Thomson. — Geology of Middle Clarence and. Vie Valleys. 335 



within it disconforniities or planes corresponding to periods of non- 

 deposition. The junction between the limestone and the phospliatic green- 

 sand at the base of the Weka Pass stone in North Canterbury and South 

 Marlborough appears to represent such a plane, since, although there is 

 perfect parallelism of dip and strike of the two rocks in all exposures, 

 the surface of tlie Amuri limestone is perforated by borings filled with 

 glauconitic material and the loosened fragments of limestone lying at the 

 base of the greensand are phosphatized. 



Barrell (1918) has shown that in epicontinental beds, such as form the 

 greater part of the geological record in North America, disconformities 

 must be much commoner than was formerly supposed. These beds, how- 

 ever, were deposited in shallow epicontinental seas not often miore than 

 a hundred fathoms in depth. His conclusions cannot be made to apply 

 to rocks such as compose the bulk of the Notocene in New Zealand, which 

 have been deposited on a continental shelf fringing a deep ocean, and, for 

 the greater part, on the outer slope of this shelf. Here, as Cotton (1918) 

 has shown, when foreset beds fail through insufficiencv of waste to com- 

 pensate subsidence, or for other reasons, the deposition of pelagic beds 

 immediately commences. It is difficult to imagine conditions which can 

 bring about a disconformity between pelagic and foreset beds, such as 

 exists at the above-mentioni?d junction. If the pelagic beds are in large 

 part a chemical deposit, however, it is not difficult to imagine a change 

 of conditions which would put a stop to chemical deposition, and the 

 formation of purely organic ooze might be so slow as to allow time for tlic 

 boring of the last-formed bed and the phcispliatization of its u})per surface 

 before the deposition of the foreset greensand began. 



The absence of other phospliatic layers in the Amuri limestone makes 

 it probable that other similar disconformities do not exist, and the enor- 

 mously greater thickness of the formation in the Mead area than of that 

 at Amuri Bluff and farther south proves without any doubt that in the 

 former area it must have taken much longer to accumulate. If, however, 

 the limestone is in large part a chemical deposit, and if chemical deposition 

 was more active in forming the lower part than the upper part, the time 

 taken to form the greater thickness may not have been much more than 

 twice that taken for the lesser thickness, although the ratio of the thicknesses 

 is as four to one. The fact that in both areas the limestone is covered by 

 the phosphatic greensands and the Weka Pass stone makes it probable 

 that the top of the limestone is of the same age in both areas. Consequently 

 the base of the limestone must be much older in the Mead area than it is 

 at Amuri Bluff. 



To believe with Woods (1917) that the Amuri limestone of the Mead- 

 Coverham area is disconformable to the Clarentian and that it is Eocene 

 places a much greater strain on the imagination than to believe that it is 

 conformable and ranges in age from the top of the Clarentian to the base of 

 the Oamaruian. It would mean that the Clarentian beds were emergent with- 

 out tilting all through the Cenomanian, Turonian, Senonian, and Danian, and 

 yet suffered no subaerial erosion such as would truncate their structures 

 or dissect their surface, and that in the succeeding Eocene transgression so 

 rapid was depression that there was no marine erosion or deposition of clastic 

 beds, but instead the immediate formation of pelagic beds. It implies 

 also that a similar succession of events happened at Anuiri Bluff, except 

 that emergence lasted only throughout the Danian and lower Eocene ; but 

 the depression, which was large and sudden, must have been much later 



