562 A. W. GRABATJ LOWER ORDOVICIC FORMATIONS 



III. Sailmhor group. 

 II. Eilean Dubh group. 

 I. Ghrudaidh group. 



Eriboll quartzite series: 



Feet 



Serpulite grit 30 



Fucoid shales 40-50 



Pipe-rock 300 



Basal sandstone and conglomerate 200 



Unconformity. 

 Lewisian gneiss or Torridonian sandstone. 



The Fucoid shales carry a considerable Lower Cambric fauna, includ- 

 ing four species and two varieties of Olenellus, as well as such character- 

 istic species as Kutorgina (Iphidea) labradorica Bill, and others found 

 in the American Lower Cambric of the Appalachian province. The 

 Serpulite grit contains Salterella maccuttochi (Salter) and Olenellus 

 lapworthi. The succeeding Ghrudaith limestone contains three species 

 of Salterella — S. maccuttochi (Salt), S. pulchella Bill., and S. rugosa 

 Bill. — the last two being American species. The Salterella are distrib- 

 uted in two bands in the Ghrudaith limestone, one at the base and one 

 almost 30 feet above it. The upper part is mottled dolomite, "the mot- 

 tling being due to the great abundance of worm-casts of the nature of 

 Planolites." 6 



The Eilean Dubh group of fine-grained, white, flaggy limestones is 

 destitute of fossils, except those markings which have been referred to 

 worm-casts. This appears to have been a shallow-water accumulation 

 with occasional hardening of the layers on exposure and the production 

 of intra-formational breccias. An examination of the beds of this group 

 on Eilean Dubh (The Black Isle), in Balnakiel Bay, near Durness, which 

 I was able to make in 1910, disclosed such a zone about 10 feet thick on 

 the eastern side of the island, continuing with irregular thickness and 

 showing no stratification. The rock is a calcilutite and is succeeded 

 irregularly by a fine calcarenite. 



On the west side of the island, just above high-water mark, is an inter- 

 bedded conglomerate, with worn pebbles of limestone generally less than 

 an inch in diameter, the bed varying in thickness from a few inches to a 

 foot. There is also a green clay streaking the conglomerate in places. 

 The rock above this clay layer likewise appears conglomeratic or brecci- 

 ated; but this structure is not very distinct, and since the pebbles and 

 matrix are essentially of the same material, it is often difficult to differ- 



6 B. Peach and J. Home : The geological structure of the northwest highlands of 

 Scotland. Great Britain, 1907. 



