A NEW BRITISH ROCK. BY T. BARRON 97 



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 covered, 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 comparison, 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 

 eegirine, 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 por- 

 phyritic 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. 

 Snriall rectangular sections, much altered, were also observed 

 giving the same polarization colours. From the very close 



