Address by Professor B. N. Peach. 463 



that formation. On the other hand, the Cambrian strata of Wales seem 

 to have been deposited along the southern limit of this marine depres- 

 sion. The Archaean rocks that now constitute the central plateau of 

 France may have formed part of its southern boundary. The extensioi? 

 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 my opinion, this southern land area in Western 

 Europe was continuous across the Atlantic with Appalachia. For the 

 life sequence found in the Cambrian rocks of New Brunswick i? 

 practically identical with that of Wales and the Baltic provinces, thus 

 showing that there must have been continuous intercourse between these 

 areas. Along this shore-line the migration of forms seems to have 

 been from Eiu'ope towards America. On the other hand, along the 

 northern shore the tide of migration seems to have advanced from 

 Amei'ica 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 Mississippian sea, which was con- 

 nected with the Pacific Ocean, and stretched northwards towards the 

 Arctic regions. Eeference has already been made to Walcott's dis- 

 covery in Nevada of the primitive Trilobite Nevadia Weeksi, from 

 which he derives both branches of the Mesonacidse, one branch linking 

 Nevadia, through Callavia, Holmia, and Wanneria, with Paradoxides, 

 the other connecting Nevadia with Olenellus, through Mesonacis, 

 Elliptocephalus, and Pcsdumias. 



In Nevada the genus Solmla, as already shown, is associated with 

 the primitive type Nevadia. Wanneria is found in Nevada, in Alabama, 

 and in Pennsylvania, thus showing that this genus is common to the 

 Mississippian sea and to the long trough north-east of Alabama. 

 Mesonacis has been obtained in the submarine depression at Lake 

 Champlain, at Bonne Bay, Newfoundland, and at the north side of the 

 Straits of Belle Isle. ElliftocephaUis has been recorded from the New 

 York State. Olenellus has been found in 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 Newfoundland, and in derived pebbles in 

 a conglomerate in Quebec. Two species have been recorded in Shropshire. 

 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 connexion with the Mississippian S3a, for in Middle 

 Cambrian time, as already indicated, the Paradoxides fauna is found 

 in the trough on the east side of North America, 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 Dikelocephalus 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 expect 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 northward recession of the shore-line, which 



