Notices of JTeinoirs — G. Ahhott — Magnesian Concretions. 35 



3V. — The Coxcretionary Types in the Cellular Magnesian 

 Limestone of Durham.^ By G. Abbott, M.R.C.S. 



A SSOCIATED with the Cannon-ball bed near Sunderland is 

 j\_ a cellular limestone which is much more extensive, and exhibits 

 ^till more remarkable physical features. Although described by 

 Professor Sedgwick more than sixty years ago with other magnesiau 

 beds in the North of England, it is still compai'atively unknown. He 

 divided the concretions in these strata into four classes, but I have 

 been unable to find any classified collection except the one in the 

 Newcastle Museum, and even in this series it is only partially done. 



My own studies at Fulwell and Hendon lead me to suggest a new 

 classification, with five primary forms, viz. : (1) rods, (2) bands, 

 (3) rings, (4) balls and modified spheres, (5) eggs. Combinations of 

 these forms constitute the major part of these massive beds, and 

 frequently a bed of less than a foot thick shows examples of several 

 different combinations. These I place in ten classes, though they may 

 have to be added to. The chief types are (1) tubes, (2) ' cauliflowers,' 

 (3) basaltiform, (4) irregulai', (5 and G) troughs and bands (two 

 kinds), (7) ' floral,' (S and 9) ' honeycomb ' or coralloid (two kinds), 

 (10) pseudo-organic. 



I exhibit photographs on the screen showing both the primary 

 forms and the combinations as seen (wherever possible) in the 

 undisturbed rock sections. 



My own conclusions are as follows : — 



1. That the rod structure is secondary to the formation of the 

 conspicuous bands which run across the beds at various angles. 

 (These bands need to be distinguished from the bands mentioned 

 among the ' primary forms.') The conspicuous bands act as planes 

 of origin for the ' rods,' and do not cross through the long axes of 

 the rods themselves. They appear never to cross the bedding 

 planes, though occasionally they follow them and also the outline 

 of the joints. The question therefore arises, whether this does not 

 give us a clue to the age and sequence of the changes which have 

 occurred in these beds, and whether the previous existence of joints 

 does not mean that the beds were already above the sea-level when 

 the changes commenced. 



2. The rods invariably start from the last-mentioned bands, and 

 may be seen at every possible angle. As they have grown upwards 

 and obliquely as well as downwards the term ' stalactitic ' is a very 

 misleading one to use. As Mr. Garwood stated long ago, these beds 

 " present many points which appear irreconcilable with the theory of 

 their stalactitic origin." 



3. The first step in the series of changes which have taken 

 place was probably an orderly but unsymmetrical arrangement of 

 amorphous molecules of calcium carbonate which separated them- 



. selves from those of the carbonate of magnesia. 



4. The internal architecture is due to such arrangement of 

 amorphous particles of lime which has since been coated with an 



^ Eeud before the British Association, Sectiou C (Geology), Bradford, Sept., 1900. 



