( 163 ) 



XL 



—On Borolanite — an Igneous Rock intrusive in the Cambrian Limestone of Assy nt, 

 Sutherlandshire, and the Torridon Sandstone of Ross-shire. By J. Horne, F.E.S.E., 

 and J. J. H. Teall, F.R.S., of the Geological Survey. (Communicated by 

 permission of the Director-General of the Geological Survey.) (With a Plate.) 



(Read 21st May 1892.) 



CONTENTS. 



PAGE 



I. Previous References to the Igneous Rocks 

 associated with the Torridon Sandstone and 

 Cambrian Strata in Assynt, .... 163 

 II. Physical Relations of the Igneous Rocks intru- 

 sive in the Torridon Sandstone and Cambrian 

 Strata, 166 



1. Evidence in favour of their being intru- 



sive sheets, 166 



2. Horizons, 167 



3. Area of distribution, .... 167 



4. Date of intrusion, .... 167 

 III. Intrusive Mass of Cnoc-na-Sr6ine, Loch Borolan 



and Ruighe Cnoc, 167 



1. Physical relations of this intrusive mass, 



and area of distribution, . 



2. Summary of the evidence regarding the 



geological relations of the mass, 



3. Area of the prevalent granitic type of 



Cnoc-na-Sr6ine, .... 



4. Area of the group of rocks included 



under Borolanite, .... 

 IV. Petrological Description of Borolanite, 



1. Macroscopic characters of the rocks, 



2. Description of the minerals, . 



' '3. Microscopic characters of the rocks, 



4. Affinities of Borolanite, .... 



PAGE 



167 



170 



170 



170 

 171 

 171 

 171 

 175 

 177 



The remarkable development of igneous rocks associated with the Torridon 

 sandstone and Cambrian strata in Assynt, Sutherlandshire, forms one of the striking 

 geological features of that region. In the various papers descriptive of the ancient 

 sedimentary formations of the North- West Highlands by former observers, references are 

 made to the lithological characters of these crystalline rocks and to their mode of 

 occurrence. 



I. Previous References to the Igneous Rocks associated with the Torridon Sandstone and 



Cambrian Strata in Assynt. 



In 1856, Professor Nicol, in his paper " On the Red Sandstone and Conglomerate, 

 and the Superposed Quartz-rocks, Limestones and Gneiss of the North- West Coast of 

 Scotland," notes the occurrence of a bed of greenstone in the cliff of limestone to the 

 south of the Inn at Inchnadamff.* He further states that in the area to the east of 

 Ledmore, the relation of the quartzite to the gneiss bounding it on its eastern side was 

 not visible on the line followed by him, as a mass of red felspar porphyry intervenes 

 near Loch Borolan. 



In 1858, Sir Roderick Murchison, in his paper "On the Succession of the Older 

 Rocks of the Northernmost Counties of Scotland, with some observations on the Orkney 

 and Shetland Islands," refers to the band of red porphyry with large crystals of felspar, 

 detected by Mr C. W. Peach and traced by him round the flank of Canisp, which is 

 there interposed between the gneiss and the Torridon sandstone.t 



* Q. J. G. Soc, vol. xiii. p. 25. 

 VOL. XXXVII. PART I. (NO. U> 



t Q. J. G. Soc, vol. xv. p. 365. 

 2 C 



