3 2 



NA TURE 



{Nov. 13, 1884 



C. Calcareous Series ... 



VTT TIiirink Croup 5 Fine-grained, light gray limestones, with an occasional dark fossiliferous 



band 



VI. Croisaphuill Group 



V. Balnakeii. Group 



IV. Sangomore Group. 



III. Sailmhor Group 



II. Eilean Dubh Group 



I. Ghrudaidh Group 



c. Fine-grained, cleaved, lilac-coloured limestones, full of flattened 



worm-casts ; fossils distorted by cleavage. 

 b. Alternations of black, dark gray, and white limestone, with an occa- 

 sional fossiliferous band, like zone [a] of this group. 

 a. Massive, dark gray limestone, chiefly composed of worm-casts which 

 project above the matrix on weathered surfaces. Near the base 

 are several lines of small chert nodules. This is one of the most 

 highly fossiliferous zones in the Durness Basin. 

 [ Alternations of dark and light gray limestone, highly fossiliferous, with 

 \ occasional impure, argillaceous, unfossiliferous bands. Most of the 



( beds are distinctly cleaved, and contain few worm-casts. 



( Fine granular dolomites, alternating near the top with cream-coloured 

 < or pink limestone. Near the base are two or more bands of white 



( chert, one of which is about 5 feet thick. 



I Massive, crystalline-granular, dolomitic limestones, occasionally fossili- 

 ) ferous, charged with dark worm-castings set in a gray matrix ; 



} large spheroidal masses of chert near the base. This limestone is 



( locally known as "the Leopard Stone." 



( Fine-grained, white, flaggy, argillaceous limestones and calcareous 



\ shales. As yet no fossils have been found in this division. 



Dark leaden-coloured limestones, occasionally mottled, alternating near 



the top with white limestone. About 30 feet from the base there 



is a thin band of limestone charged with Serpulites Maccullochii, 



and a similar band occurs at the base. 



B. Middle Series (partly 

 calcareous and partly 

 arenaceous) 



Middle Zone 



Lower Zone 



A. Arenaceous Series. 



Lower Zone 



' At the base lies a massive band of quartzite and grit, passing upwards 

 ) into carious dolomitic grit, crowded in patches with Serpulites 



1 Maccullochii, more especially in the decomposed portions (Serpu- 



lite-Grit). 

 I Alternations of brown, flaggy, calcareous, false-bedded grits and 

 j quartzites with cleaved shales. 



Calcareous mudstones and dolomitic bands, weathering with a rusty 

 brown colour, traversed by numerous worm-casts, usually flattened, 

 and resembling fucoidal impressions. These beds are often highly 

 cleaved. This and the overlying zone form the " Fucoid-beds" of 

 previous authors. 



Fine-grained quartzites, perforated by vertical worm-casts and burrows 

 becoming more numerous towards the top of the zone ("pipe- 

 rock " of previous authors). These beds pass downwards into 

 massive white quartzites. 



False-bedded flaggy grits and quartzites, composed of grains of quartz 

 and feldspar. At the base there is a thin brccciated conglomerate, 

 varying from a few inches to a few feet in thickness, containing 

 pebbles of the underlying rocks, chiefly of quartz and orthoclase, 

 the largest measuring about 1 inch across. 



The Silurian strata in the Durness area are arranged 

 in the form of a basin, truncated on the east side by a 

 fault that brings them against the Archaean gneiss, and 

 thus disconnects them from the Eriboll area, with which 

 they were certainly at one time united. Of the identity 

 of these strata in the two areas there cannot be the 

 smallest doubt. We have recognised them zone by zone, 

 completing the proofs of this identity by detecting in the 

 south and central parts of the Durness Basin the represent- 

 atives of the middle series, viz. the " Fucoid-beds" and 

 " Serpulite-grit," which had not previously been noted in 

 that area. Though subject to local variations in thick- 

 ness, these zones are singularly persistent, and, from their 

 marked lithological characters and fossil contents, consti- 

 tute admirable horizons in unravelling the complicated 

 geological structure of the region. A rich assemblage of 

 fossils has been obtained by the Survey from the various 

 fossiliferous bands indicated in the foregoing table, com- 

 prising Trilobites, Annelids, Cephalopods {A'au/iltts, Lit- 

 nites, Orthoceras, Piloceras, &c), Heteropods, Gastero- 

 pods, Lamellibranchs, Brachiopods, Corals, Sponges, and 

 Foraminifera. As yet this collection has not been ex- 

 amined in detail, but from observations in the held it is 

 probable that some of the limestone zones will be found 

 to be characterised by particular fossils. 



A striking feature of the Durness Basin is the amount 

 of displacement of the strata by faults ; indeed, this 

 feature is so characteristic of the highest limestone zones 

 that it is difficult satisfactorily to compute their thickness. 

 But from the base of the quartzite to the top of the cal- 

 careous series the total thickness of Silurian strata cannot 

 be less than 2000 feet. In Sangomore Bay, near the vil- 

 lage of Durine, the highest limestone zone is overlain by 

 shattery quartzite, striped fissile schist, frilled schists, and 

 gneiss. Though unquestionably resting upon the lime- 

 stone and sharing in the normal faulting of the district, 

 these crystalline strata do not prove a conformable upward 

 succession, as has been naturally enough supposed. The 

 key to the reading of this and of the corresponding sec- 

 tion at Farrid Head is to be sought in the Eriboll tract. 



The Silurian rocks of the Durness Basin are separated 

 from those of Loch Eriboll (b in the Section) by a pro- 

 minent ridge of Archaean gneiss, the eastern slope of 

 which is covered by a cake of quartzite. Along the crest 

 of the chain the basal breccia is exposed, overlain by the 

 lower zone of false-bedded grits (No. 3) and the upper zone 

 of " pipe-rock " (No. 4 in Section). As the eastern de- 

 clivity of the ridge is greater than the dip of the quartz- 

 ites, the observer, on descending the slope, crosses the 

 basset edges and dip-slopes of the latter strata, and eventu- 



