738 



PRINCIPLES OF STRATIGRAPHY 



the middle portion of the intercalated formation is a constantly de- 

 creasing one from section i to section 5. In each section it covers 

 the interval of time hetween the emergence of the sea hottom at that 

 point during the regressive movement and its resubmergence during 

 the succeeding transgressive movement. This relationship may be 

 expressed in the preceding diagram. (Fig. 150.) The following 

 diagrammatic sections (Fig. 151) show the relation of a series of 

 strata recording a transgressive movement followed by a regressive 

 and again by a transgressive movement, x-a is the basal sandstone 

 bed of the transgressive series, x-y the intercalated sandstone of the 



Fig. 151. Diagram showing the relationships of the strata in a transgressive- 

 regressive-transgressive series. Basal sandstone .r c and inter- 

 calated sandstone .r y are both present. 



regressive-transgressive series. Beds i to 3 are the first series of 

 transgressive formations, beds 4 to 7 the regressive formations, and 

 beds 8 to 12 the second series of transgressive formations. 



Examples of Intercalated Sandstones from the Paleozoic 

 AND Mesozoic Formations- of North America. 



TJic Saint Peter Sandstone. This is an exceedingly pure quartz 

 sandstone of rounded, well-worn and pitted grains of nearly uni- 

 form size in any given region. It is widely distributed in central 

 North America, lying generally between a member of the Chazyan 

 and one of the Beekmantownian formations of the Ordovicic. In 

 Minnesota it lies beneath the upper beds of the Stones River (Up- 

 per Chazyan), grading upward into and containing some of its 

 characteristic fossils in the upper layers. Its base rests upon lower 

 Beekmantown (Shakopee). into which it also grades in many sec- 

 tions, though in others it forms a sharp contact or even rests on an 



