54 



NATURE 



[September 12, 19 12 



It should be borne in mind that in Middle Cambrian 

 time the eastern and western parts of the western 

 life-province were evidently separated from each other 

 by a land barrier, owing to crustal movement, which 

 was probably connected with the elevation of the 

 Lower Cambrian rocks in the region where they were 

 subjected to erosion. 



In the upper division of the Cambrian system in 

 North America there is a marked change in the fauna. 

 Its characteristic features are thus clearly summarised 

 by Schuchert : " In a general way it may be said that 

 the Ozarkic period of Ulrich (Upper Cambrian) begins 

 with the trilobite genus Dikclocephalus and the first 

 distinct molluscan fauna. . . . The trilobites and in- 

 articulate brachiopods (greatly reduced in species) are 

 still Cambrian in aspect, while the new faunal feature 

 consists in a rapid evolution, in form and size, of the 

 coiled gasteropods, and of both straight and coiled 

 cephalopods. The latter are distinguished from those 

 of subsequent periods by the exceedingly close arrange- 

 ment of the septa." ^ 



The distinctive trilobite genus of the Upper Cam- 

 brian strata of the western life-province is DikeJoce- 

 plialus, where it is associated with an American facies 

 of fossils. The eastern or Atlantic province is char- 

 acterised by Olenids, though Dikclocephalus also 

 occurs, and by typical European forms. In Minne- 

 sota and Wisconsin, where the strata consist of 

 sandstones, dolomites, and shales, two species 

 of Dikelocephalus have been obtained, together with 

 other genera of trilobites such as Agnostiis and 

 lUaenurus ; the limuloid Aglaspis ; and the gastero- 

 pods Holopea, Ophileta, and Raphistoma. 



In certain areas this period is characterised bv a 

 great succession of calcareous deposits, comprising 

 parts of the Shenandoah limestone and Kittatinny 

 dolomite in New Jersey, portions of the Knox dolo- 

 mite in Tennessee, and of the dolomite and limestone 

 in Oklahoma. In some of these localities, at least, 

 the lower portions of this calcareous series are grouped 

 with the Upper Cambrian sediments, while the upper 

 parts are classed with Lower Silurian or Ordovician 

 strata. The researches of American pateontologists 

 have shown that in certain areas there is a mixed 

 Cambrian and Ordovician fauna in some of the beds, 

 as in the Tremadoc rocks of Wales. This com- 

 mingling of faunas is exemplified in the case of the 

 Beekmantown limestone, which is grouped with the 

 Ordovician (Lower Silurian) rocks by most -American 

 geologists. Ulrich and Schuchert, on the other hand, 

 regard it as a formation (the Canadic) distinct from 

 the overlying Ordovician system. 



The type areas of the Beekmantown limestone are 

 Lake Champlain, the Mingan Islands, and New- 

 foundland, where the strata consist mainly of a succes- 

 sion of limestones and dolomites more than looo feet 

 thick. The fossils are chiefly molluscan, comprising 

 lamellibranchs, gasteropods, and cephalopods. The 

 lamellibranchs are represented, among others, by the 

 genera Euchasma and Eopteria; the gasteropods bv 

 Ophileta, Maclurea, EuompliaUis, Holopea, Hormo- 

 tnma, Eciomaria, Murchisonia, Lophospira, Euconia, 

 Raphistoma, Helicotoma; the cephalopods by Ortho- 

 rrras, Cyrtoceras, Gomphoceras, Piloceras, Trocholites. 

 Of the foregoing genera many of the species are 

 common to this region and the north-west Highlands 

 of Scotland. 



The trilobites associated with this fauna comprise 

 the genera Dikelocephalus, Bathyriis, Asaphus, 

 Harpcs, and Nileus. 



In northern Newfoundland, in zones F to N of 

 Billings, this fauna, with localised species, is found 

 in great development in limestones and dolomites re- 



9 (9/.,//., r. 524. 



NO. 2237, VOL. 90J 



sembling those of Durness. Its upper limit is there 

 clearly defined, for the limestones and dolomites are 

 overlain by dark shales containing graptolites of 

 undoubted Arenig age. 



.\ careful comparison of the faunas of the Durness 

 and Beekmantown limestones shows that the assem- 

 blage of fossils in the Balnakeil and Croisaphuill 

 groups of Durness is practically identical with that in 

 the zones F to N of Billings, as developed in New- 

 foundland. These groups must therefore be older 

 than the .Arenig rocks of Wales, and must represent 

 at least the Welsh Tremadoc strata, if not part of the 

 Lingula Flags, both of which, according to the 

 English classification, are grouped with the Cambrian 

 system. 



But even in the purely European province of North 

 .\merica, in New Brunswick, where the Beekman- 

 town calcareous fauna is entirely absent, and where 

 the faunal sequence and type of sedimentation are 

 almost identical with those of North Wales, the basal 

 Ordovician or Lower Silurian rocks of .American 

 geologists include the Peltura scarabaeoides and the 

 Parabolina spiuitlosa zones, which, in Wales, are 

 classed with the Lingula Flags. It is obvious, there- 

 fore, that the boundary-line between the Cambrian and 

 Ordovician (Lower Silurian) systems is not drawn at 

 the same stratigraphical horizon by .American and 

 British geologists. In fixing the age of the Durness 

 dolomites and limestones the English classification 

 has been adopted. 



The palaeontological evidence now adduced regard- 

 ing the relation of the Cambrian fauna of the north- 

 west Highlands to that of North America leads to 

 the following conclusions : — 



1. The Lower Cambrian fauna of the north-west 

 Highlands, distinguished by the genus OlcneUiis and 

 its associates, is almost identical in character with 

 that of the Georgian terrane of the western life- 

 province of North America, and essentially different 

 from the Lower Cambrian fauna of the rest of 

 Europe. 



2. No forms characteristic of the Middle Cambrian 

 division, either of Europe or North America, have as 

 yet been found in the north-west Highlands; but this 

 division may be represented by the unfossiliferous 

 dolomites and limestones of the Ghrudhaidh, Eilean 

 Dubh, and the lower Sail Mhor groups. 



3. The fossiliferous bands of the Sail Mhor group 

 mav be the equivalents of the lower part of the Upper 

 Cambrian formation. 



4. The Balnakeil and Croisaphuill grou])s of the 

 Durness dolomites and limestones contain a typical 

 development of the molluscan fauna of the Beekman- 

 town limestone, belonging to the western life-province 

 of North -America. As the Beekmantown limestone 

 is succeeded by shales, with .Arenig graptolites, it 

 follows, in accordance with British classification, that 

 these groups must be of L'pper Cambrian age. 



5. The highest subdivision of the Durness limestone 

 (Durine) has not yielded fossils of zonal value, and 

 the members of this group are not overlain in normal 

 sequence by graptolite-bearing shale or other sedi- 

 ments. 



Cambrian Palaeogeographv betiveen North America 

 and North-West Europe. 

 In attem]3ting to restore in outline the distribution 

 of land and sea in Cambrian time between North 

 America and north-west Europe reference must be 

 made to various investigators whose researches in 

 palaeogeography are more or less familiar to geo- 

 logists. .Among these mav be mentioned Suess, Dana, 

 De Lapparent, Freeh, Walcott, Ulrich, Schuchert, 

 Bailey Willis, Grabau, Hull, and Jukes Browne. The 



