Tlte Traprain Law Fhonolite. 519 



coupled with the evidence of the peculiar reticulated cracks already 

 mentioned, showed that part of the isotropic material of Traprain 

 Law was analcite. 



An especially good example of the joint occurrence of analcite and 

 sodalite was afforded by a vein of isotropic material 04 mm. wide, 

 which traversed one of the slides. The differential action of the 

 nitric acid and silver nitrate here showed clearly that the vein con- 

 sisted chiefly of analcite, but partly of sodalite. Before the micro- 

 chemical test the presence of only one isotropic mineral was 

 suspected. 



It is hoped that the simple qualitative test for chlorine on the rock 

 powder will prove a useful means of detecting the presence of sodalite 

 in any rock, and that further work will show that the reticulated 

 system of cracks is characteristic of analcite among isotropic minerals 

 under the conditions specified above. 



Olivine.- — The optical characters of the supposed olivine left some 

 slight doubt that the mineral might be an augite. This doubt was 

 removed by marking and drawing as before about fifteen pieces of 

 supposed olivine (crystals with olivine shape, irregular grains, and 

 ophitic patches) in different slides and treating with concentrated 

 hydrochloric acid for about ten to fifteen minutes. In every case the 

 miaeral was at first attacked and then entirely removed by the acid. 

 This is characteristic of an olivine not rich in forsterite. The 

 irregular cracks and cleavages were often left running across the 

 space where the mineral had been, owing to the magnetite deposited 

 in them on the commencement of serpen tinization. The green 

 augites were unaffected by the acid. The former mineral was thus 

 proved to be an olivine fairly rich in iron. 



Note. — For success in all the microcliemical work it was found essential to 

 locate carefully and to draw a sketch of each patch of the mineral under 

 investigation, and the other minerals in its neighbourhood, after the cover- 

 glass, but not the balsam, had been removed. Without these precautions 

 it was found extremely difficult definitely to locate portions of the slide seen 

 before the microcliemical treatment, owing to movement during removal of 

 the cover-glass, and the changes due to the action of the acid and the absence 

 of the balsam. The stain of fuchsine is not stable in joresence of hot balsam, 

 so it is not convenient to replace cover-glasses on slides so stained. 



Appendix I. 

 Notes on Previous WoPvK. 



Nepheline. — Dr. Hatch, who discovered that Traprain Law was 

 a phonolite, states in his description of the rock (4 and 5) that he had 

 great trouble in determining the nepheline, but that Rosenbusch, 

 who examined some of the slides, detected small four- and six-sided 

 sections of that mineral. 



Analcite. — Dr. Hatch (5) states that analcite and natrolite are 

 ju'esent in Traprain Law as alteration products of the nepheline. 



In Mr. Bailey's description of the Traprain Law rock (^) evidence 

 is given that the isotropic material (described as analcite) is primary 

 in origin. 



