of the Scottish Highlands. 171 



the real base of the system, that being covered by the later over- 

 lapping Old Red Sandstone series, the clay-slates often being seen 

 to pass downwards before plunging beneath the Old Red Sandstone 

 basal conglomerates into grits and greywackes similar to those 

 which overlie it. These argillaceous rocks are therefore intercalated 

 between the grits, greywackes, and conglomerates of the lower 

 arenaceous group, and represent a period of sedimentation, during 

 which the land-surface had reached a maximum of depression beneath 

 the waters of the ocean. 



The principal rocks of this zone are clay-slates, phyllites, black 

 shales, and limestones, the typical rock of the whole being the clay- 

 slate, which is simply a highly indurated mud upon which a fine 

 fissile structure has been superinduced by metamorphism. As a rule, 

 the cleavage of these rocks coincides with their bedding ; this is 

 shown by the manner in which these slates are intercalated between 

 the bedding planes of the grits and conglomerates of the arenaceous 

 zone. There is, therefore, seldom evidence of any more than the one 

 set of divisional planes. Occasionally, however, evidences of other 

 divisional planes cutting across the former are to be met with. 



Sometimes the clay-slates pass into a fine lustrous or micaceous 

 variety, to which the term phyllite has been applied. These 

 phyllites seem to me to be simply further stages in the process of 

 metamorphism, and probably link the clay-slates with such highly 

 foliated crystalline rocks as the sericite schist of Ben Lawers. These 

 phyllites show an occasional tendency to foliation, as seen in the 

 beds above Loch Lubnaig. 



Towards the north-east this argillaceous zone, which is thus found 

 to form the outer edge of the Southern Grampians, dips inwards, and 

 is seen to be overlain by the main mass of the Grampian range. 

 On tracing this zone, however, towards the south-west, a gradual 

 reversal of the dip towards the south-east is seen to occur. It will 

 be evident, however, from sections to be described, that the order 

 occurring in Perthshire and further to the north-east is the normal 

 order, while that seen at Loch Lomond and further westwards 

 represents a reversal of the beds. Black graphitic shales are also to 

 be met with upon this horizon, though they do not seem to be as 

 extensively developed as in the upper argillaceous zone, which will 

 be noticed later on. 



Limestone is also to be found on this horizon, as at the Pass of 

 Leny, above Kilmahog old toll-house, which will be described in the 

 Pass of Leny section. I am also led to understand that it has been 

 seen further to the east, near Birnam. 



IV. Lower Arenaceous Zone. 



Underneath and immediately above the lower argillaceous zone 

 just described we find a series of rocks, varying from a fine-grained 

 greywacke, through quartzites and grits, to coarse conglomerates, to 

 the whole of which, taken together, we have given the name lower 

 arenaceous zone. As before remarked, the lower argillaceous 

 zone is intercalated between these rocks, and is seen at some places 



