THE PRE-CAMBRIAN ROCKS 3 



not been traced to sufficiently great distances to introduce important 

 errors upon this account. 



3. Likeness of formations. — Where in different districts there are 

 Hke formations, this is of assistance in correlation. Thus, if in 

 several districts of a geological province but a single limestone forma- 

 tion is observed in any one, and the limestone of the different districts 

 has the same pecuharities, there is a natural tendency to suppose all 

 the limestone to be part of a single formation. However, the criterion 

 of lithological likeness alone is not sufficient to establish identity. 

 This is illustrated by the three iron-bearing formations of the Lake 

 Superior region. Because these formations were of such an excep- 

 tional and pecuhar character, and w^re so remarkably ahke, it was 

 supposed for a long time that they were of the same age. For a 

 number of years this mistaken belief was a serious hindrance to an 

 understanding of the succession and structure in this region. The 

 weakness of lithological likeness in correlation is due to the fact that 

 the same set of physical conditions has frequently occurred during 

 geological time, and thus formations practically identical even in the 

 combinations of their variations, including color, banding, nature 

 of beds, etc., have been produced again and again. 



4. Like sequence of formations. — Similar sets of formations in the 

 same order furnish a criterion for correlation, of much greater con- 

 sequence than the likeness of a single formation. But even this 

 criterion has severe limitations, for similar sets of formations in the 

 same order may have been deposited a number of times during a 

 geological era; for instance, when a sea transgresses over a land area 

 there are normally formed in order a psephite, a psammite, a pelite, 

 and a non-clastic formation, and frequently over this, another pelite. 

 Several such similar sets of formations are known in the pre-Cambrian 

 in a single geological province. 



5. Suhaerial or subaqueous deposits. — Closely connected with the 

 third and fourth criteria is the question as to whether the deposits were 

 laid down under air or under water. It is clear that the conditions 

 of deposition of these two classes of rocks are so different and the 

 nature of the formations which may be contemporaneous so variable, 

 that there is great difficulty in correlating the two. Also it is plain 

 that the difficulties in correlating disconnected continental deposits 



