?52 



THE GEOLOGY OF TASMANIA— THE PRE-CAMBRIAN. 



groupings now fully recognised in the regions in which 

 they are represented. It has been estimated that these 

 groupings of the Pre-Cambrian are comparable in im- 

 portance to the systems (Cambrian, Ordovician, Silu- 

 rian, etc.) into which the Palaeozoic has been divided. 



Hence Messrs. T. C. Chamberlin and R. D. Salisbury 

 hav€ in their Geology (i) divided the Pre-Cambrian 

 rocks into two main groups of systems — the Proterozoic 

 ■and the Archaeozoic — ^which rank more nearly with the 

 main subdivisions of the upper portion of the Geological 

 Record, viz., Palaeozoic, Mesozoic, and Cainozoic. 



By thes€ authors the Proterozoic and Archaeozoic 

 are again divided in the manner indicated in the table 

 of classification which has been prepared to show the 

 various systems of nomenclature. 



The Pre-Cambrian succession in North America has 

 been specially studied by a committee appointed by the 

 Geological Surveys of the United States and Canada, 

 and the classification adopted by the members of this 

 committee (2) is given in the accompanying comparative 

 table. 



It will be seen that the succession stated by this com- 

 mittee, to be applicable to the North American areas, 

 differs from that of Messrs. Chamberlin and Salisbury. 

 The chief difference lies in the transposition of the two 

 lowest groups. Messrs. Chamberlin and Salisbury 

 agree with other American geologists in placing the 

 Laurentian above the great schist series (which includes 

 the Keewatin), for the reason that in many cases the 

 granites and gneisses of the Laurentian occur as intru- 

 sions into the schist series. However, this question has 

 no direct bearing on the Tasmanian developments, and 

 will not be further discussed. 



On the whole, therefore, it will be conceded that a 

 definite succession of groups has been established for 

 the North American region. 



There are, however, differences of opinion as to how 

 these Pre-Cambrian series may best be included in 

 major groupings. 



(i) Loc. cit.. Vol. II., p. 139. 



(2) Ibidem, Vol. II.. p. 161. Also Journal of Geology, Vol. 

 XIII., No. 2, 1905, pp. 89-104. 



