Barteum. — Recent Sediments of Auckland Harbour. 427 



limestones,* and without doubt it has led to the oft-made statements, such 

 as that quoted above from Chamberlin and Salisbury, to the effect that 

 many concretions are contemporaneous in origin with the beds enclosing 

 them. 



The writer has been unable to find any description of concretions 

 mentioned by Chapman as occurring in muds at Melbourne, f but imagines 

 that they are comparable with those of the Auckland Harbour. 



Cause of the Precipitation of the Calcium Carbonate. 



The cause of the precipitation of the calcium carbonate in the Auckland 

 Harbour concretions is often uncertain, for many of them lack definite 

 nuclei. Some pencil-like ones probably formed around a decaying twig or 

 some such organic remnant, when the organic compounds liberated induced 

 precipitation of the carbonate. The commonest nuclei are shells of the 

 mollusc Atrina zealandica and of a small crab ; the skeletal parts are almost 

 invariably filled by the material of the concretion, and this fact is perhaps 

 evidence that the soft parts had disappeared before the shells came to rest. 

 Probably precipitation was initiated by the decomposition of the organic 

 matter in the epidermis of molluscs such as Atrina and the hard parts of 

 the crabs. This seems the more probable since molluscs lacking a horny 

 epidermis are very abundant in the harbour sands and yet seldom, if ever, 

 form the true nuclei of the nodules described. 



Summary and Conclusion. 



The writer has described certain calcareous concretions forming in 

 present-dav sands and other silts of the shallow, sheltered waters of Auckland 

 Harbour (N.Z.). and contemporaneous with these sediments. He considers 

 that they result largely from direct precipitation of calcium carbonate from 

 sea-water. He wishes to bring forward this example of such nodules in order 

 to supplement the somewhat vague available information about concretions 

 which have formed as primary component parts of a stratum. 



In conclusion, the writer wishes to 'thank Mr. M. Ongley, New Zealand 

 Geological Survey, for considerable help in looking up literature. 



Postscript. 



Through the courtesy of the members of the Geological Section of the 

 Wellington Philosophical Society, the above paper was discussed at the 

 June meeting (20th June, 1917) of the section, and several very relevant 

 criticisms and suggestions were put forward. J 



It was suggested, inter alia, that (i) the concretions may have been 

 dredged from the underlying mid-Tertiary Waitemata standstones ; (ii) the 

 evidence for the Recent age of the concretion-bearing sands and silts set 

 forth in the paper was inconclusive. 



Mr. Hamer, Engineer to the Auckland Harbour Board, has very kindly 

 allowed the writer to see the mapped records of many scores of bores, and 



* Cf. A. J. Jukes Irowne, Concretions in Magnesian Limestones, Geol. Mag. (n.s\ 

 dec. iii. vol. 8, 1891, p. 528. 



t F. Chapman, On Concretionary Nodules with Plant-remains in the Old Bed of 

 the Yarra at South Melbourne, Geol. Mag. (n.s.), dec. v, vol. 3, 1906, pp. 553-56. 



% Communicated to the writer by Dr. C. A. Cottcn, Victoria University College. 



