THE ARCHAEAN KOCK8 39 



i < hit ion to the gneiss is uncertain because the planes of contact 

 seem to be shear-planes with bands of crushed rock, but they are 

 clearly older than the Torridon sandstone. 



This schistose series consists for the most part of mica-schist, 

 gniphite-schist, and crystalline limestone, with bands that may 

 represent altered quartzite and chert ; and there are also broad out- 

 crops of hornblende-schist which are probably dykes or sills of 

 basic igneous rock. There are several bands of limestone, varying 

 in thickness from 3 to 30 feet; one of the best known is the 

 Letterewe limestone, which is a cream-coloured rock containing 

 In nips of actinolite ; it was formerly extensively quarried. Another 

 band at Ardlair is .1 white saccharoid dolomite. 



The existence of this tract of metamorphosed sediments to the 

 west of the great thrust-plane is important, because it may be 

 regarded as a remnant of a mass of such rocks which was originally 

 much more extensive, and seems to occupy an intermediate position 

 between the Hebridean gneiss and the Torridon sandstone. We 

 shall presently see that the more highly metamorphosed complex 

 of rocks to the eastward includes representatives of a similar 

 schistose series, and it has been reasonably supposed that the 

 Loch Maree Series represents some of the sedimentary rocks into 

 which the Hebridean gneisses were intruded. 



Hebridean gneisses are found on the western border of Inverness, 

 opposite Skye, and in the peninsula of Sleat ; they form the islands 

 of Coll and Tiree and occur in lona and Soa ; they are also 

 exposed in the north-east part of Colonsay and again in the south- 

 western part of Islay which terminates in the precipices of Rhinn* 

 Point. The gneisses of Islay are similar to those of the north -west, 

 but Dr. Teall observes 4 that under the microscope they show 

 evidence of profound modification by pressure, " the general result 

 of which has been to crush the original constituents and thus to 

 produce microscopic breccias, not holocrystalline schists." 



The features of the belt of faulted and overthrust masses of rock 

 which separates the western and eastern areas of the Northern 

 Highlands are sufficiently illustrated in Figs. 2 and 3, drawn by 

 the Officers of the Geological Survey, and the details of its structure 

 need not be described in this place It is only necessary to not* 

 that the rocks which occupy the country to the east of the great 

 Moine thrust have been so crushed, crumpled, and metamorphosed 

 that they now have special aspects of their own. 



Part of the great median area of schistose rocks is shown on 

 the map (Fig. 1) ; it comprises the greater parts of Sutherland, 

 Ross, Cromarty, and Inverness ; it extends across the " great glen " 

 into Nairn and Elgin, and southwards through the Forest of Athol 



