THE PRE-CAMBRIAN ROCKS 99 
2. Continuity of formations—Where formations in different 
districts are found to be continuous, they are supposed to be of the 
same age. It is realized that this conclusion is not absolute, for in 
the case of a great slanting transgression of the sea, the basal clastic 
deposits of the early part of the transgression may be considerably 
earlier than those in the later part, although the formations may be 
continuous. However, as yet given pre-Cambrian formations have 
not been traced to sufficiently great distances to introduce important 
errors upon this account. 
3. Likeness of formations.—Where in different districts there are 
like formations, this is of assistance in correlation. ‘hus. at an 
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 peculiarities, 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 peculiar character, and were so remarkably alike, 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. 
