III4 



PRINCIPLES OF STRATIGRAPHY 



two quadrangles. This distinctness is partly due to the fact that 

 formations represented in one are wanting in the other, owing to 

 discontinuity of sequence (represented by either unconformities or 

 disconformities). It would, however, be just as true if the forma- 

 tions were complete in both. The greater number of subdivisions 

 is found in the more western nearer shore phase of the lower 

 (Ordovicic) system, while the Siluric and Mississippic systems have 

 their nearer shore phase in the eastern sections. The following dia- 



FiG. 263. A. Section showing variation of strata from shore seaward. B. 

 The same section after folding and erosion. C, D. Maps of 

 the same region, representing the two end quadrangles which 

 have scarcely any formations in common. 



grams (Fig. 26^) illustrate the change in formations away from 

 shore, and the resultant differences in the cartographic units of two 

 quadrangles separated by an interval of one quadrangle only. In 

 the eastern portion of the section only sands were deposited, con- 

 stituting but one formation. Owing to repeated oscillation during 

 the deposition of these sands, a series of intercalations of the more 

 off-shore clays and the still more distant limestones occurred. 

 Two anticlinal folds were formed which subsequently suffered 

 erosion and exposed the succession of beds. The strike of the eroded 

 strata is northeasterly, though the section is due east and west at 



