334 Transactions. 



not more than might be expected if it represents a downward migration 

 and accumulation at the base of all the magnesium carbonate originally 

 present in the whole mass of limestone, but it creates a considerable 

 difficulty if the flint-beds are considered as siliceous oozes of organic origin 

 reprecipitated i/( s?<i<. . «.r :^^^^s.%, Si^ 



(3.) Its lack of fossils. Although the Glohigerina oozes of the present 

 oceans do not afford a suitable bottom for some forms of benthic life — 

 e.g., brachiopods — 3^et they are by no means devoid of other forms. 

 Moreover, the European Chalk, which presumably represents a pelagic 

 deposit laid down in not very deep water on the continental slopes, is not 

 conspicuously poor in fossils but can be zoned by means of them. The 

 Amuri limestone, on the other hand, is practically without any fossils of 

 benthic organisms from bottom to top, and this can hardly be due to 

 their destruction after consolidation, since the tests of Glohigerina and 

 also sharks' teeth have endured. This means, then, that benthic life was 

 practically absent during its accumulation. The presence of bands of 

 alternations of marly and chalky limestone proves that the limestone is 

 not an abyssal deposit, but was laid down on the slopes of the continental 

 shelf. The absence of benthic life is in accord with a hypotliesis of the 

 precipitation of a calcareous mud by the presence of solutions in the 

 bottom waters inimical to animal-life. It is not necessary to postulate 

 the presence of sulphur bacteria to produce such solutions, for it has 

 recently been shown that albuminous bacteria can produce similar effects. 



There is reason to believe that the formation of the flint took place soon 

 after dej^osition, and before the beds were folded and jointed. The flints 

 are themselves much jointed, and, moreover, in the overturned syncline 

 between the upper Ure and Coverham the flint-beds overlie the limestone 

 in the reversed upper limb and underlie it in the lower limb. Supposing 

 the flint-beds to result from a downward migration of silica, it is very 

 doubtful whether silica-bearing or other solutions could have passed freely 

 through the marly limestones before these were jointed. It is noteworthy 

 that the flint-beds are always associated with massive chalky limestones, 

 and where these fail, as in the Herring River, there are no flint-beds. If, 

 then, the alternations of chalky and marly limestone imposed an obstacle 

 to the downward migration of silica, the flint-beds in the Mead section 

 must result from the redeposition of the silica of only the lower 1 ,500 ft. 

 of limestone, of which they compose the greater part. The amount of silica 

 in these lower beds is so great that it could not be explained, apart from 

 the chemical theory, without concluding that these beds were originally 

 in large part radiolarian or diatomaceous oozes. Why such oozes should 

 be so completely altered as to retain no evidence of a former organic 

 origin is a problem which must be answered b}' the opponents of the 

 chemical theor}-. 



The above reasons seem to me to establish a prima facie case for the 

 chemical precipitation of calcium carbonate during the deposition of the 

 Amuri limestone. If this took place it is not difficult to believe that there 

 was deposition also of magnesium carbonate or of dolomite in a similar 

 manner. The chemical deposition of silica presents a greater difficulty 

 to the imagination, and I do not consider that the evidence for this is so 

 strong, althougli, as I have shown above,, there are also difficulties in 

 accepting any other explanation. 



In seeking to infer the age of the Amuri limestone, the question arises 

 whether it is a continuously-formed deposit or whether there may be 



