MR J. HORNE AND MR J. J. H. TEALL ON BOROLANITE. 171 



type of this rock is exposed in knolls by the side of a small stream. The rock is dark 

 grey, with abundant white knots and black garnets, effervescing very freely with acid. 

 The foliated and unfoliated varieties just referred to are traceable at intervals in the 

 stream sections to the south of Aultivullin as far as the slope of Cnoc-na-Glas-Choille. 



These exceptional varieties to which special attention is called in this paper have been 

 traced across an area upwards of two miles in length from Loch-a-Mheallain to Cnoc-na- 

 Glas-Choille, and about a mile in breadth from east to west. 



But, in addition to these localities, abnormal types which will be referred to on a 

 subsequent page occur not far to the south of the junction of the Ledmore and Ledbeg 

 Rivers, and also at the base of the north-west slope of Cnoc-na-Sr5ine. At the latter 

 locality it forms the margin of the igneous mass, and the marble occurs not far distant 

 on the south bank of the Ledbeg River, about half a mile to the east of the Loyne 

 shepherd's house. 



But perhaps the most interesting sections showing the relation of this peculiar 

 type of rock with the pink and white knots to the marble, occur in the small streams on 

 the south-west slope of Sgonnan Mor. 



IV. Penological Description of Borolanite. 



1. The prevailing type is a medium-grained rock of a dark grey colour. It frequently 

 contains whitish or pinkish patches, usually more or less spherical or ellipsoidal in form, 

 but occasionally showing polyhedral boun- 

 daries. These patches vary considerably in 

 size. The smallest are only just distinctly 

 visible to the naked eye ; the largest measure 

 an inch or more across. They also vary con- 

 siderably in relative abundance. Sometimes 

 they are absent altogether, whereas at other 

 times the main mass of the rock is composed 

 of them. The general appearance of a 

 polished surface is represented in fig. 1. 



Where the rock has been subjected to ,.„,,,.-,«■,_,„. 



J Fig. 1. — About Two-thirds Natural Size. 



deformation during or subsequent to consoli- 

 dation, the white patches take the form of lenticles or streaks, as may be seen in fig. 2. 



2. The principal interest of these rocks centres in their peculiar mineralogical 

 composition. The dominant constituents are orthoclase, plagioclase, a substance which 

 gelatinises with hydrochloric acid, melanite, pyroxene and biotite. The small black 

 spots seen in fig. 1 are due to the garnet. Apatite, sphene and iron ores occur as 

 accessory constituents. The secondary products include a peculiar blue substance which 

 is probably an alteration product after a mineral of the sodalite group, white mica and 

 possibly calcite. Many of the specimens effervesce freely with acid, but this is probably 



VOL. XXXVII. PART I. (NO. 11). 2 D 



