790 Geological Society : — 



the above form, the quantity directly affected when the dis- 

 turbance is slow, is the reciprocal of the spring, which in 

 general undergoes (i.) a change in its mean value proportional 

 to the pressure, and (ii.) a periodic variation : the latter gives 

 an increase in frequency, the former either an increase or 

 diminution. Thus we can assert only that there is a change 

 of frequency proportional to the pressure. 



Reductions proportional to the pressure have been observed 

 in the case of iron vapour *, but the hypothesis does not 

 appear of marked value in this instance owing to the absence 

 of specially selective effects. Evidently the statical influence 

 of the pressure is predominant. 



In the case of sodium many resonance series appear to 

 have the same frequency difference. From the present 

 standpoint it seems probable, therefore, that the vapour 

 would strongly absorb long waves of this frequency, v/2ir, 

 and show selective refraction in its neighbourhood. Such 

 absorption would be accompanied by displacements of the 

 series towards the violet end of the spec.rum under the 

 influence of the intensified periodic variation in the reciprocal 

 of the spring. 



It is hardly necessary to point out that slow normal 

 oscillations within the vibrating system are sufficient to 

 account for the complexity of linear spectra, and the varia- 

 tions in relative brightness at different pressures. 



July 1910. 



LXXXVIII. Proceedings of Learned Societies. 

 GEOLOGICAL SOCIETY. 

 [Continued from p. 544.] 



February 23rd, 1910.— Prof. W. W. Watts, Sc.D., M.Sc, F.R.S., 



President, in the Chair. 



^HE following communication was read : — 



' Metamorphism around the Ross of Mull Granite.' By 

 Thomas Owen Bosworth, B.A., B.Sc, F.G.S. 



The Ross of Mull granite is a coarsely crystalline plutonic mass, 

 forming the western portion of the Ross of Mull and extending 

 over some 20 square miles. 



The intrusion is conspicuously later than the Moine rocks, and 



* W. G. Duffield, " On the Effect of Pressure on the Arc Spectrum of 

 Iron," Phil. Trans. 1907. 



