824 



PRINCIPLES OF STRATIGRAPHY 



conformity if fully established has even a more far-reaching signifi- 

 cance. Where a marine formation is abruptly succeeded by a con- 

 tinental formation, the existence of a possible hiatus between the 

 two must be taken into consideration. Recently the tendency has 

 manifested itself in certain quarters to greatly multiply the number 



Fig. 20 1. 



Basal Palaeozoic sandstone resting unconformably upon gneisstiid 

 granite, Williams Canyon, Colorado. (After Hayden.) 



of disconformities. (Ulrich-22a.) In many cases the apparent 

 absence of a formation between two others is merely due to a 

 change in facies so that the formation is actually present, but in a 

 diflferent lithic or faunal facies or both. 



Unconformitv (Clinunconformity, Discordanz). (Figs. 201-202.) 



Fig. 202. 



Unconformity at Siccar Point, Scotland, a a, Ordovicic strata, 

 d, d, d, Old Red Sandstone. (After Lyell.) 



The Structural unconformity is readily recognized and the one 

 generally detected wherever it occurs. This type of uncon- 

 formity involves the folding of the older strata, and the subsequent 

 erosion of the folds follow^ed by the deposition of the later strata 

 upon the eroded edges of the older beds. This type of unconformity 



