498 Mi.^s 31. B. Chapman— 



Cretaceous and older formations. In the Waipara district, where 

 the terrestrial heds of the Oamaruian are absent, the physical break 

 between the Cretaceous and Tertiary is not well marked, but the 

 ])al8eontological break is complete. In all other parts of New 

 Zealand wherever the terrestrial beds of the Oamaruian are present, 

 the stratigraphical break between the Cretaceous and Tertiary is as 

 conspicuous as the palaeontological. 



IV. — The Chemical ExAMI^fATION■ of the Cakboniferous Limestones 

 OF THE Avon Gorge. 



By Mildred B. Chapman, B.Sc.D.I.C. 



IN view of the attention which has been drawn of recent years to 

 the mode of origin of the purer limestones, 1 have been led 

 to prosecute some inquiries under this head on the Carboniferous 

 Limestones of the Avon Gorge, near Bristol. I have paid greatest 

 attention to the chemical composition of the rocks, particularly tlie 

 proportion of insoluble residue remaining after treatment with hydro- 

 chloric acid ; at the same time I have not wholly neglected the fossil 

 contents, although for information on this point I have mainly relied 

 on other observers. 



The method of analysis which I adopted was similar to that used 

 by Professor Skeats in his investigations on the Dolomites of the 

 Tyrol, so that the results can easily be comjiared. Briefly it was as 

 follows : — 



The finely powdered limestone, after solution in strong hydrochloric acid and 

 evaporation to dryness, was again moistened with the acid and evaporated. 

 The mass was lixiviated with water containing a few drops of hydrochloric acid 

 and the weight of residue determined. This residue was examined under the 

 microscope, but I have not attempted to give a detailed description, as this has 

 already been done by Mr. E. Wethered (Q.J.G.S., 1888). Iron and aluminium 

 hydroxides were precipitated from the solution by means of ammonia, and the 

 content of iron oxide and alumina determined as a whole. 



The percentage of lime was ascertained by titration with potassium 

 permanganate after precipitation by means of ammonium oxalate. Magnesia 

 was weighed as magnesium pyrophosphate, obtained by precipitation with 

 sodium phosphate and ammonia. 



A separate estimation was made of the carbon dioxide in the usual manner, 

 using soda lime for absorption. 



Each specimen was also carefully tested for the presence of phosphoric acid. 



Except wheu otherwise stated the figures given are the average of 

 several analyses. 



Sections of the same specimens of the rocks were examined micro- 

 scopically and were treated with Lemherg's solution to distinguish 

 the dolomite if present. 



Specimens were collected throughout the Avon Gorge section 

 except from the shale and argillaceous bands, which were neglected, 

 as they do not coiue within tlie scope of this paper. Special care was 

 taken to obtain samples from all marked coral bands. 



The classitication of the beds is that laid down by Di'. Yanghan 

 (Q.J.G.S., 1905). 



