98 CHARLES RICHARD VAN HISE 
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: (1) Lithological character; (2) Continuity of formations; 
(3) Likeness of formations; (4) Like sequence of formations; (5) 
Subaerial or subaqueous deposits; (6) Unconformities; (7) Relations 
to series of known age; (8) Relations with intrusive rocks; (g) Amount 
of deformation; (10) Degree of metamorphism. 
1. Lithological character.—The first step in the study of rocks 
from a physical point of view is to determine the character of the: 
formations, series, and groups—whether igneous or sedimentary; 
if igneous, whether plutonic or volcanic, acid or basic; if sedimentary, 
whether psephite, psammite, pelite, limestone. While according to 
definition a formation is essentially a lithological unit, usually this 
unit is more or less composite, consisting of many somewhat variable 
beds and often of several members of different character. Because 
of the variability of the elements constituting a formation, there are 
an indefinite number of permutations and combinations of these 
factors. ‘This results in giving a given formation, series, or group 
special peculiarities which often enable one to recognize it even when 
actual connections of the various outcrops have not been observed. 
Accepting any: of the current theories as to the history of the earth, 
the rocks of the earliest time are dominantly of igneous origin, and 
those of later time dominantly sedimentary. Since the earliest 
Cambrian rocks contain remains of all the great types of life, it is 
certain that antecedent to this time the more fundamental and 
greater part of organic evolution took place. Hence in a full 
pre-Cambrian succession we should expect the rocks of the early 
pre-Cambrian to be dominantly igneous and those of the later 
pre-Cambrian to be dominantly sedimentary. In accordance with the 
natural expectation, in practically all of the great regions of the world 
in which the pre-Cambrian have very extensive exposures, and in 
which close studies have been made, we find that the basal series of 
rocks is dominantly igneous, and the superior series dominantly 
sedimentary. 
