The Traj^rain Law FJionolite. 515 



the petrography of Traprain Law, and sodalite seems to have been 

 described in only one other rock from the British Isles (14).^ 



In Traprain Law all the isotropic mineral content was previously 

 taken for analcite. Now that some of it proves to be sodalite, the 

 question must be faced, how far the analcite of other Scottish rocks 

 will survive chemical investigation. 



After Part 1 of this paper had been written Dr. Flett very kindly 

 arranged for a re-analysis of the Traprain Law phonolite in the 

 Geological Survey laboratory. Mr. Ennos undertook the work and 

 his results furnish the material of Part II. 



Optical Examination. 

 Nepheline. — During the examination of the slides under the 

 microscojie a mineral was noticed to occur fairly abundantly, and 

 with the following characteristics : — 



1. Verif low polari-.ation colours. — Between crossed nicols the mineral had 

 a pale indigo-blue-grey colour. This polarization colour was decidedly lower 

 than that of the felsjiars, and by means of it the larger patches of the mineral 

 could be picked out fairly readily, after some practice, with the 1 inch objective. 

 The smaller patches were located by a careful examination with the J in. 

 objective. 



2. Refractive Index, as shown by the Becke test, decidedly higher tiian 

 that of the surrounding alkaH-felspar laths or tlie groundmass in contact with 

 it, and approximately the same as that of the balsam of the slide. 



3. Clearage. — Indications of cleavage were generally present, one fairly 

 well defined, and another much less clear and approximately at right angles 

 to the first. With reference to the better defined cleavage the mineral showed 

 straight extinction and negative character. 



4. Decompositioyi. — The felsi)ars of the groundmass were in general made 

 cloudy by brown powdery decomposition products, but this was not the case 

 as regards the mineral in question, which stood out, in ordinary light, clear, 

 colourless, and transparent among the cloudy felspar laths. Very occasionally 

 isotropic material occurred in such a way that it might be interpreter) as 

 replacing this mineral (neioheline). Tiny very sporadic fibres (a zeolite ?) 

 could often be seen in the mineral. 



5. Habit. — In nearly every case the mineral showed ophitic relations to the 

 felsjaar laths of the groundmass, and good idiomorj)hic outlines were never 

 seen. Very occasionally the mineral was included as small irregular patches 

 within the felspar phenocrysts. Occasionally approximation to squarish 

 rectangular outlines was seen, but idiomorphism was never complete. 

 Exceptionally there was a doubtful suggestion of a felspar lath moulded on the 

 mineral. 



6. The size of the patches ranged from those hardly determinable with a 

 J in. objective, to irregular patches about the order of 0-6 mm. The size 

 varied greatly even in the same slide. 



7. Interference figure. — Many attempts were made to get an interference 

 figure from the mineral, but the best were very ill-defined, and it was even 

 difficult to decide whether a biaxial or a uniaxial figure was being dealt with. 

 In some cases the figure could be doubtfully interpreted as a very broad, ill- 

 defined uniaxial black cross giving negative character with the gypsum plate. 

 The mere failure to give a distinct figure, when combined with the very low 

 double refraction, suggested a distinction between the mineral and a felspar. 



From the above characteristics it was suspected that the m,ineral 

 was nepheline. 



^ Figures in parentJieses refer to the Bibliography at the end. 



