MR J. HORNE AND MR J. J. H. TEALL ON BOROLANITE. 177 



4. The affinities of borolanite are unmistakable. It is a member of the foyaite (elseolite- 

 syenite) family. The occurrence of melanite as an important accessory in certain rocks 

 belonging to the nepheline-leucite group has long been recognised. In our rock we have 

 melanite raised to the rank of an essential constituent. Borolanite, as we have already 

 shown, is intrusive in the Cambrian rocks of Sutherlandshire. The nearest rocks in any 

 way allied to it are the elseolite- syenites of the Christiania district, which are also intrusive 

 in Lower Palaeozoic strata. 



Appendix. 



So far as our own observations go, we have met with borolanite only in the neighbour- 

 hood of the granitic mass of Cnoc-na-Sr5ine. Our colleague, Mr Hinxman, has observed 

 a patch of borolanite intercalated in the thrust Eilean Dhu limestone at Elphin (Group 

 II., Durness series, Cambrian). The rock is well exposed at the back of the Weaver's 

 Cottage, south of the Elphin Schoolhouse ; it is in places highly decomposed, grey, with 

 white knots and abundant melanite. Our colleague, Mr Gunn, has found dykes of the 

 same type of rock in the area he has surveyed in West Ross-shire. He has kindly 

 furnished us with the following note : — " In the Coigach district of West Ross-shire, about 

 five miles to the north-west of Achiltibuie, there are found at Camas Eilean Ghlais two 

 vertical dykes of borolanite intruded into the Torridon sandstone. They run in a general 

 W.N.W. and E.S.E. direction, and vary considerably in width — the widest one east of 

 the house being nearly thirty feet across, but further west only about six feet. This, 

 which is the most southerly of the two, is also the longest, and can be traced for a length 

 of half a mile or so/' 



A hand specimen of the rock is medium-grained, brownish-grey and massive. Lath- 

 shaped cleavage faces of felspar may be seen with the naked eye. Numerous minute 

 black specks (melanite), uniformly scattered through the rock, are visible with a pocket 

 lens. 



Under the microscope the rock is seen to be composed of orthoclase, nepheline 

 (partly fresh and partly altered to a substance giving aggregate polarisation), melanite, 

 segirine and biotite. The main mass is an aggregate of orthoclase and nepheline or its 

 alteration product. Traces of idiomorphism may occasionally be seen in both constituents, 

 but, as a rule, the outlines of the individuals are not crystallographic faces. Melanite is 

 scattered through the orthoclase-nepheline aggregate in small crystals of the usual form. 

 In thin sections the crystals are either pale yellow or very deep brown. Not unfrequently 

 a pale external zone surrounds a deeply coloured nucleus. 



iEgirine occurs in long prisms idiomorphic in the prismatic zone. The prisms are 

 crowded together in certain portions of the slide, not uniformly scattered through it. This 

 is the only rock in which we have detected typical segirine. In the other rocks the 

 corresponding mineral is a green pyroxene with high extinction angles. The biotite 

 occurs in the form of six-sided tablets. It is nearly opaque in thin sections when 



