Thomson. — (hohxjn of Middle (Jlarence and. Ure Vallei/.<. 331 



under the limestone in the Herring River area throws new light on the 

 problem. 



In 1917 Park suggested that the fossils foujid by Speight and myself 

 in the Trelissick Basin, and earlier reports by von Haast and himself of 

 Recent brachiopods from the " concretionary greensands "' in the Waipara 

 district, proved that the latter were uiiconformal^le to tlie Senonian 

 " saurian beds," and that they, with the overlying Amuri limestone, were 

 Oamaruian. In the same year I showed that the top of the " concretionary 

 greensands "' was Senonian, and that if any disconformity exists — for there 

 is no unconformity of the beds — it should be looked for above the " con- 

 cretionary greensands " in the dark carbonaceous mudstone into which the 

 Amuri limestone passes down. Probably a collection of Foraminifera 

 could be made from this mudstone which might settle its age if a large 

 enough fauna were found. With regard to the supposed Wahlheiniia letdi- 

 cularis from the "" concretionary greensand," unfortunately the specimens 

 do not appear to be preserved, and I have searched this horizon in vai)i 

 for further specimens. No reliance is to be placed on the identification, 

 for determinations of brachiopod species even by Hutton as late as 1904 

 were very unreliable, and Neothyris lenticidaris does not occur fossil below 

 the \Vanga)niian, and even there it is not at all common. The only Neofhijris 

 species known to me from the Oamaraian is Neotht/ris novara (von Ihering). 



The problem of the age of the Amuri limestone is closely bound up 

 with that of its mode of deposition. In 1916 I pointed out that the 

 hard, chalky type consists pi-edominantly of exceedingly fine-grained calcite, 

 in which matrix isolated but unbroken tests of Foraminifera, chiefly 

 Globigerina, occur sporadically, but skeletons of siliceous organisms appear 

 to be absent. That they were formerly present but have disappeared 

 by solution seemed improbable, since there are no spaces of dissolution 

 such as exist in the English Chalk ; and although the pressures to which 

 the limestone has almost everywhere been subjected might be considered 

 capable of closing such spaces, this could hardly have happened without 

 the crushing of the delicate tests of Globigerina. Although the fact that 

 the flint-beds are thickest where the limestone is thickest would find an 

 easy explanation were the flint the result of a downward migration of silica 

 derived from siliceous organisms in the overh'ing limestone, the absence 

 of casts of these organisms seemed to prevent the acceptance of such 

 hypothesis, and I put forward the suggestion that the silica of the flint 

 and also the dolomite frec|uently found with it were chemically precipitated 

 and that the limestone itself was in large part a chemical deposit, but in 

 part composed of foraminiferal tests settling gently into the chemically 

 formed calcareous mud. At the same time I stated, " Much of the above 

 is speculative, but will serve a useful purpose if it calls attention to the 

 peculiar nature of the Amuri limestone, the flint-beds, and the sulphur 

 sands, and proA'okes an alternative explanation." This hope has been 

 partially realized by the observations published by Marshall (1916, 1917) 

 and by Speight and Wild (1918) on the presence and origin of the flint, 

 but as none of these writers appears to have considered the possibility of 

 chemical precipitation of any of the silica or carbonates of the limestone, 

 and as none of them has discussed the subject from the point of view of 

 the whole problem, it seems desirable to restate it in the light of the 

 evidence now obtainable. 



Marshall (1916) agrees with me that by far the greater part of the 

 Amuri limestone over the whole of North Canterbury and Marlborough 



