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



one of us he sa3 T s : — " In preparing a specimen of your melanite rock, I cut through some 

 of the white aggregates and was struck by the tendency to polyhedral outlines, which is 



not apparent on a broken surface but is quite distinct on the plane saw-cut face 



This to me is very suggestive of the pseudo-crystals of leucite in some of the related 

 Brazilian rocks,* and suggests an interesting subject for investigation." In the same 

 letter he says that he has found a micro-pegmatitic intergrowth of orthoclase and 

 ncpheline in some of the pseudo-leucites. We have re-examined the whole of the material 

 at our disposal, but are not able to add anything to what has been stated above. In 

 addition to the constituents already mentioned as occurring in the white patches, we find 

 also melanite, calcite and white mica. The melanite, however, is rare. It is always 

 much less abundant in the patches than in the main mass of the rock. 



The matrix in which the white patches are embedded, and the entire rock-mass 

 when these are absent, are composed of two or more of the constituents already enumerated. 

 The type rock is essentially composed of orthoclase and melanite. A good idea of its 

 microscopic character may be obtained from a glance at fig. 1, Plate XXXVII. 



As frequently happens when any extensive • mass of plutonic rock is examined there 

 is considerable variation in the relative proportions of the different constituents, but this 

 is not sufficient to take away from the orthoclase-melanite combination its dominant 

 character. As illustrating the extremes of variation which have come under our notice, 

 we may mention a rock from a point about one mile east of Aultnacallagach which is 

 mainly composed of large individuals of orthoclase with a small quantity of the peculiar blue 

 substance wedged in between the more or less idiomorphic crystals of the former mineral ; 

 and one from the burn near Ledmore which consists of pyroxene, melanite, biotite, 

 apatite and pyrite. One of these varieties consists, therefore, entirely of alumino-alkaline 

 silicates ; the other almost entirely of ferro-magnesian minerals. The former occurs as a 

 pegmatitic vein in typical borolanite. 



Another exceptional type was obtained on the north-west slopes of Cnoc-na-Sr5ine. 

 It is essentially composed of orthoclase and pyroxene ; with biotite, sphene, apatite and the 

 doubtful substance which gelatinises with hydrochloric acid as subordinate or accessory 

 constituents. 



In the majority of cases the roeks are massive, but in some instances a well-marked 

 foliation may be observed. In the foliated varieties the white patches have been 

 orientated or even pulled out into lenticles and streaks. The movement probably took 

 place during the final stages of consolidation. 



We have, then, a group of rocks especially characterised by the association of 

 orthoclase and melanite. They are extensively developed in the neighbourhood of Loch 

 Borolan, and as a new name appears to be required, we propose to call them borolanites. 

 The typical rock is a crystalline granular aggregate of orthoclase and melanite. Biotite, 

 pyroxene, alteration products after nepheline and sodalite, sphene and apatite, occur as 

 subordinate and variable constituents. 



* See "On Nepheline Rocks in Brazil," Quart. Jour. Geol. Soc, vol. xlvii. (1891), p. 251. 



