T. Barron — A British Rock containing Nepheline, etc. 375 



altered, and some were in an advanced stage of zeolitization. In 

 extremely thin sections the difference in the polarization-colours of 

 this mineral and the felspar microhtes was much more plainly seen. 

 From these optical characters I have concluded that the mineral is 

 nepheline. As a confirmatory test, another thin section was 

 prepared, and exposed to the vapour of strong hydrochloric acid for 

 half an hour. It was then washed with warm water, covered with 

 a saturated solution of malachite-green, and set aside for fifteen 

 minutes. (This material was chosen in preference to fuchsine, 

 because, according to Behrens, it does not give accidental stains — 

 a great drawback to the use of the latter substance.) The section 

 was now washed once again with warm water, a camel's-hair brush 

 being used to remove any superfluous staining material. After the 

 section was mounted in canada balsam and covei'ed, it was found 

 on examination under the microscope that the mineral moulding 

 the felspars had been etched and stained deeply, while the latter 

 remained untouched. 



By way of compai-ison, a thin section of the phonolite-trachyte 

 from Traprain Law was prepared and treated in a similar way. 

 The staining showed the relations between the nepheline and felspar 

 in this rock to be different to those existing in that from Eildon ; 

 for, while in the latter, nepheline moulded the felspars, in the 

 former it seemed to be present in more or less idiomorphic plates. 



In general structure the two rocks are very similar, except that 

 in the Traprain Law rock the ferro-magnesian mineral is asgirine, 

 which is present in a granular condition, while in that from Eildon 

 it is riebeckite, which is often ophitic. 



On the south side of the hill, the rock assumes a much more 

 felsitic character : its fracture is conchoidal ; and porphyritic felspars 

 are very rare. A blue mineral can be seen scattered in patches 

 through the rock, which by the aid of a lens can be recognized as 

 riebeckite. 



Under the microscope, the rock is seen to consist of felspar 

 microlites, and patches of riebeckite or pseudomorphs after it. 

 Ferruginous alteration-products are scattered thickly through it, 

 giving a pinkish colour to the section, and rendering the rock so 

 dense that it requires great care to obtain a section thin enough to 

 be transparent. A few granular aggregates of felspar (sanidine) 

 are present in the slide, some of which show zoning, and a rude 

 attempt at perthite structure when examined between crossed nicols. 

 The riebeckite exhibits the same characters as in the previous rock, 

 except that its pleochroism is more intense. Between crossed nicols, 

 the base of this rock is seen to be under strain; it also breaks up 

 into patches which polarize in two distinct tints. One set of patches 

 gives the extremely low colours of nepheline, and when examined 

 in ordinary light shows greater alteration than the other. Small 

 rectangular sections, much altered, were also observed giving the 

 same polarization colours. From the very close resemblance of 

 these patches to those in the rock previously described, there seems 

 to be little doubt that they are composed of nepheline. 



