■September 12, 19 12] 



NATURE 



51 



sedimentation. This layer, which is singularly devoid 

 of organisms, ushers in the great succession of dolo- 

 mites and limestones, upwards of 1500 feet in thick- 

 ness—perhaps the most remarkable type of sedimenta- 

 tion among the Cambrian rocks of the north-west 

 Highlands. The Geological Survey has divided this 

 calcareous sequence into seven well-marked groups, 

 some of which have as yet yielded no fossils beyond 

 worm casts. Attention will presently be directed to 

 the absence of calcareous forms in many of the bands 

 of dolomite and to the probable cause of their dis- 

 appearance. 



The thin calcareous shale just referred to is fol- 

 lowed bv dark blue dolomite limestone, forming the 

 basal portion' of the Ghrudhaidh group. It contains 

 sparselv scattered, well-rounded sand grains, with a 

 bed about three feet thick, near the bottom, charged 

 with Salterella pulchella and 5. ritgosa. In the over- 

 lying twentv feet of dolomite the sand grains gradually 

 disappear, and the rock assumes a mottled character, 

 due to innumerable worm casts of the Planolites type, 

 Here a second layer, yielding S. pulchella and S. 

 riigosa. supervenes, both forms occurring in the 

 Olenellns zone of North America. 



The brief summary of the palseontological evidence 

 which has just been given clearly shows that the 

 strata ranging from the middle of the pipe-rock zone 

 to the upper Salterella band of the Durness dolomites 

 represent in whole or in part the Olenellus zone of 

 North America. Owing to the absence of fossils we 

 have no means of deciding more definitely the base and 

 top of the Lower Cambrian rocks of the north-west 

 Highlands. All the quartzites lying below the middle 

 of the pipe-rock, notwithstanding the absence of zonal 

 forms, have been included in the Lower Cambrian 

 division. This correlation receives some support from 

 the remarkable discovery of Dr. VValcott, who found 

 primitive trilobites several thousand feet beneath the 

 beds yielding Olenellus Gilberti, the form closely allied 

 to the Highland trilobites. 



On the other hand, when we pass upwards for a 

 certain distance from the Salterella bands the evidence 

 is insufficient to establish the stratigraphical horizon 

 of the beds. For in the overlying strata, comprising 

 the remainder of the Ghrudhaidh group, the whole of 

 the Eilean Dubh group, and the lower part of the 

 Sail Mhor group, and consisting of dolomites, lime- 

 stones, and cherts, with little or no terrigenous mate- 

 rial, the onlv fossils that can be shown to be due to 

 organisms are worm casts of the nature of Planolites, 

 although the limestone and chert may have originated 

 from the debris of the calcareous and siliceous organ- 

 isms of the plankton. A noticeable feature of the 

 Ghrudhaidh and Eilean Dubh groups is the occurrence 

 in them of bands of brecciated dolomite on several 

 horizons, which do not imply any break in the con- 

 tinuous sequence of deposits. The total thickness of 

 this portion of the Durness dolomites and limestones, 

 yielding no fossils beyond worm casts, amounts to 

 •^:;o feet. 



^'But in the upper part of the Sail Mhor group 

 siliceous and calcareous organisms of a higher grade 

 make their appearance. Among the former we find 

 the Rhabdaria of Billings. The calcareous forms are 

 represented bv (i) gasteropods, including a single 

 specimen of a'murchisonid, two species of a pleuroto- 

 marid {Euconia Ramsayi and E. Etna) of a type occur- 

 ring in the calciferous rocks of Newfoundland and 

 Canada ; (2) cephalopods, comprising two slightly bcrit 

 forms with closely set septa and wide endogastric 

 siphuncles, showing affinities with those of Endoceras 

 and Piloceras; (3)" arthropods, represented by the 

 epitome of a large asaphoid trilobite resembling that 

 of Asaphus canalis of Conrad. This evidence is in- 



NO. 2237, VOL. 90] 



sufficient to determine the exact horizon of these beds, 

 but clearlv indicates that we are no longer dealing 

 with Lower Cambrian strata. The cephalopods are 

 like those found in the Ozarkic division of L Irich 

 (Upper Cambrian), in North America. According to 

 Schuchert, the cephalopods with closely set septa are 

 of Cambrian type and older than those of the Beek- 

 mantown terrane of American geologists. On the 

 other hand, the asaphoid type of trilobite is sugges- 

 tive of a somewhat higher horizon. 



No fossils have been found in the overlying Sango- 

 more group, about 200 feet thick, which consists 

 mainly of granular dolomite, with bands of chert, 

 some ' being oolitic, together with thin fine-grained 

 limestones near the top. 



Above this horizon, at a height of more than 800 

 feet above the top of the Olenellus zone, we encounter 

 the great home of the fossils peculiar to the Durness 

 limestone in the Balnakeil and Croisaphuill groups. 

 The former consists mostly of dark limestones, with 

 nodules of chert, and, w'ith a few alternations, of 

 white limestone bands. A few thin layers are charged 

 with worm casts. The overlying group is more 

 varied, the lower part being composed of dark grey 

 limestones full of worm casts, and with some_ small 

 chert nodules arranged in lines ; the middle portion, of 

 dark granular and unfossiliferous dolomite ; and the 

 upper "part, of massive sheets of fossiliferous limestone 

 full of worm casts. The total thickness of these two 

 groups in Durness is about 550 feet. 



These two subdivisions have yielded more than 

 twenty genera and about one hundred species. In 

 Durness sixty-six species have been obtained from the 

 Balnakeil group alone, fifteen of which have not as yet 

 been found in the overlying Croisaphuill group, thus 

 leaving fif tv-one species common to both divisions. The 

 Ben Suarcial limestones in Skye, which were mapped 

 bv the Geological Survey as one division, are regarded, 

 on palseontological grounds, as the equivalents of both 

 these groups. Owing to the number of species com- 

 mon to both subdivisions, the fauna will be here 

 referred to as a whole. 



Both siliceous and calcareous organisms are present 

 in this fauna. Among the former we find 

 Archaeoscvphia (Hinde), described by Billings 

 as Arch'aeocvathus, an early Cambrian coral, 

 but shown bv Hinde to be a siliceous sponge.' 

 The genus 'Calathium is represented by four 

 species. Other genera and species of sponges occur, 

 so that the siliceous nodules, which are very common 

 in both groups, may be in great part due to them. 

 In this connection it may be mentioned that Hmdc 

 obtained sponge spicules 'from some of the nodules. 

 Hinged brachiopods have 'also been collected from 

 these beds, and include Nisusia {O)1hosina) feslinnta. 

 N. grandaeva, and Camarella. 



But the characteristic feature of the fauna is the 

 assemblage of calcareous mollusca comprising lamelli- 

 branchs, "gasteropods, and cephalopods, showing a 

 wide range of variation, and consequently a long 

 ancestrv. The lamellibranchs, though represented 

 only by two genera, Eucliasma and Eopieria of Bill- 

 ing's, with several intermediate forms, are of extreme 

 interest, as thev are only known to occur elsewhere 

 in Newfoundland and ea'stern Canada. The gastero- 

 pods, however, furnish the largest number of species 

 —about 48 per cent, of the whole. The primitive 

 euomphalids, Maclurea and Ophileta. arc most char- 

 acteristic. The former genus has a large number of 

 species, manv of which are to be found in the Beek- 

 mantown limestone of Newfoundland and eastern 

 North .America. Only one of the species (Maclurea 

 Pcachi) is peculiar to Durness. Several species of 



i Quart. Jour. Geol. Soc, vol. xlv., p. 125, 1889. 



