2 CHARLES RICHARD VAN HISE 



to series of known age; (8) Relations with intrusive rocks; (9) 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 hthological 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 variabihty 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 eariiest 

 Cambrian rocks contain remains of all the great types of life, it is 

 certain that antecedent to this time the more fundamental and 

 the 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. 



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 



