426 Transactions. 



Examination with the microscope shows that the calcareous material 

 of the concretions was deposited as a microgranular aggregate, which is so 

 fine in grain that it often has infilled minute diatom capsules which are 

 even yet arranged in their characteristic bead-like strings. The included 

 quartz-grains are somewhat sparsely but uniformly scattered throughout 

 the calcareous matrix, and the only direct evidence of their having been 

 outcrowded during growth of the concretion is that the somewhat incoherent 

 surface zones are crowded with sand-grains, which, however, can equally 

 well be accounted for by mechanical intermixture during pumping opera- 

 tions. Concentric structures and bedding-planes are absent. 



It is evident from the facts already stated — the largely uncarbonized 

 nature of many included wood fragments and the preservation of the natural 

 colours of the shells in particular' — that these concretions not only are 

 contemporaneous with the beds in which they exist, but, further, have 

 been formed at no very distant date. 



Origin op the Concretions. 



It is agreed by practically all geologists that the manganese nodules 

 of deep-sea deposits, the phosphatic concretions near shore, and the minute 

 pisolites and oolites of many limestones, with many other similar chemic- 

 ally precipitated accretions, are contemporaneous in origin with the beds 

 in which they lie. With respect to numerous accretionary nodules of 

 other types there is less agreement, and the criteria of primary origin set 

 forth by one writer may be construed by another as criteria of secondary 

 origin. For example, the passage of bedding-planes through a concretion 

 is considered by Geikie* to be evidence of contemporaneous origin, but by 

 Chamberlin and Salisbury} - to indicate secondary origin. The last-named 

 authorities indicate the general lack of decision on this point in the following 

 statement : " Some concretions probably form during the accumulation of 

 the beds in which they lie. "J 



Because such indecision is shown by many authors in dealing with 

 concretions of this type the writer feels justified in emphasizing the 

 indubitably contemporaneous origin of those described herein. There 

 is no possibility that they have come by virtue of wave-action from the 

 soft mid-Tertiary sandstones forming the prominent cliffs of the shores of 

 Auckland Harbour, for these beds contain very few molluscan fossils, and, 

 further, the concretions themselves have not been subjected to any erosive 

 action. 



The large quantity of calcium carbonate present in the matrix of the 

 nodules cannot have been derived from the inferior amount constituting 

 the included shells. Percolation cannot be effective under the conditions 

 obtaining in this instance, so that the calcareous material must have been 

 precipitated directly from sea-water by much the same process as that 

 explaining the formation of the " coal-balls "§ of the English Coal Measures 

 and similar accretionary bodies. 



This long-established principle of direct precipitation has been suggested 

 by one or two writers in explanation of the concretions in many magnesian 



* Sir A. Geikie, Text-book of Geology, vol. 1, 4th ed., 1903, p. 647. 

 t T. C. Chamberlin and R. D. Salisbury, Geology, vol. 1, 2nd ed., 1909, p. 496. 

 ± Lor. cit., p. 439. 



§ M- C. Stopes and D. M. Watson. Phil. Trans. Roy. Soc. Loud., ser. B, vol. 200,. 

 1997, pp. 167-218. 



