558 HISTORY OF THE OUTER HEBRIDES. 



cally of a different formation from the rest of the Outer 

 Hebrides. The prevalent " strike " or direction of 

 ridges, where the rock rises above the general slope, is fr 

 north-west to south-east. This direction, as seen best ii 

 the Harris hills, is at right angles to the general directi< 

 of the mountain chains of later formation on the mainlam 

 Indeed, this " strike " or direction is characteristic of tl 

 most ancient formation. As the strata, however, is 

 turning round arches of gentle curve, the " strike " in man] 

 places is from north-east to south-west. According to tl 

 late Professor Nicol of Aberdeen, the whole of the Central 

 and North- West Highlands is composed of these Archaean 

 rocks. The Gneiss, as this rock is named, is composed of 

 the following minerals : felspar, mica, hornblende an< 

 quartz. In the Outer Hebrides the hornblende prevail 

 over the mica as a constituent of the Gneiss. The struc- 

 ture varies from granitic, where the lines of foliation nearly 

 disappear, to the coarse schistose or flaggy, where the lines 

 are distinctly visible. These lines are no indication of the 

 gneiss having been laid down in water as sedimentary 

 rocks, though in all probability they were so formed. The 

 lines of " foliation " are rather indications of the rock havii 

 been changed by heat and pressure, so that the segregation 

 of the minerals which form these lines took place by the 

 relative cooling into the crystalline form of the separate 

 minerals which compose the gneiss. The gneiss passes, as 

 it has been above said, from the foliated state to the 

 amorphous or shapeless granitic state. This is well seen 

 in Lewis, where the flaky gneiss of the east side of the 

 island passes into the compact gneiss or granite of Beinn 

 Bhragair. With the exception of this granite, which, by 

 the way, compares well with the best Aberdeen grey granite, 

 the gneiss of Lewis weathers rapidly, and hence is not 

 durable as a building stone. Lewis Castle, at Stornoway, 

 is built of the Bragar Granite. The stone used in the best 

 class of houses in Stornoway is taken from the Torridon 

 Sandstone of Isle of Martin at the entrance of Loch Broom 

 across the Minch. 



