DISCUSSION 635 



formed by the rolling of mud balls down cla-y slopes, with the result that 

 they carry on their surfaces a sort of WTapping of clay layers about the 

 central parts. 



Author's reply: Professor Quirke's observations on marlekor have 

 been made in an area somewhat remote from the one in which my own 

 studies were made, and it is possible that some of the specimens referred 

 to by him may have a different genetic history from that of the marlekor 

 of the Ottawa district. I am not prepared to explain just how the fis- 

 sures in the interior of the mmielcor originated, but the clay pebbles 

 which may be seen in abundance along the Ottawa Eiver in late summer 

 certainly show no such features; neither do any of the mud balls wdiich 

 have come under my notice along the Bay of Fmidy estuaries possess any 

 features which suggest in any way a comparable origin for the marlekor. 

 The best evidence, perhaps, that the marlekor were produced by the same 

 general agencies which are responsible for the claystones occurring at a 

 lower level in the clays, is found in the fact that, like them, they are 

 confined to that portion of the clays which lies within a few feet of the 

 surface. 



Bibliography 



George Abbott : Water zones ; their influence on tlie situation and growth of 

 concretions. Report of the British Association for the Advancement 

 of Science, 1899, page 741. 



Tubular and concentric concretions. Report of the British Association 

 for the Advancement of Science, 1900, pages 741-742. 



Concretionary cellular limestone of Durham. Naturalist (London), 1905, 

 pages 231-233. 



Concretions. S. E. Naturalist (England), 1907, pages 67-76, plates ix-xviii. 



Notes on concretions. Proceedings of the Geological Association, volume 

 XXVII, 1916, pages 192-197. 



Tubular structures in rocks which are probably due to osmetic action. 

 Transactions of the S. E. Union of Scientific Societies (England), 

 1916, page 20. 



A collection of concretions at Tunbridge Wells (privately printed), Sep- 

 tember, 1918, pages 1-3. 

 C. Baker Adams : Second Annual Report on the Geology of the State of Ver- 

 mont. Concretions, pages 111-118; Clay deposits, etc., pages 140-142; 

 Analysis, page 255 ; 1846. 

 Professor Andra : Phosphatic concretions from Waldbockelheim. Verb. nat. 



Ver. preuss. Rheinl., Jahrg. XXXIII, Sitz., 1876, page 121. 

 '3'hos. Andrews : On some curious concretion balls derived from a colliery 

 mineral water. Chemical News, volume XL, 1879, pages 103-104. The 

 concretions were found in the feed-tank of boilers at the Worthley 

 Silkstone Colliery. They contained 62.86 per cent of peroxide of iron. 



