Correspondence — Rev. A. Irving. 381 



rough calculations take the alumina as 14 per cent., and use the 

 proportions given by Nicol in his Mineralogy : then there will be in 

 anorthite ff x 14 of silica, and I--?- x 14 of lime, i. e, the amounts of 

 silica and of lime in the felspar will be nearly 16-3 and 7'6 respec- 

 tively. But the amount of CaO in the analyses is only 3-05 or 3-22. 

 Moreover, the total of the constituents in the anorthite would be 

 nearly 37; or more than a third of the rock would be felspar, which 

 is certainly far too much for any slide that I have seen. If the 

 amount be calculated from the lime, 5-5 of the alumina would be 

 needed, and 6-5 of the silica, and the felspar would be 26 per cent, 

 of the rock, — still too much, and there would be 8-5 of the alumina 

 left. In both these cases also there is not magnesia enough for the 

 remaining silica, if, as seems certain, another principal constituent 

 has been olivine. Suppose, however, the felspar be labradorite; then, 

 calculating in the same way, and supposing all the lime to be a 

 constituent of that mineral, we require 8 per cent, of the alumina, 

 leaving 6 per cent., so that the rock should be rather rich in such a 

 mineral as spinel, which it is not. In this case also the proportionate 

 amount of felspar seems considerably in excess of the amount of the 

 mineral which has been claimed. 1 have made various other trial 

 calculations from the analysis, and in no case can I obtain results 

 which seem to accord with the microscopic structure of the rock, 

 even in matters on which I believe we should be in agreement. 



I may indeed add that I have more than once found a similar 

 apparent discrepancy between the microscopic and the chemical 

 analysis of a picrite, and had reason to suspect that the alumina was 

 mainly present as the constituent of a mineral other than felspar. 

 So, notwithstanding the apparently conclusive evidence of the 

 chemical analysis, on which I frankly admit Mr. Teall is entitled to 

 claim a verdict in his favour, I still feel very strongly the difficulties 

 as to the identification of the mineral alleged in my former commu- 

 nication, and am not sure that the question is even yet decided 

 beyond all appeal. T. G. Bonney. 



THE BAGSHOT SANDS. 



Sir, — I do not think Mr. E. S. Herries (Gkol. Mag. April, 1887, 

 p. 192) has found such a 'mare's nest' as he seems to imagine. 

 The note he has quoted from vol. iv. of the Memoirs of the Geo- 

 logical Survey, of a pebble-bed somewhere near Barkham, has long 

 been familiar to me ; but I have never succeeded in finding the pit 

 to which the description would apply. Short of the identification 

 of the pit, which I have described in my paper in the Geol. Mag. 

 for March last, by the original writer of the note quoted, I cannot 

 admit its application to the case in question. If the author of that 

 note is prepared to vouch for the supposed identification, the in- 

 accuracy of the description will go far to vitiate the evidence of 

 similar notes from the same source. I leave my critics to choose 

 between the horns of this dilemma. 



In speaking of an "unmappped outlier," it was simply intended 

 to imply that the beds under consideration liad not been mapped out 



