September 12, 19 12] 



NATURE 



55 



views now presented seem to me to be reasonable in- 

 ferences from the palasontological evidence set forth 

 in this address. 



In the north-west Highlands there is still a rem- 

 nant of the old land surface upon which the 

 Torridonian sediments were laid down. There is con- 

 clusive evidence that the pre-Torridonian land was one 

 of high relief. As the Torridonian sediments form part 

 of a continental deposit it may be inferred that the 

 Archaean rocks had a great extension in a north- 

 westerlv direction. The increasing coarseness of the 

 deposits towards the north-west suggests that the 

 land mav have become more elevated in that direction. 

 At anv rate, the pile of Torridonian sediments points 

 to a subsidence of the region towards the south-east, 

 and probably to a correlative movement of elevation 

 towards the north-west. 



The sparagmite of Scandinavia is an arkose re- 

 sembling the dominant type of the Torridon sand- 

 stone ; is of the same general age, and has evidently 

 been derived from similar sources in the Scandinavian 

 shield. In eastern North America coarse sedimentary 

 deposits form part of the newer Algonkian rocks, 

 which are still to be found rising from underneath the 

 Cambrian strata in the region of the great lakes. 

 These materials were obtained from the great 

 Canadian shield, which must have formed a large 

 continental area during their deposition. 



It is reasonable to infer that these isolated relics 

 of old land surfaces were united in pre-Torridonian 

 time, thus forming a continuous belt from Scandi- 

 navia to North America. During the period which 

 elapsed between the deposition of the Torridon sand- 

 stone and the basement members of the Cambrian 

 svstem a geosyncline was established which gave rise 

 to a submarine trough, trending in an east-north- 

 east and west-south-west direction, both in the British 

 and North American areas. In the latter region it 

 extends from Newfoundland to Alabama, its south- 

 eastern limit being defined by the old land surface of 

 Appalachia. The extension of this Appalachian land 

 area in a north-east direction beyond the limits of 

 Nova Scotia and Newfoundland was postulated by 

 Dana and other American writers. This geosyncline 

 remained a line of weakness throughout palceozoic 

 time, both in Britain and North America, which re- 

 sulted in the Caledonian system of folding in Britain, 

 and in the Taconic, Appalachian, and Pennsylvanian 

 systems in North America. Hence it is manifest that 

 the original shore-lines of this trough are now much 

 nearer each other than they were in Cambrian time. 



The Cambrian rocks of the north-west Highlands 

 were laid down along the north-west side of this 

 trough during a period of subsidence, for the great 

 succession of Durness dolomite and limestone, with 

 little or no terrigenous material, is superimposed on 

 the coarser sediments of that formation. On the 

 other hand, the Cambrian strata of Wales seem to 

 have been deposited along the southern limit of this 

 marine depression. The .Archaean rocks that now 

 constitute the central plateau of France may have 

 formed part of its southern boundary. The extension 

 of this land area towards the north-east may have 

 given rise to the barrier that separated the Baltic 

 life-province from that of Bohemia, Sardinia, and 

 Spain. In mv opinion this southern land area in 

 Western Europe was continuous across the Atlantic 

 with .\ppalachia. For the life sequence found in the 

 Cambrian rocks of New Brunswick is practically 

 identical with that of Wales and the Baltic provinces, 

 thus showing that there must have been continuous 

 intercourse between these areas. Along this sliore- 

 line the migration of forms seems to have been from 



NO. 2237, VOL. go] 



Europe towards America. On the other hand, along 

 the northern shore the tide of migration seems to 

 have advanced from America towards the north-west 

 Highlands. The question naturally arises, what cause 

 prevented the migration of the forms from one shore 

 of this trough to the other? American geologists are 

 of opinion that this is probably due to the existence 

 of land barriers ; but, in my opinion, it can be more 

 satisfactorily accounted for by clear and open sea, 

 aided by currents. 



The south-western extremity of the American trough 

 in Lower Cambrian time opened out into the Missis- 

 sippian sea, which was connected with the Pacific 

 Ocean, and stretched northwards towards the Arctic 

 regions. Reference has already been made to Wal- 

 cott's discovery in Nevada of the primitive trilobite 

 Nevadia Wceksi, from which he derives both branches 

 of the Mesonacidae, one branch linking Nevadia, 

 through Callavia, Holmia, and Wanneria, with Para- 

 doxides, the other connecting Nevadia with OleueUus. 

 through Mesonacis, Elliptocephalus and Paedumias. 



In Nevada the genus Holmia, as already shown, 

 is associated with the primitive type Nevadia. Wan- 

 neria is found in Nevada, in Alabama, and in Penn- 

 sylvania, thus showing that this genus is common to 

 the Missisippian sea and to the long trough north- 

 east of .-Xlabama. Mesonacis has been obtained in the 

 submarine depression at Lake Champlain, at Bonne 

 Bav, Newfoundand, and at the north side of the 

 Straits of Belle Isle. ElUptoccphalus has been re- 

 corded from the New York State. Olenelhis has been 

 found ip Nevada, in Vermont, and in the north-west 

 Highlands. All the genera now referred to may have 

 migrated along the north-western shore of this trough. 



-As regards the distribution of the genus Callavia, 

 this form has been met with in Maine, in Newfound- 

 land, and in derived pebbles in a conglomerate in 

 Quebec. Two species have been recorded in Shrop- 

 shire. These forms probably moved along the 

 southern shore of this sea from Wales to North 

 America. 



Reference has already been made to the fact that, 

 in the interval between Lower and Middle Cambrian 

 time, in certain areas in North .America the Lower 

 Cambrian rocks were locally elevated and subjected to 

 erosion. During this interval the southern end of 

 the trough seems to have had no connection with the 

 Mississippian sea, for in Middle Cambrian time, as 

 alreadv indicated, the Paradoxides fauna is found 

 in the trough on the east side of North .\merica, 

 whereas on the west side it is represented by the 

 Olenoides fauna. 



In Upper Cambrian time a great transgression of 

 the sea towards the north supervened. The DikeJo- 

 cephahts fauna is found on both sides of .America, 

 thus showing that the previous land barrier had been 

 submerged. While this genus occurs in Wales and 

 the Baltic provinces, it has not as yet been recorded 

 from the north-west Highlands, but I quite e.xpect 

 that this discovery may be made at some future time. 



.Along the northern side of the .American trough 

 clear water conditions prevailed, owing to the north- 

 ward recession of the shore-line, which led to the 

 accumulation of a great succession of calcareous de- 

 posits, including the Beekmantown limestone, to 

 which reference has already been made. Schuchert, as 

 already stated, has pointed out that, in the lower part 

 of the Ozarkic (Upper Cambrian) system, in Minne- 

 sota and Wisconsin, the gasteropod genera Hoopea, 

 Ophileta, and Raphistoma are associated with two 

 species of Dikelocephalus. This molluscan fauna is 

 evidently the precursor of that of the Beekmantown 

 limestorie. It was probably from this central region 



