6 THE JOURNAL OF GEOLOGY. 



The oldest or doleritic dykes form a wonderful feature in the 

 gneiss, from their abundance, persistence and uniformity of trend 

 in a west -northwest direction. They have no parallel in British 

 Geology until we reach the crowded dykes of older Tertiary 

 time. 



Throughout this remarkable complex of eruptive material, 

 though its different portions present many features that may be 

 compared with those of intrusive bosses and sheets belonging to 

 later geological periods, there is no trace of any superficial 

 volcanic manifestation. No tuffs or agglomerates or slaggy 

 lavas have been detected, such as might serve to indicate the 

 ejection of volcanic materials to the surface. All the phenomena 

 of the Lewisian gneiss point to the consolidation of successively 

 protruded portions of eruptive material at some depth within the 

 crust. 



Nevertheless it may yet be possible to show that these deep 

 seated masses have been injected into rocks of older date and 

 of sedimentary origin, and that ^they have communicated with 

 the surface in true volcanic eruptions. I have already alluded 

 to one limited area where various rocks exist, distinctly different 

 from the prevalent types in the Lewisian gneiss. In the area 

 which is traversed by the long valley of Loch Maree in western 

 Rosshire, there occur clay-slates, fine mica schists, graphitic 

 schists, and saccharoid limestones. These rocks remind us of 

 some of the prevalent members of a series of metamorphosed 

 sediments. The minerals enclosed in the marbles are just such 

 as might be expected in the metamorphic aureole of a granite 

 boss, piercing limestone. But the relations of this group of rocks 

 to the ordinary gneiss of the region are not quite so clear as 

 could be desired, though they seem to point to these rocks 

 being surrounded by and enclosed within the gneiss. 



The detailed field-work of the officers of the Geological Sur- 

 vey has made known the remarkable amount of mechanical 

 deformation which the various rock-masses composing the 

 Lewisian gneiss have undergone. These rocks have been com- 

 pressed, crushed, and drawn out, until what were originally mas- 



