504 MR. M. ODLING OX THE [Oct. I913, 



i 



and oolitic grains arranged with their long axes parallel to the 

 walls of the tube, so as to present a smooth surface internally ; the 

 fragments all show evidence of having been rolled and coated, so as 

 to form ooliths with one thin coat ; typical ooliths also occur. 



Towards the base of the tubes the fragments are arranged in 

 such a manner as still to present a smooth surface internally ; 

 that is, the fragments forming the extreme base of the tube are 

 arranged with their long axes at right-angles to those that form 

 the sides. 



These tubes are very numerous in the Chipping-Xorton Lime- 

 stone of the Ardley section, and an apparently similar structure 

 occurs in the Great Oolite of the Portland-Cement Works Quarry. 



It is impossible to state definitely how these tubes were formed ; 

 but they bear a certain resemblance to the tubes constructed by 

 such varieties of annelids as Terebella and Sabella, so abundant 

 on recent sea-shores. 



Y. Chemical Composition. 



The chemical examination of the rocks has in certain instances 

 greatly facilitated the microscopic examination. In cases where 

 the matrix is more or less opaque, it is impossible to distinguish 

 between clay and amorphous carbonate of lime. The ' Cream- 

 Cheese Bed,' for instance, in thin section appears to be extremely 

 marly ; but the analysis has shown that 93*5 per cent, is composed 

 of carbonates of calcium and magnesium. 



The proportion of magnesium carbonate was found to be so 

 small (usually less than 1 per cent.), that after the first thirty 

 analyses only the insoluble residue, iron, and alumina were calcu- 

 lated ; it was found that the iron and alumina, which were esti- 

 mated together, were always in proportion to the amount of 

 insoluble residue if it consisted of clay, but not if it consisted of 

 quartz-grains. The most interesting results are obtained by con- 

 sidering the insoluble residue, iron, and alumina together. When 

 this was done, it was ascertained that the Cornbrash from two 

 exposures, namely : Islip and the Portland-Cement Works, has 

 approximately the same composition, containing the following per- 

 centages of clay, iron, and alumina : — 



Cornbrash, Islip 8'53 



Cornbrash, Portland-Cement Works 7 - 927 



In the Porest Marble the sequence in different parts of the 

 same quarry is so variable, that no result of any value could be 

 expected, even from numerous analyses. 



The marly bands from the Upper Green-Hill Quarry, namely, 

 C and D 3 , yielded the following residue, iron and alumina, which 

 was almost entirely clay. 



Per cent. 



C, 51-24 



D 3 21-01 



