Handbook of Paleontology 59 



sils and comparison with the known succession of the 

 series elsewhere. 



Unconformities and disconformities may be of large 

 extent or regional or of small extent or local. A 

 regional unconformity may be recognized, as pointed 

 out above, by discordance in bedding at the line of 

 contact. There are, however, other distinguishing 

 characters to be looked for, such as the presence of a 

 basal conglomerate or arkose at the contact line; 

 faults, dikes and other igneous intrusive bodies in the 

 lower series that are truncated by the upper series of 

 beds ; greater amount of folding or metamorphism in 

 one formation than in the other, etc. The number of 

 geologic formations missing in an unconformity or 

 disconformity constitutes its hiatus. One must be 

 careful not to confuse cross-bedding with angular un- 

 conformity, nor contemporaneous erosion with discon- 

 formity. Sometimes after sedimentation has been go- 

 ing on for some time a change in conditions causes 

 erosion to take place. The deposition of the sediments 

 may begin again after a relatively short time and a 

 break or unconformity is developed of so small an ex- 

 tent that it is only a local unconformity. As the ero- 

 sion, however, took place in unconsolidated sediments 

 during a short cessation of deposition it is referred to 

 as contemporaneous erosion. 



Age Relations of Sedimentary Rocks 

 The beds of sedimentary deposits are built up layer 

 by layer. Therefore, each succeeding stratum is 

 younger than the one just below and older than the 

 one deposited next above. This is true likewise for 

 the consolidated strata in most cases, but when there 



