66 ME. A. GEIKIE ON THE AGE OF THE 



ward, keeping close to the shore until it widens out south of the 

 church, ascending inland as far as Sithein and disappearing towards 

 the mouth of the Sound of Scalpa. The Liassic band which crosses 

 Strath may be described as a long, flat, synclinal trough with the 

 basement-beds coming to the surface on both the north-west and 

 south-east sides, but with such a general predominant westward in- 

 clination that at the south-western end of the tract higher beds 

 appear than are to be found to the north-east. 



To the north and north-west of this belt of Liassic limestone, 

 shale and sandstone, the older or so-called " unstratified " limestone 

 covers most of the ground up to the flanks of the chain of Red Hills, 

 which stretches across the island from the head of Loch Slapin to 

 the Sound of Scalpa. The tracts not occupied by the limestone are 

 covered with eruptive bosses of granophyre (MaccuUoch's " syenite") 

 except the district of Loch Lonachan and Beinn Suardal, where red 

 sandstone (Cambrian or Torridon sandstone) makes its appearance. 

 Even from a distance the areas occupied by the limestone are easily 

 recognizable by the brightness of their verdure and by the innumer- 

 able knobs and pinnacles of bare white or grey stone that project 

 from the ground. On closer examination, the rock is seen to have 

 a geueral lead-grey colour, more or less veined with white. Some 

 portions are darker, almost black in tint, while others are nearly 

 pure white. This range of colour, so unlike anything to be seen 

 among the Liassic beds, exactly resembles that of the Durness lime- 

 stone and of its prolongation southwards through the counties of 

 Sutherland and Ross. 



Though on the whole more crystalline than any portions of the 

 Lias, the limestone throughout most of its extent is not more 

 altered than most Palaeozoic limestones are. Certainly it does not 

 deserve the name of " marble," except locally around the eruptive 

 bosses that have broken through it. It is for the most part firm, 

 close-grained, and compact, breaking with a splintery, semi- 

 conchoidal fracture. Many of the more sandy beds are crowded 

 with dark wav)- plant-like stripes, most of which are probably 

 worm-casts. Much of the limestone is tolerably pure ; some parts 

 are dolomitic. It weathers with a clean smooth surface. Many 

 of the dark beds are crowded with nodules and nodular layers 

 of black or blackish-grey chert. These concretions protrude in 

 enormous quantities from the weathered surface of the rock, to 

 which they give a singularly rough aspect. As they break up 

 under the influence of subaerial disintegration, they gather on the 

 slopes into curious cinder-like heaps. IN'ow, I have not succeeded 

 in finding a single concretion of chert in any of the Lias limestones 

 so well exposed for several miles along the shores of Broadford Bay 

 and Loch Eishort *. But every geologist who has seen the rocks of 

 Durness will remember how fuU are some of the limestones there 

 of similar dark irregular chert-concretions. 



* The occurrence of pieces of chert derived from the older liraestone, as at 

 Boreraig, obviously does not kivalidate this statement. 



