388 J. J. E. Teallr—Nepheline- Syenite in N. W. Scotland. 



are often twinned on the Carlsbad plan. Under the microscope 

 the rock is seen to be composed of nepheline and alkali- felspar in 

 approximately equal proportions. A greenish biotite and melanite 

 occur as accessory constituents. Both felspar and nepheline are 

 present as large individuals, measuring half an inch or more across. 

 The occurrence of melanite as an accessory constituent serves to 

 emphasize the close relation between nepheline - syenite and 

 borolanite. 



The typical borolanites have been described in the paper above 

 referred to. They are extensively developed to the east of 

 Aultnacallagach in the low ridge on the north-eastern side of the 

 road leading from Oykell Bridge to Inchnadampf, and occur also 

 at other points on the borders of the plutonic mass. One specimen 

 (3,737) may be briefly referred to as a type. It is a massive rock 

 composed of white patches, measuring one-quarter to three-quarters 

 of an inch across and often polygonal in outline, embedded in a dark, 

 almost black matrix. The white patches correspond to the pseudo- 

 leucites described by Derby, Hussak, and J. F. Williams. They are 

 composed mainly of orthoclase, which is often micrographically 

 intergrown with an alteration product after nepheline. Biotite 

 and Eegirine-augite occur sparingly in these patches. The matrix 

 is composed of orthoclase and melanite with some segirine-augite 

 and biotite. Micrographic intergrowths of orthoclase and the 

 alteration product after nepheline are also present. 



The rocks most nearly allied to these peculiar borolanites are 

 unquestionably the ' leucite-syenites ' from the igneous complex 

 of Magnet Cove, Arkansas, described by the late J. F. Williams,^ 

 whose early death was such a severe loss to American petrology. 

 Williams speaks of these and other rocks of the complex as forming 

 dykes ; but the geological map of the district accompanying his 

 memoir clearly shows that they do not occur as ordinary dykes. 

 According to Dr. Washington ^ the complex represents a laccolite in 

 which differentiation has produced a mass varying in composition 

 from a highly basic jacupirangite at the centre to foyaite at the 

 margin. The different types — foyaite, leucite-porphyry, shonkinite, 

 ijolite, biotite-ijolite, and jacupirangite — succeed each other in 

 concentric zones from the margin to the centre, the rock most nearly 

 allied to borolanite coming second in the series, reckoning from the 

 outside. This rock is described by Williams as " a hypidiomorphic 

 granular combination of pseudo-leucite, eleolite, orthoclase, and 

 basic silicates, which presents a more or less perfect granitic 

 structure and is genetically connected with the eleolite - syenite 

 dike rocks." The pseudo-leucites are composed mainly of orthoclase 

 and nepheline, as are those of our borolanite, but they are more 

 perfect in form. It must, however, be remembered that the borolanite 

 area has never been searched for specimens showing the forms of 



^ " The Igneous Rocks of Arkansas " : Annual Report of the Geological Survey of 

 Arkansas for 1890, vol. ii. 



* "Igneous Complex of Magnet Cove" : Bull. Geol. Soc. Amer., vol. ii (1900), 

 pp. 389-416. 



