Vol. 6$.~] CONSTANTS IN MINERALS AND IGNEOUS ROCKS. 161 



10. Soxlas, W. J. ' A Process for the Mineral-Analysis of Rocks ' Quart. Journ. 



Geol. Soc. vol. lviii (1902) pp. 163-76. 



11. Teall, J. J. H. 'British Petrograplvy ' 1888, chapt. xiii ; and 'Presidential 



Address' Quart. Journ. Geol. Soc. vol. lvii (1901) pp. lxx et seqq. 



12. Vogt, J. H. L. 



(i) ' Die Silikatschmelzlosungen ' Vidensk. Selsk. Skrifter, No. 8 (1903) pt. i. 

 (ii) ' Physikalisch-Chemische Gesetze dei 1 Krystallisationsfolge in Eruptiv- 

 gesteinen ' Tscherm. Min. & Petr. Mitth. vol. xxiv (1905) p. 437. 



Rock-analyses (references) : — (i) Quart. Journ. Geol. Soc. vol. xlvii (1891) p. 276; 

 (ii) J. Peestwich, 'Geology' vol. i (1886) p. 41; (iii, iv) W. J. Solias, Quart. 

 Journ. Geol. Soc. vol. lviii (1902) pp. 163-76 ; (v-xii) F. H. Hatch, ' Textbook of 

 Petrology ' 2nd ed. (1892). 



Discussion. 



The Chairman (Dr. A. Strahan) commented on the skill with 

 which these difficult experiments had heen carried out. The 

 increase in volume undergone by some of the rocks on passing into 

 the glassy condition came to him as a surprise. 



Mr. J. Y. Elsden called attention to the Author's determination 

 of the melting-points of various glasses. He believed that it was 

 generally admitted that, from a physical point of view, glasses had no 

 definite melting-point. They behaved as liquids of high viscosity. 

 He would suggest, therefore, that the Author's figures represented, 

 not melting-points, but rather the relative rates of diminution of 

 viscosity. 



Prof. "W. W. Watts congratulated the Society on being the 

 recipient of an exceedingly-important contribution to experimental 

 geology. He called especial attention to the Author's results with 

 regard to the expansion of certain acid rocks, which expanded as 

 much as 10 per cent, in passing into the glassy condition. The 

 still greater expansion in liquefying would provide an explanation 

 of the enormous force developed in volcanic eruptions. 



Prof. Sollas remarked that the extreme lucidity with which the 

 Author had communicated his results fittingly corresponded with 

 the precision which marked his work as an experimenter. The 

 specific gravity of a rock afforded a most important indication as to 

 its nature ; but hitherto full advantage could not be taken of this, 

 owing to the fact that rocks were not all in the same physical state, 

 some being glassy, some crystalline, and some both crystalline and 

 glassy. To render specific gravity a test of general application, all 

 rocks must be reduced to a common state ; and, since a universal 

 crystalline state was not attainable, recourse must be had to the 

 glassy. The Author's method furnished a simple and accurate 

 means of accomplishing this, and should prove of great practical 

 value, especially in determining the affinities of volcanic rocks. For 

 this reason the reinstatement of Prof. Joly's meldometer as an instru- 

 ment of precise research was especially welcome. The results of 

 the Author, and those obtained by Messrs. Day & Allen with much 

 more expensive apparatus, were in mutual confirmation. 



The Author thanked the Fellows for their kind reception of his 

 paper. 



