CHAPTER I 



PRINCIPLES OF CLASSIFICATION AND CORRELATION OF THE 



PRE-CAMBRIAN ROCKS 



CHARLES RICHARD VAN HISE 



A half-hour summary of the principles of classification and correla- 

 tion of the pre-Cambrian rocks can give no more than the barest 

 outline of the subject. 



In the classification and correlation of the pre-Cambrian forma- 

 tions we lack the guide of fossils. While Hfe existed in pre-Cambrian 

 times, and a few fossils are found in several areas, they are not suffi- 

 ciently abundant to serve either for the purposes of classification 

 or correlation. How far-reaching this handicap is will be reahzed 

 when this paper is contrasted with those that follow. In considering 

 the questions of classification and correlation of the later formations, 

 fossils occupy a paramount position. It is true that the faunal breaks 

 are often and probably are generally dependent upon physical causes, 

 and the latter are frequently considered; but when the determinations 

 are made, the fauna rather than the physical factors are given first 

 place. 



In the classification and correlation of the pre-Cambrian our sole 

 criteria are physical. Therefore we have for the discriminations only 

 those guides which for the fossiliferous rocks are commonly regarded 

 as subordinate. It follows that with the pre-Cambrian rocks we are 

 on less certain ground than with the later formations. However, 

 the very fact that fossils are not available in studying the pre-Cambrian 

 has led the workers in this field to a careful consideration of the 

 physical criteria and their relative value. 



Among the physical factors which have been used in the classi- 

 fication and correlation of the pre-Cambrian, the following are the more 

 important: (i) Lithological character; (2) Continuity of formations; 

 (3) Likeness of formations; (4) Like sequence of formations; (5) 

 Subaerial or subaqueous deposits; (6) Unconformities; (7) Relations 



