188 HALL AND SARDESON — THE MAGNESIAJ*] SERIES. 



shaly at the bottom and top. It is further fair to assume that a forma- 

 tion like the Jordan, still from 75 feet to 200 feet in thickness, originally 

 must have been a succession of sands and shaly sandstones. A typical 

 modern section of such a succession may be seen in the upper portion of 

 the marine division of the New Jersey Cretaceous,* since a marine fauna 

 has been found in the Jordan sandstone at Osceola, Wisconsin, and Rap- 

 idan, JMinnesota.f It is evident that calcareous beds also were laid 

 down with the sandstones and shales, since the faunas already discov- 

 ered give strong assumption of still others, now entirely obliterated 

 through the chemical changes of which the present condition of the rocks 

 bears such strong indications. 



With the above considerations in mind, it seems highly probable that 

 the present condition of the sandstones has been reached through pro- 

 longed physical and chemical action, the physical action being chiefly 

 effected through flowing waters and transported solids, the chemical 

 through the dissolving and redistribution of mineral compounds, includ- 

 ing the removal of those more soluble portions which are naturally de- 

 posited in the building of every sandstone bed. 



THE SHALES. 



Touching the origin of the shales, but little need be said in addition 

 to what was stated concerning the lithologic characters of the sandstones 

 in the preceding paragraphs. The shales are the most natural deposits 

 between sands and calcareous organic remains; indeed, it is hard to 

 understand how an alternation of sandstones and limestones could occur 

 without the interposition of a shale. As the carbonates in subsequent 

 alterations become changed by reduction and dolomitization, so |the 

 shales, to some extent carbonates in chemical composition, should be- 

 come reduced in thickness and altered chemically to more alumino- 

 silicious beds than were the original deposits. This view of the origin 

 of shales is far from new; indeed, Grandjean, in 1844, J suggested this 

 origin by exfiltration of the carbonates. He called attention particularly 

 to the lack of schistosity in those layers which contained a large per- 

 centage of calcium carbonate and in which the fossils were in a good 

 state of preservation. To show that the same physical conditions pre- 

 vailed throughout the entire period of formation-building, Grandjean 

 points out the fact that the same fossils occur, however different the 

 present conditions of the rocks may be. This fact is a significant one 

 when carried over into the study of the various members of the Magnesian 



* C. A. White : Correlation Papers, Bull. no. 82, U. S. Geol. Survey. 

 fSee ante, p. 177. 



X Die Dolomite und Braunstein : Lagerstatten in untern Lahn-Thale, Neues Jahrbueh fiir Min- 

 eralogie, u. s. w., 1844, p. 551. 



