CHAPTER II 

 GENERAL PRINCIPLES — CONCLUDED 



Correlation of Rock Formations 



By Stratigraphy is meant that branch of geologic science 

 which " arranges the rocks of the earth's crust in the order of their 

 appearance, and interprets the sequence of events of which they 

 form the records" (A. Geikie). All stratified rocks may be sub- 

 divided into formations or groups of strata, each of which is marked 

 either by a characteristic facies or assemblage of fossils, or, to 

 greater or lesser extents, by similarity of lithologic features, or 

 by both. A rock formation is generally considered to be a map- 

 pable unit, that is its area can be delimited upon a geologic map. 

 Subdivisions of formations are usually called members. By cor- 

 relation of formations is meant the determination of the equiva- 

 lence, or practical equivalence, of rock groups or formations in 

 various parts of the earth. Exact contemporaneity for widely 

 separated districts cannot be expected as above explained in 

 chapter 1. In general the criteria of correlation may be divided 

 into two classes, namely, geological (physical) and paleontological 

 (biological). 1 



I. Geological (physical) criteria. In many cases forma- 

 tions carry no fossils or very few, and it is then necessary to seek 

 means of correlation without their aid. None of the geological 

 (physical) methods can, however, be applied over wide areas such 

 as opposite sides of a continent, or different continents. For such 

 wide correlations, criteria derived from a study of fossils only can 

 be used. 



1. Continuity of deposit. If, as shown in the accompanying 

 diagram (Fig. 15), continuity can be traced from A to B, it is quite 

 certain that the rock masses at A and B are of the same, or very 

 nearly the same, age. There is probably no more important means 

 of correlation used by the geologist except over wide areas. 



1 The criteria of correlation as here presented are based largely upon uni- 

 versity lectures by Dr. W. B. Clark. 



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