50 



NATURE 



[September 12, 1912 



.Survey, consists mainly of igneous rocks, or of 

 gneisses and scfiists of igneous origin. But, in 

 addition to these materials, we find, in tfie Loch 

 Maree region, schists o sedimentary origin, com- 

 prising siliceous schist, mica-schist, "graphite-schist, 

 limestone, chert, and other sediments. The associa- 

 tion of graphite-schist with limestone and chert sug- 

 gests that we are here dealing with roclis that were 

 formed at or near the extreme limit of sedimentation, 

 where the graphite, the limestone, and the chert were 

 probably accumulated from the remains of plankton. 

 But this assemblage has been so completely altered 

 into crystalline schists that all traces of original 

 organic structure in them have been destroyed. 



The Torridonian strata were evidently accumulated 

 under desert or continental conditions, and could 

 therefore furnish little or no evidence bearing upon 

 the development of marine life. That life existed, 

 however, is clear from the presence of phosphatic 

 nodules, containing remains of cells and fibres of 

 organic origin, in the upper division of the system, 

 and from the presence of worm burrows and casts 

 in the Diabaig beds (Lower Torridon). 



Geologists are familiar with the fact that the 

 Cambrian faunas all over the globe present highly 

 specialised types belonging to most of the great 

 groups of marine invertebrate life. Scotland is no 

 exception to this general rule. For the fossils prove 

 that their ancestors must have had a long historv 

 in pre-Cambrian time. 



The Camhrian Fauna of Scollaiid. 



Beginning with the false-bedded quartzites forming 

 the basal sub-division of the Cambrian strata in the 

 north-west Highlands, we find no traces of organic 

 remains in them, except at one locality, where worm 

 casts (Scolithiis linearis) were obtained. In the upper 

 subdivision of the quartzites — the pipe-roclis — the 

 cylinders of sand are so numerous that the beds have 

 been arranged in five subzones, based on a definite 

 order of succession of different forms probably of 

 specific value. One of them, Arenicolites of Salter, 

 may be of generic importance. Worms of this habit 

 are confined to comparatively shallow water, and there- 

 fore near the shore line. Their occurrence helps to 

 confirm the belief that the quartzites were laid down 

 on an ancient shelving shore line during a period 

 of gentle subsidence. Their presence also indicates 

 the existence of plankton, from which they derived 

 nourishment. Besides the relics of these burrowing 

 annelids, one of the subzones of the pipe-rocli has 

 \ielded specimens of Saiterella (Serpiilites Maccid- 

 lochii) — a tubicolar annelid, which becomes more 

 abundant in ^the overlying fucoid beds, serpulite grit, 

 and basal limestone, where it is associated with 

 Olenellus and other typical Lower Cambrian forms. 



The fucoid beds, whicli immediately overlie the 

 |)ipe-rocks, consist chiefly of shales " and brown 

 dolomitic bands, with intercalations of grit locally 

 developed. This type of sedimentation indicates that 

 I he mud line was superimposed on the shore line by 

 subsidence. With this -change of conditions there is 

 a change of organistns, for though the burrowing 

 forms (Scolithus) are still to be found in the sandy 

 layers, the most characteristic types are those occur- 

 ring along the bedding planes, known under the 

 name of Planohtes (Nicholson). They are very varied 

 forms, and were probably produced bv many tvpes 

 of errant annelids. The tubicolar annelids are repre- 

 sented by Salterella, CoJeoloides, and Hyolithes — an 

 organism which perhaps links the worms with the 

 hingeless brachiopods. This suggestion gains addi- 

 tional support from the researches of Dr. Walcott 

 in the Middle Cambrian rocks of Canada. It is 

 NO. 2237, VOL. 90] 



interesting to note that small annelids seem to have 

 bored the spines of dead trilobites. Walcott has 

 found similar borings in the chetae of annelids in the 

 Middle Cambrian rocks of Canada.' 



The researches of Dr. Walcott have proved beyond 

 doubt that representatives of nearly all the divisions 

 of the annelids are entombed in the Middle Cambrian 

 rocks of Mount Stephen, in British Columbia. We 

 may therefore reasonably infer that the worm casts 

 of Scolithus type found in the north-west Highlands 

 are due to annelids. He has also shown that worm- 

 like holothurians are to be found in the same beds.^ 

 In this connection it may be observed that some of 

 the recent holothurians have much the same habit 

 of obtaining nourishment from the sands and silts 

 containing organic matter. 



Fragments showing the characteristic microscopic 

 structures of the plates and ossicles of echinoderms 

 have been found in the fucoid beds. These are 

 possibly Cystidean. Hingeless forms of brachiopods 

 also occur, among which may be mentioned Paterina 

 lahradorica and Acrothcle subsidua. The tvpe of 

 Acrothele suggests a genetic descent from such a 

 tubicolar worm as Hyolithes. Of the gasteropods, 

 only one specimen, belonging to a subgenus of 

 Murchisonia, has been obtained at one locality in 

 Skye. Helenia hclla, a curved calcareous tube, open 

 at both ends, doubtfully referred to the Dentalidae 

 by W'alcott, is comparatively plentiful. It occurs also 

 in the Olenellus zone in Newfoundland. 



But the organic remains that render the fucoid 

 beds of exceptional interest and importance are the 

 trilobites, because they clearly define the horizon of 

 this zone in the Cambrian system and display strong 

 affinities with .American types. They are represented 

 by five species and varieties of Olenellus, very closely 

 resembling the forms in the Georgian terrane, or 

 Olenellus zone, on the east and west sides of the 

 North .American continent. The genus Olenelloides 

 has also been recorded from these beds. The Crus- 

 tacea are represented by phyllocarids, among which 

 we find Aristo::oe rotundata. likewise characteristic 

 of the Olenellus zone of North America. 



Next in order comes the serpulite grit, which indi- 

 cates a recrudescence of the pipe-rock conditions of 

 deposition, and presents the Scolithus type of annelid 

 borings. From the diameter of the pipe and the 

 depth of the burrow it is probable that the worm 

 may have belonged to a different species from anv 

 of those the casts of which are to be found in lower 

 horizons. This large variety is associated with 

 smaller and more irregular worm casts which have 

 often weathered out and leave the rock honeycombed 

 with liollow casts. The characteristic form from 

 which the zone takes its name is Salterella (Serpulitrs 

 Maccullochii). It occurs abundantly along certain 

 calcareous layers that mark pauses in the deposition 

 of the sand. This calcareous type culminates at the 

 top of the zone, where there is a thick, carious, 

 weathering band, crowded with specimens of 5a?- 

 terella. forming a passage bed into the calcareous 

 shales at the base of the Durness dolomites. .At one 

 locality near Loch an Nid, Dundonnell Forest, Ross- 

 shire, thin shales, intercalated in the serpulite grit, 

 yielded a fine carapace of Olenellus Lapworthi — a 

 form of frequent occurrence in the underlying fucoid 

 beds. Prof. Lapworth recorded the finding of 

 Orthoceras and linguloid shells in the top part of 

 this zone at Eireboll.' 



Immediately above the serpulite grit in Eireboll and 

 .Assvnt we find a few feet of dark calcareous shale, 

 with iron pyrites, probably deposited at the limit of 



■ SmUhsonian MIscell. Collect., vol. Ivii., No. 5, p. 125, 101,. 

 ^ Wrf.. No. 3, loti. :» F s. « 



^ 0V(?/. .1/a^., vol. X., new series, p. 126, 1883. 



