Speight and Wild. — Weka Pass Stone and Amuri Limestone. 71 



which have an origin quite distinct from the true phosphatic nodules referred 

 to previously. The nodular limestone, though easily recognized in the hand- 

 specimen, differs little from the true nodules under the microscope, except 

 that it is less glauconitic and approaches very closely to normal Amuri 

 limestone. There can be no doubt that for a considerable. period the lime- 

 stone formed the ocean-floor (as is indicated by the phosphatic nodules), 

 and that it was honeycombed by the borings and burrows of marine 

 organisms operating at that depth, and that the additional phosphatic 

 material was obtained from the ordinary limestone by a process of concen- 

 tration, and from remains of those organisms responsible for the burrows. 

 It is quite intelligible that during a period of halt in the deposition the 

 solvent action of sea-water would cause a disappearance of a portion of the 

 floor, and, as the phosphatic material is less soluble than the calcareous, 

 some concentration of the phosphate would result. 



This idea finds strong support in the following partial analyses of speci- 

 mens obtained at Weka Pass. At this particular section the Amuri lime- 

 stone is seen to be perforated to a depth of 4 ft. 6 in., the cavities being 

 filled with the calcareous greensand that represents the overlying Weka Pass 

 stone at this locality. The upper 18 in. of the Amuri limestone is much 

 honeycombed with burrows, and completely detached fragments are to be 

 found lying within the Weka Pass stone as much as 6 in. above the present 

 surface of the Amuri limestone. It is to be understood that we look upon 

 these nodular fragments as remnants of the original upper portion of the 

 Amuri limestone which, during a halt in the deposition, was broken down 

 by the combined action of boring-animals and solution by sea-water, some 

 at least of the phosphate so set free being concentrated in the residual 

 portions of limestone. 



Table IV. 



(1.) 

 Insoluble in acid . . . . 12-08 



CaO .. .. 48-65 



P 2 5 0-16 



(1.) Sample 2 ft. from present surface of Amuri limestone. 



(2.) Sample from upper 6 in. of honeycombed portion of Amuri lime- 

 stone. 



(3.) Detached nodules of Amuri limestone lying in the Weka Pass 

 stone a few inches above the present surface of the Amuri 

 limestone. 



(4.) Lower 2 ft. of Weka Pass stone. 



In both types of nodules fehere is little difference from the associated 

 limestone in the character of the Foraminifera and general structure of the 

 rock, and they seem to have been formed under similar conditions. The 

 description applies to specimens from Amuri Bluff and Stonyhurst equally 

 with those from Weka Pass. 



Under the microscope the material that fills the borings appears to 

 be composed of much the same material as the associated limestone, and 

 resembles in texture the Amuri limestone rather than the Weka Pass stone. 

 There is, however, more granular glauconite, and there are more numerous 

 shreds of mica and fragments of quartz. The glauconite does not fill the 

 cavities in the Foraminifera so markedly, though undoubtedly some are 

 filled. The genera of Foraminifera appear to be the same as in the Amuri 

 limestone, Globigerina, Nodosaria, and Rotalia being clearly recognizable. 



