11IK ARCHAEAN HO. 



phyllites are of Arch jean age at all, or are iin-tamm phosed aediiurnt.s 

 "f much later date. 



All ,'igree, however, in admitting that in the region* where 

 large tracts of pro-Cambrian rocks are exposed, there IB a general 

 succession from an older to a younger series, and that in some area* 

 then- is ;t distinct break and unconformity between the older and 

 younger series. Thus there is no doubt that the Archaean rock* 

 can be divided into two distinct systems, and some continental 

 geologists advocate the restriction of the name Archaean to the 

 older series and apply that of pre-Cambrian to the newer and more 

 distinctly stratified series. This, however, is not a satisfactory 

 nomenclature, for the term pre-Cambrian is an indefinite and 

 inconvenient name for a system or a period, though it is a useful 

 adjective in a general sense. It is better, therefore, to call all pre- 

 Cambrian rocks Archcean, using the term Protarchaean for tin- 

 oldest and Nearchcean or Eparchcean for the younger series. 



Difficulties, however, still remain, because in some countries, 

 and notably in America, the older crystalline rocks have been 

 divided into two and even three series, while in some areas two 

 series of distinctly stratified rocks are believed to exist Ultimately 

 it may be possible to divide the whole into three distinct systems, 

 as suggested by Professor Bouney, which may then be called 

 Protarchaean, Mesarchaean, and Eparchaean. 1 a 



Meanwhile at the present time the rock-groups which have been 

 recognised in different areas are known by different local names ; 

 so that not only every country, but almost every separate district 

 in which Archaean rocks are exposed, has its own special nomen- 

 clature, and it is seldom possible to correlate the groups established 

 in one country with those of another. 



Under these circumstances it is obvious that the Archaean rocks 

 cannot be described in separate chapters on a stratigraphical basis, 

 but must be dealt with regionally, those of each large area being 

 described separately and successively. As regards their total 

 thickness exaggerated ideas seem to prevail in some quarters. 

 Attempts to estimate the thickness of crystalline schists are quite 

 untrustworthy, and where that of the stratified series can be 

 estimated their thickness is not more than that attained by 

 Cambrian or Silurian strata in some district*, t.. from 10,000 to 

 16,000 feet 



" References will be fouml at the end of the ohnpUr. 



