 
6b. 
Wall-case' 
6c. 
Between 
Wall-cases 
1 & 2 . 
Wall-case 
7a, 7b. 
