166 MR J. HORNE AND MR, J. J. H. TEALL ON BOROLANITE. 



rock-forming minerals.* He indicated some of the remarkable lithological varieties of 

 these intrusive rocks, and gave analyses of three specimens, viz. : — 1. Hornblende porphy- 

 rite, intrusive in quartzite ; 2. Porphyritic diorite ; 3. Plagioclase — pyroxene — hornblende 

 rock near Inchnadamff, intrusive in limestone. The last, which is the most basic, differs 

 from the others in containing a large amount of colourless pyroxene. The author suggested 

 that "in all probability the pyroxene is a nearly pure lime-magnesia bisilicate, and one is 

 tempted, therefore, to ask whether it may not be due to the absorption by the igneous magma 

 of a certain amount of the dolomitic limestone into which the rock has been intruded." 



In 1888, in the report on the recent work of the Geological Survey in the North- 

 West Highlands of Scotland, special reference was made to the intrusive igneous rocks 

 associated with the Torridon sandstone and Cambrian strata in Assynt, brief descriptions 

 being given of the geological features which they present in the field, t 



II. Physical Relations of the Igneous Rocks intrusive in the Torridon Sandstone and 



Cambrian Strata. 



1. Before proceeding to the description of the particular group of rocks that form the 

 subject of this communication, it may be desirable to refer generally to the physical 

 relations which the igneous materials, as a whole, present in the field. Perhaps their 

 most characteristic feature is their occurrence in the form of intrusive sheets injected 

 along the planes of bedding of the sedimentary strata. The remarkable parallelism of 

 the igneous bands, varying in thickness as a rule from 10 to 50 feet, and the manner in 

 which they cling to the same horizon for considerable distances, have led one or two 

 observers to the conclusion that they are contemporaneous lava flows. But a careful exam- 

 ination of the physical relations of these igneous rocks reveals certain phenomena which 

 are characteristic of intrusive sheets. First, when the igneous bands are traced along 

 the line of outcrop, they pass transgressively from lower to higher members of the same 

 group, and occasionally plunge downwards into a lower platform. A striking example of 

 these phenomena is to be found on the western face of Canisp, where a mass of porphy- 

 ritic felsite rises from the old platform of Archaean gneiss, passing upwards into the 

 overlying Torridon sandstone and eventually spreading along the bedding planes. Second, 

 where the sheets reach a considerable thickness, both the overlying and underlying 

 strata are altered by contact metamorphism. The zone of marble surrounding the great 

 igneous mass to the east of Ledbeg admirably illustrates this local metamorphism, and 

 even in the case of the thinner sheets, the quartzites are hardened and welded to the 

 igneous rock. Third, there is a marked absence of cellular structure in the various types 

 of igneous materials. Fourth, they occasionally contain fragments of the sedimentary 

 rocks which they traverse. 



* Notes on some hornblende-bearing rocks from Inchnadamff. J. J. H. Teall, Geol. Mag., 1886, p. 346. 



t " ['i [i i' "ti the Recent Work of the Geological Survey in the North-West Highlands of Scotland, based on the 

 Field Notes and Maps of Messrs B. N. Peach ; J. Horne ; W. Gunn ; C. T. Clough ; L. Hinxman and H. M. Cadell," 

 Q. J. G. Soc, xliv. p. 378. 



