12 . IOWA ACADEMY OF SCIENCES. 



at all is simply one of convenience. Furthermore, any lines that can be drawn 

 between the divisions, if divisions are to be made, must be to a large extent purely 

 arbitrary. The upper portions of the Dakota merge gradually into the Fort Ben- 

 ton, while the Fort Benton group passes by gradual transition paleontologically, 

 and in some places lithologically, into the calcareous beds of the Niobrara. 



Farther west, where the sea was deeper and the conditions presumably more 

 uniform, the distinctions between some of the groups cannot be maintained, and 

 King has combined the deposits of the Fort Benton, Niobrara and Fort Pierre 

 epochs under the single designation of the Colorado group. Hayden acquiesces in 

 this arrangement in his annual report for 1874; but later, in his report for 1877, 

 he makes the Colorado group include <'nly the Fort Benton and the Niobrara, 

 while the two upper divisions, the Fort Pierre and the Fox Hills, are united under 

 the name of the Fox Hills group. The Dakota group with its coarse sandstones 

 and leaves of forest trees is still recognized as a distinct division. 



And this leads to another consideration that is of wide reaching importance in 

 the correlation of synchronous geological deposits. The sandstones at the base of 

 the Dakota group near Sioux City owe their physical and even their paleontologi- 

 cal characters to conditions prevailing near the shore. As the bottom subsided 

 and the shore was moved farther to the east, the character of the deposits at Sioux 

 City changed, but coarse, cross-bedded sandstones and other littoral deposits, 

 charged with leaves and twigs of forest trees, must still have been formed in prox- 

 imity to the new shore Ime. Even while the chalk and limestone of the Niobrara 

 epoch were being precipitated over western Iowa from solution in clear sea water 

 that contained no trace of sediments, sandstones and shales, containing numerous 

 impressions of leaves and branches of terrestrial plants, must still have been piled 

 up along that most remote eastern shore. But if the shore deposits of the Nio- 

 brara epoch could now be found, it is probable that every competent geologist or 

 paleontologist would refer them unhesitatingly to the Dakota group. Deposits 

 absolutely synchronous may present very wide extremes of lithological and paleon- 

 tological characteristics. It is possible, 1 think, to recognize a law which I have 

 not seen expressly formulated, but which may run something in this wise: Si/n- 

 chronous deposits of the same geologic basin ate more likeJi/ to present uniform 

 lithological and pnleontological characters, if the geologist ti aces them along a line 

 parallel to the shore of the basin. If the observations are made along the line 

 that is radial to the geologic basin, or at right angles to the trend of the shore, the 

 different parts of absolutely synchronous beds are almost certain to vary in litho- 

 logical and paleoiitological characteristics so much as sometimes to tnake it appear 

 that different parts of the same bed belong to dfferent geologic epochs. 



This law may have greater force in connection with the study of Mesozoic and 

 Cenozoic strata than in the study of the more ancient terranes, but even among the 

 Paleozoics it must frequently have an important application. 



