160 MR. J. A. DOUGLAS ON CHANGES OF PHYSICAL [May 1907, 



•" In conclusion, I must thank Prof. Sollas for his kind supervision 

 and advice in the course of the preparation of this paper. 



Since writing the above, I have received a paper by A. Bygden : 

 { Ueber das quantitative Yerhaltniss zwischen Feldspath & Quarz 

 in Schriftgraniten ' Bull. Geol. Inst. Univ. Upsala, vol. vii, 1904-05 

 (1906) pp. 1-18 : this paper, after giving a short outline of Prof. 

 Yogt's theoretical conclusions on graphic granites, proceeds with an 

 account of analyses of various pegmatites, the results of which 

 seem to show that these rocks may vary considerably in the pro- 

 portion of felspar and quartz which they contain, ranging from 81*71 

 per cent, of felspar and 18*29 per cent, of quartz to 62*07 per cent, 

 of felspar and 37*93 per cent, of quartz, or in molecular proportions 

 from 1 : 1 to 1 : 2*71. The specific gravities of the rocks analysed — 

 which were taken by two methods, namely, (1) weighing in air and 

 water, (2) by the use of a ' Westphal balance ' employing mixtures 

 of bromoform and benzol as heavy fluids — seem hardly to corre- 

 spond to the mineral composition. For example, two specimens of 

 specific gravity 2*58 were found by analysis to have the com- 

 positions 1 of felspar to 1*2 of quartz, and 1 of felspar to 2*36 of 

 quartz. If, then, pegmatites be found to vary so much in com- 

 position, the production of eutectic mixtures must be controlled by 

 some, at present, unknown agency. From the presence of water 

 included in cavities in the quartz, and the analogy of the silica- 

 hydrates, it would appear that water was a dominant factor. 



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