Bartrum. — Recent Sediments of Auckland Harbour. 425 



Art. XXXIII. — Concretions in the Recent Sediments oj the Auckland 



Harbour, New Zealand. 



By J. A. Bartrum, Auckland University College. 



[Read before the Auckland Institute, 13th December, 1916; received by Editors, 

 ■30th December, 1916 ; issued separately, 30th November, 1917.] 



Plate XXIX. 



During dredging operations recently carried out in St. George's Bay, 

 Auckland Harbour, a considerable number of concretions were brought 

 up, along with fine sands and numerous molluscan shells, from a depth of 

 about 28 ft. to 35 ft. below mean high-water level. 



It is no new discovery that concretions can form contemporaneously 

 with the beds in which they are found : deep-sea manganese oolitic grains 

 and nodules are classical examples. It is, however, seldom that one finds 

 definite mention in literature of contemporaneous concretions of the type 

 to which these at Auckland belong. 



Description of the Concretions. 



The majority of the concretions are irregular nodular masses varying in 

 size from | in. up to 6 in. or more in diameter. (See Plate XXIX, fig. 1.) 

 Their material is very hard and compact limestone (about 70 per cent, 

 calcium carbonate) enclosing numerous angular grains of quartz, a few of 

 glauconite, numerous specks of carbonaceous matter, small fragments of 

 partially carbonized wood, and, as a rule, numbers of Recent fossils— diatoms, 

 a few foraminifera, tmall crabs, and molluscs. In some concretions there 

 are comminuted fragments of the more delicate shells, but the stronger 

 shells are intact. Small crabs are exceedingly common nuclei of small 

 rounded concretions, such as the smallest of Plate XXIX, fig. 1, and the 

 mollusc Atrina zealamlica frequently the nucleus of the larger ones. (See 

 Plate XXIX, fig. 2.) It shows its peculiar tooth-like ornamentation un- 

 impaired, and the prisms of its outer calcareous layer are still uncemented. 

 Few of the other molluscs found in the concretions can be regarded as 

 nuclei, as the photograph (Plate XXIX, fig. 3) shows well. 



No attempt has been made to list the shells found in the nodules, but all 

 belong, so far as the writer is able to determine them, to Recent species. 

 Many, such as Cominella maculosa and Fxdgoraria gracilis (depicted in 

 Plate XXIX, fig. 3), show unbleached natural colours. Amphibola crenata 

 (Plate XXIX, fig. 3), various species of Turritella (Plate XXIX, fig. 1), 

 and small Pecten valves are frequent. 



In a few of the larger concretions some very irregular drusy cavities 

 up to \ in. in diameter are present. They are arranged without any relation 

 to the boundaries of the concretion, or its nucleus. Their irregularity pro- 

 hibits their being considered hollow casts of some since-removed organism ; 

 nor do they show the characters of shrinkage cracks, although this last 

 explanation seems to be the only feasible one. They cannot be regarded 

 as solution cavities, because there is ample evidence that the harbour 

 deposits containing the concretions have not been subjected to any change 

 of conditions such as would be necessary to permit percolation and solution 

 to be effective. 



