Correspondence — F. R. G. Reed. 191 



On p. 379 of my paper on the Gunong Bakau rocks I pointed 

 out that without segregation one could not expect to have a rock 

 very rich in topaz. In a pure orthoclase magma the 18*4 per cent 

 of alumina could only produce 32*6 per cent of topaz if attacked by 

 fluorine unaccompanied by more alumina. Dr. Jones produces 

 evidence to show that alumina was introduced into some greisens. 1 

 I do not know how the rock-sampling was carried out in the cases 

 quoted, nor on how many analyses the results are based ; but I do 

 not wish to question the increase of alumina in any of the altered 

 rocks in the table on p. 260 as compared with the unaltered 

 granite. The greatest increase is 2 , 09 per cent, which, added to the 

 alumina of a pure orthoclase rock, gives a possible 36-3 per cent of 

 topaz, which is still very far short of 90 per cent, and we are not 

 dealing with pure orthoclase rocks. There can be no question that 

 topaz does occur as an original rock constituent. The Meldon 

 aplite, for instance, has been described anew recently, 3 in which 

 topaz is associated with lepidolite, tourmaline, and fluorspar, among 

 other minerals. There is no doubt in my mind that it occurs also as 

 a pneumatolytic alteration product. Each case must be decided on 

 the local evidence. 



J. B. Sceivenor. 



YUNNAN CYSTIDEA. 



Sie, — A few comments are necessary on Dr. Bather's letter in the 

 March number of this Magazine (p. 143) in reply to my remarks on 

 his articles on Yunnan Cystidea. Especially is this the case with 

 regard to the diplopores in Sinocystis. Firstly, it must be borne in 

 mind that the figured specimens which were lent to him for a short 

 time for the purpose of making casts for the British Museum 

 constitute the only material on which he can base his conclusions, 

 while I had three times the number of specimens for study for two 

 years. Secondly, it has not been mentioned that these figured 

 specimens before being drawn or sent to him had been cleaned under 

 my eyes with a weak acid solution, which I then observed attacked 

 and partially dissolved a few of the tubercles, so as to remove the 

 thin covering layer of epistereom in some cases and thus expose the 

 pores. Thirdly, the other specimens of Sinocystis, numbering over 

 twenty, which Dr. Bather never saw, were examined by me as they 

 came fresh from their limestone matrix, unaffected by weathering, 

 untouched by any solvent, and often only partly exposed. These 

 did not show any pores on the hundreds of tubercles which 

 I scrutinized, except where the tubercles were obviously injured. 

 Fourthly, his statement that on removing a piece of the matrix 

 from one of my figured specimens there was disclosed a tubercle 

 exhibiting the minute pores completely confirms my experience that 

 there is extreme difficulty in getting rid of the closely adherent 

 matrix without damaging the surface, and thus his discovery is of 



1 Op. cit., pp. 259-60. 



2 Geol. Mag. 1919, pp. 41-2. 



