24 



HISTORICAL GEOLOGY 



2. Similarity of materials. Rock formations not actually 

 continuous, though not too widely separated, are often correlated 

 by noting similarity or identity of lithologic character, especially 



if there are any locally peculiar 

 features. Older geologists were 

 inclined to overwork this method 

 of correlation by applying it over 

 areas of too great extent, in 

 some cases even suggesting iden- 

 tity of age of deposits on oppo- 

 site sides of the ocean by this 

 means. The danger of such 

 application is apparent when we 

 realize that, for example, a sand- 

 stone of very early (Cambrian) 

 age may be exactly like sand- 

 stone of much later (Tertiary) 



of 



Fig. 15 

 Diagram to illustrate correlation 



rock formations by continuity of » 

 deposit. (W. J. M.) 



3. Similarity of sequence. A 

 succession of strata in two places 

 like A and B (Fig. 16), and not continuous on the surface, may 

 be correlated on the basis of similarit}?' of sequence, particularly 

 when each formation at one place 

 (A) shows little or no difference 

 in lithologic character or thick- 

 ness as compared with each 

 formation at the other place (B). 

 4. Similarity of degree of 

 change, or structural relations. B} r 

 finding similarly changed or 

 metamorphosed rocks in the 

 same vicinity, they may thus be 

 correlated. For instance, in the Fi 16 



accompanying diagram (Fig. 17) Diagmm to mu ^ ate correlation of 

 it is evident that the rocks of 

 group A are older than those of 

 group B, and these in turn older 

 than C. Outcrops over limited areas at least can thus be placed in 

 one of these three groups. By way of illustration, the (pre-Pale- 

 ozoic) rocks of the Highlands of the Hudson in southeastern New 





















"1" _~_~_" 





~_~_ — -~— 



_r~-» ~ _"~.r 



INI 



MM 



INI 







A 





B 





rock formations by similarity of 

 sequence. (W. J. M.) 



