THE METAMORPHOSIS OF DOLEKITE INTO HORNBLEXDE-SCHIST. 133 



17. The Metamorphosis of Dolerite into Hornblende-schist. 

 Ey J. J. H. Teall, Esq., P.G.S. (Read January 14, 1885.) 



[Plate 1L] 



The object of this commimication is to describe the gradual 

 change from dolerite to hornblende-schist as it may be observed 

 in two more or less parallel dykes which occur in the Archaean 

 gneiss of the north-west of Scotland, near the village of Scourie in 

 Sutherlandshire. 



The northern promontory of Scourie Bay is named on the Ordnance 

 map Creag a'Mhail. ISTear this point two dykes of basic igneous 

 rock may be seen. The one to the south is about 30 yards wide ; 

 it has been more affected by denudation than the surrounding- 

 gneiss, and its course is thus marked by a depression which is 

 occupied by the sea at high tide. A short distance to the north 

 of the actual promontory is another dyke, about 20 or 30 feet wide, 

 of similar character, which terminates westward in a vertical cliff. 

 This may be traced in a south-easterl)^ direction down to a small 

 beach where the southern dyke is again seen, and from this point 

 the two dykes may be followed, running parallel to each other 

 towards the south-east, for a distance of a quarter of a mile or more. 

 The prevalent strike of the gneissic banding in this district is 

 E.N.E. and W.S.W., with a moderate dip to K.N.W. The dykes 

 therefore cut across the strike and cannot possibly be regarded as 

 bands in the gneiss. 



On the opposite shores of Scourie Lake (Loch a' Bhaid Daraich) 

 and close by the side of the road to Laxford Bridge (three quarters of a 

 mile from the Scourie Hotel), the dykes again make their appearance. 

 Prof. Blake first recognized the northern dyke at this spot on the 

 occasion of our joint excursion to the north-west of Scotland in the 

 summer of 1883. This dyke is here the wider of the two and 

 forms a portion of the conspicuous hill which overlooks the lake. 

 The southern dyke produces a marked gap to the south of the hill. 

 The distance between the two dykes is only a few yards. The 

 junction-planes of gneiss and dyke are nearly vertical, and the fact 

 that they cut across the gneissic banding is most clearly shown. 



Erom this point the dykes may be traced to the south-east for 

 more than a mile. What becomes of them ultimately in this 

 direction I do not know ; but, so far as I followed them, they keep 

 very closely to the same straight line and preserve their lithological 

 characters (subject to such exceptional variations as will presently be 

 described) with as much persistence as the dykes in the Carboniferous 

 region of the north of England. 



The rock forming these dykes O'ccurs in two strongly marked 

 varieties. The one is a moderately coarse-grained, crystalline, 



