CHAPTER IV 



THE ARCHAEAN ROCKS 



A. CLASSIFICATION AND NOMENCLATURE 



IN this chapter we shall deal with the oldest rocks which are 

 known to enter into the surface-structure of the earth's crust, rocks 

 which can be seen to emerge from below the oldest fossiliferous 

 (Cambrian) strata, not only in Britain but in many other parts of 

 the world. 



These primitive formations consist mainly of crystalline rocks 

 granite, gneiss, and crystalline schists but they also include great 

 thicknesses of stratified rocks, partly sedimentary and partly 

 volcanic, which have not been altered beyond recognition as such 

 rocks. Gneisses and crystalline schists belong to a category of their 

 own and are generally termed metamorphic rocks, yet they have 

 certainly not all been formed in the same way, and the determina- 

 tion of their relative age and manner of formation is often a very 

 difficult matter. Even the stratified Archaean rocks cannot be 

 classified and correlated in the same way as those of the Palaeozoic 

 systems can, because they are destitute of fossils, except some 

 minute microscopic bodies, and because they vary greatly in litho- 

 logical composition and in the extent of metamorphic change which 

 they have undergone. 



Consequently, when a succession of pre-Cambrian rocks has been 

 made out in any one area, it is very difficult to correlate the 

 divisions which are observable in this area with those which can be 

 made out in another. Even with regard to larger divisions, of 

 the rank of series or systems, European geologists have not yet 

 formulated any classification which has met with general acceptance. 

 They differ as to the number of rock-groups which can be recognised 

 in some of the principal areas of pre-Cambrian rocks, and also as 

 to the relative age of some of the component rock-masses ; and in 

 some cases it is a matter of dispute whether certain schists and 



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