456 Notices of Memoirs — Dundee, 1912 — 



Torridon Sandstone. The Lewisian Gneiss, as mapped by the Geological 

 Survey, consists mainly of igneous rocks, or of gneisses and schists of 

 igneous lorigin. But, in addition to these materials, we find, in the Loch 

 Maree region, schists of sedimentary origin, comprising siliceous schist, 

 mica-schist, graphite-schist, limestone, chert, and other sediments. The 

 association of graphite-schist with limestone and chert suggests that we 

 are here dealing with rocks 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 specialized 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 history in pre-Cambrian time. 



The Cambrian Fauna of Scotland. 



Beginning with the false-bedded quartzites forming the ba5al 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 (Scolithus linearis) were obtained. In the upper subdivision 

 of the quartzites — the pipe-rocks — the cylinders of sand are so numerous 

 that the beds have been arranged in five sub-zone 3, 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 therefore 

 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 sub-zones of the 

 pipe-rock has yielded specimens of Salterella {Serpulites MacctiUochii) — 

 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, which immediately overlie the pipe-rocks, consist 

 chiefly of shales and brown dolomitic bands, with intercalations of grit 

 locally developed. This type of sedimentation indicates that the mud- 

 line was superimposed on the shore-line by subsidence. With this 

 change of conditions there is a change of organisms, for though the 

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

 the most characteristic types are those occurring along the bedding 

 planes, known under the name of Planolites (Nicholson). They are very 

 varied forms, and were probably produced by many types of errant 

 Annelids. The tubicolar Annelids are represented by Salterella, Coleoloides, 

 and Hyolithes — an organism which perhaps links the worms with the 

 hingeless Brachiopods. This suggestion gains additional support from 

 the researches of Dr. Walcott in the Middle Cambrian rocks of Canada. 

 It is interesting to note that small Annelids seem to have boi'ed the 

 spines of dead Trilobites. Walcott has found similar borings in the 

 chetse of Annelids in the Middle Cambrian rocks of Canada. i 



^ Smithsonian Misc. Coll., vol. Ivii, No. 5, p. 125, 1911. 



