100 DS. J. W. EVANS ON THE EOCKS OE THE [Feb. 1906, 



The rock is a pale granitoid or haplite-gneiss, in which the 

 ferromagnesian minerals are but little developed. 1 In some places 

 it is fairly coarse-grained, and in pegmatoid veins the constituents 

 measure as much as 2 centimetres in diameter. 



On examination of a microscope- section (M4«) from one of 

 these coarser portions, it appears to be mainly composed of irregular 

 masses of quartz, microcline, and orthoclase. Sometimes the 

 border of the quartz sends out a number of rounded prominences 

 in crystalline continuity, which penetrate the felspar micro- 

 graphically. The felspar is altered in places into a colourless 

 mineral with a higher refractive index, and a negative sign of the 

 principal zone. The relative retardation of the two directions of 

 vibration is higher than in quartz, the birefringence reaching a 

 maximum of about 0*029, or 29 thousandths. 2 This is probably a 

 somewhat hydrous muscovite ; the cleavage is, however, not well 

 marked. Plagioclase intermediate between albite and oligoclase is 

 also present. 



Elsewhere the rock (M 4) shows megascopically a fine granular 

 mixture of quartz and felspar, with small aggregates and rods of a 

 dark-green mineral. Under the microscope it is seen to be a granu- 

 litic rock, consisting of a mixture of rounded quartz-grains measuring 

 up to half a millimetre in diameter, and plagioclase (albite-oligoclase) 

 of about the same size. The general appearance has a curious 

 resemblance to some quartzites where the grains are uniform in size. 

 There are a certain number of felspars that show only Carlsbad- 

 twinning or none at all ; but the refractive index appears to be 

 identical with those showing twin-lamellation, and they have 



1 Dr. Fonseca describes here the occurrence in the ' syenite '-rocks of the 

 oval and elliptical holes already mentioned. 



2 The birefringence or difference between the indices of refraction of the two 

 directions of vibration represents the relative retardation in a unit of length, 

 and might be termed the rate of relative retardation. If Jc be the total 

 relative retardation between the two directions of vibration, I the thickness of 



the section, and d the birefringence or rate of relative retardation, then d=—. 



In this particular case the observed relative retardation (Jc) was 670 microiniUi- 

 metres (millionths of a millimetre), and the thickness was 23,000 micromilli- 

 metres, so that d was equal to 670 -^ 23,000 =-029. It is, however, convenient 

 to measure the thickness of the section in microns (a thousandth of a millimetre 

 = 1000 micromillimetres)^ and to take as a practical unit of birefringence the 

 number of micromillimetres of relative retardation in a micron : thus avoiding 

 the use of small decimal fractions. If then D be the rate of relative retarda- 

 tion in micromillimetres per micron, Jc the relative retardation in micromilli- 

 metres, and L the thickness in microns, D = =- = -Kq = 29. 



The unit rate of relative retardation in micromillimetres per micron is 

 toVtj °f the absolute unit of birefringence or rate of relative retardation, in 

 which both the relative retardation and the distance are expressed in the same 

 units ; it is, therefore, conveniently spoken of as a ' thousandth/ Perhaps 

 a 'millesim' would be better; see a paper in the Mineralogical Magazine, 

 vol. xiv (1905) p. 87. The words micron and micromillimetre are here 

 used in accordance with the rules laid down by the British Association for 

 the Advancement of Science. 



